@phdthesis{Maier2017, author = {Maier, Sebastian}, title = {Quantenpunktbasierte Einzelphotonenquellen und Licht-Materie-Schnittstellen}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-152972}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {Die Quanteninformationstechnologie ist ein Schwerpunkt intensiver weltweiter Forschungsarbeit, da sie L{\"o}sungen f{\"u}r aktuelle globale Probleme verspricht. So bietet die Quantenkommunikation (QKD, engl. quantum key distribution) absolut abh{\"o}rsichere Kommunikationsprotokolle und k{\"o}nnte, mit der Realisierung von Quantenrepeatern, auch {\"u}ber große Distanzen zum Einsatz kommen. Quantencomputer (engl. quantum computing) k{\"o}nnten von Nutzen sein, um sehr schwierige und komplexe mathematische Probleme schneller zu l{\"o}sen. Ein grundlegender kritischer Baustein der gesamten halbleiterbasierten Quanteninformationsverarbeitung (QIP, engl. quantum information processing) ist die Bereitstellung von Proben, die einerseits die geforderten physikalischen Eigenschaften aufweisen und andererseits den Anforderungen der komplexen Messtechnik gen{\"u}gen, um die Quanteneigenschaften nachzuweisen und technologisch nutzbar machen zu k{\"o}nnen. In halbleiterbasierten Ans{\"a}tzen haben sich Quantenpunkte als sehr vielversprechende Kandidaten f{\"u}r diese Experimente etabliert. Halbleiterquantenpunkte weisen große {\"A}hnlichkeiten zu einzelnen Atomen auf, die sich durch diskrete Energieniveaus und diskrete Spektrallinien im Emissionsspektrum manifestieren, und zeichnen sich {\"u}berdies als exzellente Emitter f{\"u}r einzelne und ununterscheidbare Photonen aus. Außerdem k{\"o}nnen mit Quantenpunkten zwei kritische Bausteine in der Quanteninformationstechnologie abgedeckt werden. So k{\"o}nnen station{\"a}re Quantenbits (Qubits) in Form von Elektronenspinzust{\"a}nden gespeichert werden und mittels Spin-Photon-Verschr{\"a}nkung weit entfernte station{\"a}re Qubits {\"u}ber fliegende photonische Qubits verschr{\"a}nkt werden. Die Herstellung und Charakterisierung von quantenpunktbasierten Halbleiterproben, die sich durch definierte Eigenschaften f{\"u}r Experimente in der QIP auszeichnen, steht im Mittelpunkt der vorliegenden Arbeit. Die Basis f{\"u}r das Probenwachstum bildet dabei das Materialsystem von selbstorganisierten In(Ga)As-Quantenpunkten auf GaAs-Substraten. Die Herstellung der Quantenpunktproben mittels Molekularstrahlepitaxie erm{\"o}glicht h{\"o}chste kristalline Qualit{\"a}ten und bietet die M{\"o}glichkeit, die Quantenemitter in photonische Resonatoren zu integrieren. Dadurch kann die Lichtauskoppeleffizienz stark erh{\"o}ht und die Emission durch Effekte der Licht-Materie-Wechselwirkung verst{\"a}rkt werden. Vor diesem Hintergrund wurden in der vorliegenden Arbeit verschiedene In(Ga)As-Quantenpunktproben mit definierten Anforderungen mittels Molekularstrahlepitaxie hergestellt und deren morphologische und optische Eigenschaften untersucht. F{\"u}r die Charakterisierung der Morphologie kamen Rasterelektronen- und Rasterkraftmikroskopie zum Einsatz. Die optischen Eigenschaften wurden mit Hilfe der Reflektions-, Photolumineszenz- und Resonanzfluoreszenz-Spektroskopie sowie Autokorrelationsmessungen zweiter Ordnung ermittelt. Der Experimentalteil der Arbeit ist in drei Kapitel unterteilt, deren Kerninhalte im Folgenden kurz wiedergegeben werden. Quasi-Planare Einzelphotonenquelle mit hoher Extraktionseffizienz: Planare quantenpunktbasierte Einzelphotonenquellen mit hoher Extraktionseffizienz sind f{\"u}r Experimente zur Spinmanipulation von herausragender Bedeutung. Elektronen- und Lochspins haben sich als gute Kandidaten erwiesen, um gezielt einzelne Elektronenspins zu initialisieren, manipulieren und zu messen. Ein einzelner Quantenpunkt muss einfach geladen sein, damit er im Voigt-Magnetfeld ein λ-System bilden kann, welches die grundlegende Konfiguration f{\"u}r Experimente dieser Art darstellt. Wichtig sind hier einerseits eine stabile Spinkonfiguration mit langer Koh{\"a}renzzeit und andererseits hohe Lichtauskoppeleffizienzen. Quantenpunkte in planaren Mikrokavit{\"a}ten weisen gr{\"o}ßere Werte f{\"u}r die Spindephasierungszeit auf als Mikro- und Nanot{\"u}rmchenresonatoren, dagegen ist bei planaren Proben die Lichtauskoppeleffizienz geringer. In diesem Kapitel wird eine quasi-planare quantenpunktbasierte Quelle f{\"u}r einzelne (g(2)(0)=0,023) und ununterscheidbare Photonen (g(2)indist (0)=0,17) mit hoher Reinheit vorgestellt. Die Quantenpunktemission weist eine sehr hohe Intensit{\"a}t und optische Qualit{\"a}t mit Halbwertsbreiten nahe der nat{\"u}rlichen Linienbreite auf. Die Auskoppeleffizienz wurde zu 42\% f{\"u}r reine Einzelphotonenemission bestimmt und {\"u}bersteigt damit die, f{\"u}r eine planare Resonatorstruktur erwartete, Extraktionseffizienz (33\%) deutlich. Als Grund hierf{\"u}r konnte die Kopplung der Photonenemission an Gallium-induzierte, Gauß-artige Defektstrukturen ausgemacht werden. Mithilfe morphologischer Untersuchungen und Simulationen wurde gezeigt, dass diese Defektkavit{\"a}ten einerseits als Nukleationszentren f{\"u}r das Quantenpunktwachstum dienen und andererseits die Extraktion des emittierten Lichts der darunterliegenden Quantenpunkte durch Lichtb{\"u}ndelung verbessern. In weiterf{\"u}hrenden Arbeiten konnte an dieser spezifischen Probe der fundamentale Effekt der Verschr{\"a}nkung von Elektronenspin und Photon nachgewiesen werden, der einen kritischen Baustein f{\"u}r halbleiterbasierte Quantenrepeater darstellt. Im Rahmen dieses Experiments war es m{\"o}glich, die komplette Tomographie eines verschr{\"a}nkten Spin-Photon-Paares an einer halbleiterbasierten Spin-Photon Schnittstelle zu messen. {\"U}berdies konnte Zweiphotoneninterferenz und Ununterscheidbarkeit von Photonen aus zwei r{\"a}umlich getrennten Quantenpunkten auf diesem Wafer gemessen werden, was ebenfalls einen kritischen Baustein f{\"u}r Quantenrepeater darstellt. Gekoppeltes Quantenfilm-Quantenpunkt System: Weitere Herausforderungen f{\"u}r optisch kontrollierte halbleiterbasierte Spin-Qubit-Systeme sind das schnelle und zerst{\"o}rungsfreie Auslesen der Spin-Information sowie die Implementierung eines skalierbaren Ein-Qubit- und Zwei-Qubit-Gatters. Ein k{\"u}rzlich ver{\"o}ffentlichtes theoretisches Konzept k{\"o}nnte hierzu einen eleganten Weg er{\"o}ffnen: Hierbei wird die spinabh{\"a}ngige Austauschwechselwirkung zwischen einem Elektron-Spin in einem Quantenpunkt und einem Exziton-Polariton-Gas, welches in einem nahegelegenen Quantenfilm eingebettet ist, ausgen{\"u}tzt. So k{\"o}nnte die Spin-Information zerst{\"o}rungsfrei ausgelesen werden und eine skalierbare Wechselwirkung zwischen zwei Qubits {\"u}ber gr{\"o}ßere Distanzen erm{\"o}glicht werden, da sich die Wellenfunktion von Exziton-Polaritonen, abh{\"a}ngig von der G{\"u}te des Mikroresonators, {\"u}ber mehrere μm ausdehnen kann. Dies und weitere m{\"o}gliche Anwendungen machen das gekoppelte Quantenfilm-Quantenpunkt System sehr interessant, weshalb eine grundlegende experimentelle Untersuchung dieses Systems w{\"u}nschenswert ist. In Zusammenarbeit mit der Arbeitsgruppe um Yoshihisa Yamamoto an der Universit{\"a}t Stanford, wurde hierzu ein konkretes Probendesign entwickelt und im Rahmen dieser Arbeit technologisch verwirklicht. Durch systematische epitaktische Optimierung ist es gelungen, ein gekoppeltes Quantenfilm-Quantenpunkt System erfolgreich in einen Mikroresonator zu implementierten. Das Exziton-Polariton-Gas konnte mittels eines Quantenfilms in starker Kopplung in einer Mikrokavit{\"a}t mit einer Rabi-Aufspaltung von VR=2,5 meV verwirklicht werden. Zudem konnten einfach geladene Quantenpunkte mit hoher optischer Qualit{\"a}t und klarem Einzelphotonencharakter (g(2)(0)=0,24) in unmittelbarer N{\"a}he zum Quantenfilm gemessen werden. Positionierte Quantenpunkte: F{\"u}r die Herstellung quantenpunktbasierter Einzelphotonenquellen mit hoher optischer Qualit{\"a}t ist eine skalierbare technologische Produktionsplattform w{\"u}nschenswert. Dazu m{\"u}ssen einzelne Quantenpunkte positionierbar und somit deterministisch und skalierbar in Bauteile integriert werden k{\"o}nnen. Basierend auf zweidimensionalen, regelm{\"a}ßig angeordneten und dadurch adressierbaren Quantenpunkten gibt es zudem ein Konzept, um ein skalierbares, optisch kontrolliertes Zwei-Qubit-Gatter zu realisieren. Das hier verfolgte Prinzip f{\"u}r die Positionierung von Quantenpunkten beruht auf der Verwendung von vorstrukturierten Substraten mit ge{\"a}tzten Nanol{\"o}chern, welche als Nukleationszentren f{\"u}r das Quantenpunktwachstum dienen. Durch eine optimierte Schichtstruktur und eine erh{\"o}hte Lichtauskopplung unter Verwendung eines dielektrischen Spiegels konnte erstmals Resonanzfluoreszenz an einem positionierten Quantenpunkt gemessen werden. In einem weiteren Optimierungsansatz konnte außerdem Emission von positionierten InGaAs Quantenpunkten auf GaAs Substrat bei 1,3 μm Telekommunikationswellenl{\"a}nge erreicht werden.}, subject = {Quantenpunkt}, language = {de} } @article{OPUS4-15008, title = {Search for dark matter in events with heavy quarks and missing transverse momentum in pp collisions with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {2}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3306-z}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150089}, pages = {92}, year = {2015}, abstract = {This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3 fb\(^{-1}\) of pp collisions collected at \(\sqrt {s}\) = 8 TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing b-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter-nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.}, language = {en} } @article{OPUS4-15011, title = {Search for Higgs boson pair production in the b\(\overline{b}\)b\(\overline{b}\) final state from pp collisions at \(\sqrt {s}\) = 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {9}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3628-x}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150119}, pages = {412}, year = {2015}, abstract = {A search for Higgs boson pair production pp → hh is performed with 19.5 fb\(^{-1}\) of proton-proton collision data at \(\sqrt {s}\) = 8 TeV, which were recorded by the ATLAS detector at the Large Hadron Collider in 2012. The decay products of each Higgs boson are reconstructed as a high-momentum b\(\overline{b}\) system with either a pair of small-radius jets or a single large-radius jet, the latter exploiting jet substructure techniques and associated b-tagged track-jets. No evidence for resonant or non-resonant Higgs boson pair production is observed. The data are interpreted in the context of the Randall-Sundrum model with a warped extra dimension as well as the two-Higgs-doublet model. An upper limit on the cross-section for pp → G\(^{*}_{KK}\) → hh → b\(\overline{b}\)b\(\overline{b}\) of 3.2(2.3) fb is set for a Kaluza-Klein graviton G\(^{*}_{KK}\) mass of 1.0(1.5) TeV, at the 95 \% confidence level. The search for non-resonant Standard Model hh production sets an observed 95 \% confidence level upper limit on the production cross-section σ(pp → hh → b\(\overline{b}\)b\(\overline{b}\)) of 202 fb, compared to a Standard Model prediction of σ(pp → hh → b\(\overline{b}\)b\(\overline{b}\)) = 3.6±0.5 fb.}, language = {en} } @article{OPUS4-15012, title = {Search for invisible decays of the Higgs boson produced in association with a hadronically decaying vector boson in pp collisions at \(\sqrt {s}\) = TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3551-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150126}, pages = {337}, year = {2015}, abstract = {A search for Higgs boson decays to invisible particles is performed using 20.3 fb\(^{-1}\) of pp collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. The process considered is Higgs boson production in association with a vector boson (V = W or Z) that decays hadronically, resulting in events with two or more jets and large missing transverse momentum. No excess of candidates is observed in the data over the background expectation. The results are used to constrain VH production followed by H decaying to invisible particles for the Higgs boson mass range 115 < m\(_{H}\) < 300 GeV. The 95 \% confidence-level observed upper limit on σ\(_{VH}\) × BR(H → inv.) varies from 1.6 pb at 115 GeV to 0.13 pb at 300 GeV. Assuming Standard Model production and including the gg → H contribution as signal, the results also lead to an observed upper limit of 78 \% at 95 \% confidence level on the branching ratio of Higgs bosons decays to invisible particles at a mass of 125 GeV.}, language = {en} } @article{OPUS4-15013, title = {Search for invisible particles produced in association with single-top-quarks in proton-proton collisions at \(\sqrt {s}\) = 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {2}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-014-3233-4}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150134}, pages = {79}, year = {2015}, abstract = {A search for the production of single-top-quarks in association with missing energy is performed in proton-proton collisions at a centre-of-mass energy of \(\sqrt {s}\) =8 TeV with the ATLAS experiment at the large hadron collider using data collected in 2012, corresponding to an integrated luminosity of 20.3 fb\(^{-1}\). In this search, the W boson from the top quark is required to decay into an electron or a muon and a neutrino. No deviation from the standard model prediction is observed, and upper limits are set on the production cross-section for resonant and non-resonant production of an invisible exotic state in association with a right-handed top quark. In the case of resonant production, for a spin-0 resonance with a mass of 500 GeV, an effective coupling strength above 0.15 is excluded at 95 \% confidence level for the top quark and an invisible spin-1/2 state with mass between 0 and 100 GeV. In the case of non-resonant production, an effective coupling strength above 0.2 is excluded at 95 \% confidence level for the top quark and an invisible spin-1 state with mass between 0 and 657 GeV.}, language = {en} } @article{OPUS4-15014, title = {Search for metastable heavy charged particles with large ionisation energy loss in pp collisions at \(\sqrt {s}\) = 8 TeV using the ATLAS experiment}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {9}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3609-0}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150149}, pages = {407}, year = {2015}, abstract = {Many extensions of the Standard Model predict the existence of charged heavy long-lived particles, such as R-hadrons or charginos. These particles, if produced at the Large Hadron Collider, should be moving non-relativistically and are therefore identifiable through the measurement of an anomalously large specific energy loss in the ATLAS pixel detector. Measuring heavy long-lived particles through their track parameters in the vicinity of the interaction vertex provides sensitivity to metastable particles with lifetimes from 0.6 ns to 30 ns. A search for such particles with the ATLAS detector at the Large Hadron Collider is presented, based on a data sample corresponding to an integrated luminosity of 18.4 fb\(^{-1}\) of pp collisions at \(\sqrt {s}\) = 8 TeV. No significant deviation from the Standard Model background expectation is observed, and lifetime-dependent upper limits on R-hadrons and chargino production are set. Gluino R-hadrons with 10 ns lifetime and masses up to 1185 GeV are excluded at 95 \% confidence level, and so are charginos with 15 ns lifetime and masses up to 482 GeV.}, language = {en} } @article{OPUS4-15015, title = {Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at \(\sqrt {s}\) = 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3517-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150152}, pages = {299}, year = {2015}, abstract = {Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb\(^{-1}\) of \(\sqrt {s}\) = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between E\(^{miss}_{T}\) > 150 GeV and E\(^{miss}_{T}\) > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presented.}, language = {en} } @article{OPUS4-14988, title = {Measurement of the production and lepton charge asymmetry of bosons in Pb+Pb collisions at \(\sqrt {^{S}NN}\) with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {1}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-014-3231-6}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149889}, pages = {23}, year = {2015}, abstract = {A measurement of W boson production in lead-lead collisions at \(\sqrt {^{S}NN}\)=2.76 TeV is presented. It is based on the analysis of data collected with the ATLAS detector at the LHC in 2011 corresponding to an integrated luminosity of 0.14 nb\(^{-1}\) and 0.15 nb\(^{-1}\) in the muon and electron decay channels, respectively. The differential production yields and lepton charge asymmetry are each measured as a function of the average number of participating nucleons ⟨N\(_{part}\)⟩ and absolute pseudorapidity of the charged lepton. The results are compared to predictions based on next-to-leading-order QCD calculations. These measurements are, in principle, sensitive to possible nuclear modifications to the parton distribution functions and also provide information on scaling of W boson production in multi-nucleon systems.}, language = {en} } @article{OPUS4-15017, title = {Search for resonant diboson production in the ℓℓq\(\overline{q}\) final state in pp collisions at \(\sqrt {s}\) = 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {2}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3261-8}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150171}, pages = {69}, year = {2015}, abstract = {This paper reports on a search for narrow resonances in diboson production in the ℓℓq\(\overline{q}\) final state using pp collision data corresponding to an integrated luminosity of 20 fb\(^{-1}\) collected at \(\sqrt {s}\) = 8 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess of data events over the Standard Model expectation is observed. Upper limits at the 95 \% confidence level are set on the production cross section times branching ratio for Kaluza-Klein gravitons predicted by the Randall-Sundrum model and for Extended Gauge Model W′ bosons. These results lead to the exclusion of mass values below 740 and 1590 GeV for the graviton and W′ boson respectively.}, language = {en} } @article{OPUS4-14997, title = {Observation and measurements of the production of prompt and non-prompt J/ψ mesons in association with a Z boson in pp collisions at \(\sqrt {s}\)=8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {5}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3406-9}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149970}, pages = {229}, year = {2015}, abstract = {The production of a Z boson in association with a J/ψ meson in proton-proton collisions probes the production mechanisms of quarkonium and heavy flavour in association with vector bosons, and allows studies of multiple parton scattering. Using 20.3fb\(^{-1}\) of data collected with the ATLAS experiment at the LHC in pp collisions at \(\sqrt {s}\) = 8 TeV, the first measurement of associated Z+J/ψ production is presented for both prompt and non-prompt J/ψ production, with both signatures having a significance in excess of 5σ. The inclusive production cross-sections for Z boson production (analysed in μ\(^{+}\)μ\(^{-}\) or e\(^{+}\)e\(^{-}\) decay modes) in association with prompt and non-prompt J/ψ(→μ\(^{+}\)μ\(^{-}\)) are measured relative to the inclusive production rate of Z bosons in the same fiducial volume to be (36.8±6.7±2.5)×10\(^{-7}\) and (65.8±9.2±4.2)×10\(^{-7}\) respectively. Normalised differential production cross-section ratios are also determined as a function of the J/ψ transverse momentum. The fraction of signal events arising from single and double parton scattering is estimated, and a lower limit of 5.3 (3.7)mb at 68 (95)\% confidence level is placed on the effective cross-section regulating double parton interactions.}, language = {en} } @article{OPUS4-15003, title = {Performance of the ATLAS muon trigger in pp collisions at \(\sqrt {s}\)= 8 TeV}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {3}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3325-9}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150032}, pages = {120}, year = {2015}, abstract = {The performance of the ATLAS muon trigger system is evaluated with proton-proton collision data collected in 2012 at the Large Hadron Collider at a centre-of-mass energy of 8 TeV. It is primarily evaluated using events containing a pair of muons from the decay of Z bosons. The efficiency of the single-muon trigger is measured for muons with transverse momentum 25 < p\(_{T}\) < 100GeV, with a statistical uncertainty of less than 0.01 \% and a systematic uncertainty of 0.6 \%. The pT range for efficiency determination is extended by using muons from decays of J/ψ mesons, W bosons, and top quarks. The muon trigger shows highly uniform and stable performance. The performance is compared to the prediction of a detailed simulation.}, language = {en} } @article{OPUS4-15006, title = {Search for W' → tb → qqbb decays in pp collisions at \(\sqrt {s}\) = 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {4}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3372-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150068}, pages = {165}, year = {2015}, abstract = {A search for a massive W′ gauge boson decaying to a top quark and a bottom quark is performed with the ATLAS detector in pp collisions at the LHC. The dataset was taken at a centre-of-mass energy of \(\sqrt {s}\) = 8 TeV and corresponds to 20.3 fb\(^{-1}\) of integrated luminosity. This analysis is done in the hadronic decay mode of the top quark, where novel jet substructure techniques are used to identify jets from high-momentum top quarks. This allows for a search for high-mass W′ bosons in the range 1.5-3.0  TeV. b-tagging is used to identify jets originating from b-quarks. The data are consistent with Standard Model background-only expectations, and upper limits at 95 \% confidence level are set on the W′ → tb cross section times branching ratio ranging from 0.16 pb to 0.33 pb for left-handed W′ bosons, and ranging from 0.10 pb to 0.21 pb for W′ bosons with purely right-handed couplings. Upper limits at 95 \% confidence level are set on the W′-boson coupling to tb as a function of the W′ mass using an effective field theory approach, which is independent of details of particular models predicting a W′ boson.}, language = {en} } @article{OPUS4-14096, title = {Measurement of the inelastic proton-proton cross-section at √s=7 TeV with the ATLAS detector}, series = {Nature Communications}, volume = {2}, journal = {Nature Communications}, organization = {The ATLAS Collaboration}, doi = {10.1038/ncomms1472}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-140960}, pages = {463}, year = {2011}, abstract = {The dependence of the rate of proton-proton interactions on the centre-of-mass collision energy, √s, is of fundamental importance for both hadron collider physics and particle astrophysics. The dependence cannot yet be calculated from first principles; therefore, experimental measurements are needed. Here we present the first measurement of the inelastic proton-proton interaction cross-section at a centre-of-mass energy, √s, of 7 TeV using the ATLAS detector at the Large Hadron Collider. Events are selected by requiring hits on scintillation counters mounted in the forward region of the detector. An inelastic cross-section of 60.3±2.1 mb is measured for ξ>5×10-6, where ξ is calculated from the invariant mass, MX, of hadrons selected using the largest rapidity gap in the event. For diffractive events, this corresponds to requiring at least one of the dissociation masses to be larger than 15.7 GeV.}, language = {en} } @article{OPUS4-14963, title = {Determination of spin and parity of the Higgs boson in the WW\(^{*}\)→eνμν decay channel with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {5}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3436-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149632}, pages = {231}, year = {2015}, abstract = {Studies of the spin and parity quantum numbers of the Higgs boson in the WW\(^{*}\)→eνμν final state are presented, based on proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb\(^{-1}\) at a centre-of-mass energy of \(\sqrt {s}\)=8 TeV. The Standard Model spin-parity J\(^{CP}\)=0\(^{++}\) hypothesis is compared with alternative hypotheses for both spin and CP. The case where the observed resonance is a mixture of the Standard-Model-like Higgs boson and CP-even (J\(^{CP}\)=0\(^{++}\)) or CP-odd (J\(^{CP}\)=0\(^{+-}\)) Higgs boson in scenarios beyond the Standard Model is also studied. The data are found to be consistent with the Standard Model prediction and limits are placed on alternative spin and CP hypotheses, including CP mixing in different scenarios.}, language = {en} } @article{ZhangWuLietal.2015, author = {Zhang, Xin and Wu, Wei and Li, Gang and Wen, Lin and Sun, Qing and Ji, An-Chun}, title = {Phase diagram of interacting Fermi gas in spin-orbit coupled square lattices}, series = {New Journal of Physics}, volume = {17}, journal = {New Journal of Physics}, number = {073036}, doi = {10.1088/1367-2630/17/7/073036}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-151475}, year = {2015}, abstract = {The spin-orbit (SO) coupled optical lattices have attracted considerable interest. In this paper, we investigate the phase diagram of the interacting Fermi gas with Rashba-type spin-orbit coupling (SOC) on a square optical lattice. The phase diagram is investigated in a wide range of atomic interactions and SOC strength within the framework of the cluster dynamical mean-field theory (CDMFT). We show that the interplay between the atomic interactions and SOC results in a rich phase diagram. In the deep Mott insulator regime, the SOC can induce diverse spin ordered phases. Whereas near the metal-insulator transition (MIT), the SOC tends to destroy the conventional antiferromagnetic fluctuations, giving rise to distinctive features of the MIT. Furthermore, the strong fluctuations arising from SOC may destroy the magnetic orders and trigger an order to disorder transition in close proximity of the MIT.}, language = {en} } @article{OPUS4-17335, title = {Measurement of the inclusive cross-sections of single top-quark and top-antiquark \(t\)-channel production in \(pp\) collisions at \(\sqrt{s}\) = 13 TeV with the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {2017}, journal = {Journal of High Energy Physics}, number = {04}, organization = {The ATLAS Collaboration}, doi = {https://doi.org/10.1007/JHEP04(2017)086}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173357}, year = {2017}, abstract = {A measurement of the \(t\)-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+jets channel is presented, using 3.2 fb\(^{-1}\) of proton-proton collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be \(b\)-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be \({σ(tq)}\) = 156 ± 5 (stat.) ± 27 (syst.) ± 3 (lumi.) pb for single top-quark production and \(σ(\overline{t}q)\) = 91 ± 4 (stat.) ± 18 (syst.) ± 2 (lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be \(R_{t}\) = \(σ(tq)/σ(\overline{t}q)\) = 1.72 ± 0.09 (stat.) ± 0.18 (syst.). All results are in agreement with Standard Model predictions.}, language = {en} } @article{OPUS4-17336, title = {Measurement of charged-particle distributions sensitive to the underlying event in \(\sqrt{s}\) = 13 TeV proton-proton collisions with the ATLAS detector at the LHC}, series = {Journal of High Energy Physics}, volume = {2017}, journal = {Journal of High Energy Physics}, number = {03}, organization = {The ATLAS Collaboration}, doi = {https://doi.org/10.1007/JHEP03(2017)157}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173361}, year = {2017}, abstract = {We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb\(^{-1}\). The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5\% accuracy, compared with data uncertainties of less than 1\%.}, language = {en} } @article{OPUS4-17278, title = {Search for supersymmetry in final states with two same-sign or three leptons and jets using 36 fb\(^{-1}\) of \(\sqrt{s}=13\) TeV \(pp\) collision data with the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {2017}, journal = {Journal of High Energy Physics}, number = {09}, organization = {The ATLAS Collaboration}, doi = {https://doi.org/10.1007/JHEP09(2017)084}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172787}, year = {2017}, abstract = {A search for strongly produced supersymmetric particles using signatures involving multiple energetic jets and either two isolated same-sign leptons (\(e\) or \(µ\)), or at least three isolated leptons, is presented. The analysis relies on the identification of \(b\)-jets and high missing transverse momentum to achieve good sensitivity. A data sample of proton-proton collisions at \(\sqrt{s} = 13\) TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to a total integrated luminosity of 36.1 fb\(^{-1}\), is used for the search. No significant excess over the Standard Model prediction is observed. The results are interpreted in several simplified supersymmetric models featuring \(R\)-parity conservation or \(R\)-parity violation, extending the exclusion limits from previous searches. In models considering gluino pair production, gluino masses are excluded up to 1.87 TeV at 95\% confidence level. When bottom squarks are pair-produced and decay to a chargino and a top quark, models with bottom squark masses below 700 GeV and light neutralinos are excluded at 95\% confidence level. In addition, model-independent limits are set on a possible contribution of new phenomena to the signal region yields.}, language = {de} } @phdthesis{Feichtner2017, author = {Feichtner, Thorsten}, title = {Optimal Design of Focusing Nanoantennas for Light : Novel Approaches: From Evolution to Mode-Matching}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-140604}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {Optische Antennen arbeiten {\"a}hnlich wie Antennen f{\"u}r Radiowellen und wandeln elektromagnetische Strahlung in elektrische Wechselstr{\"o}me um. Ladungsdichteansammlungen an der Antennen-Oberfl{\"a}che f{\"u}hren zu starken und lokalisierten Nahfeldern. Da die meisten optischen Antennen eine Ausdehnung von wenigen hundert Nanometern besitzen, erm{\"o}glichen es ihre Nahfelder, Licht auf ein Volumen weit unterhalb des Beugungslimits zu fokussieren, mit Intensit{\"a}ten, die mehrere Gr{\"o}ßenordnungen {\"u}ber dem liegen, was man mit klassischer beugender und reflektierender Optik erreichen kann. Die Aufgabe, die Abstrahlung eines Quantenemitters zu maximieren, eines punktf{\"o}rmigen Objektes, welches einzelne Photonen absorbieren und emittieren kann, ist identisch mit der Aufgabe, die Feldintensit{\"a}t am Ort des Quantenemitters zu maximieren. Darum ist es erstrebenswert, den Fokus optischer Antennen zu optimieren Optimierte Radiofrequenz-Antennen, welche auf Gr{\"o}ßenordnungen von wenigen 100 Nanometern herunterskaliert werden, zeigen bereits eine gute Funktionalit{\"a}t. Jedoch liegen optische Frequenzen in der N{\"a}he der Plasmafrequenz von den Metallen, die f{\"u}r optische Antennen genutzt werden und die Masse der Elektronen kann nicht mehr vernachl{\"a}ssigt werden. Dadurch treten neue physikalische Ph{\"a}nomene auf. Es entstehen gekoppelte Zust{\"a}nde aus Licht und Ladungsdichte-Schwingungen, die sogenannten Plasmonen. Daraus folgen Effekte wie Volumenstr{\"o}me und k{\"u}rzere effektive Wellenl{\"a}ngen. Zus{\"a}tzlich f{\"u}hrt die endliche Leitf{\"a}higkeit zu thermischen Verluste. Das macht eine Antwort auf die Frage nach der optimalen Geometrie f{\"u}r fokussierende optische Antennen schwer. Jedoch stand vor dieser Arbeit der Beweis noch aus, dass es f{\"u}r optische Antennen bessere Alternativen gibt als herunterskalierte Radiofrequenz-Konzepte. In dieser Arbeit werden optische Antennen auf eine bestm{\"o}gliche Fokussierung optimiert. Daf{\"u}r wird ein Ansatz gew{\"a}hlt, welcher bei Radiofrequenz-Antennen f{\"u}r komplexe Anwendungsfelder (z.B. isotroper Breitbandempfang) schon oft Erfolg hatte: evolution{\"a}re Algorithmen. Die hier eingef{\"u}hrte erste Implementierung erlaubt eine große Freiheit in Bezug auf Partikelform und Anzahl, da sie quadratische Voxel auf einem planaren, quadratischen Gitter beliebig anordnet. Die Geometrien werden in einer bin{\"a}ren Matrix codiert, welche als Genom dient und somit Methoden wie Mutation und Paarung als Verbesserungsmechanismus erlaubt. So optimierte Antennen-Geometrien {\"u}bertreffen vergleichbare klassische Dipol-Geometrien um einen Faktor von Zwei. Dar{\"u}ber hinaus l{\"a}sst sich aus den optimierten Antennen ein neues Funktionsprinzip ableiten: ein magnetische Split-Ring-Resonanz kann mit Dipol-Antennen leitend zu neuartigen und effektiveren Split-Ring-Antennen verbunden werden, da sich ihre Str{\"o}me nahe des Fokus konstruktiv {\"u}berlagern. Im n{\"a}chsten Schritt wird der evolution{\"a}re Algorithmus so angepasst, so die Genome real herstellbare Geometrien beschreiben. Zus{\"a}tzlich wird er um eine Art ''Druckertreiber'' erweitert, welcher aus den Genomen direkt Anweisungen zur fokussierten Ionenstrahl-Bearbeitung von einkristallinen Goldflocken erstellt. Mit Hilfe von konfokaler Mikroskopie der Zwei-Photonen-Photolumineszenz wird gezeigt, dass Antennen unterschiedlicher Effizienz reproduzierbar aus dem evolution{\"a}ren Algorithmus heraus hergestellt werden k{\"o}nnen. Außerdem wird das Prinzip der Split-Ring-Antenne verbessert, indem zwei Ring-Resonanzen zu einer Dipol-Resonanz hinzugef{\"u}gt werden. Zu guter Letzt dient die beste Antenne des zweiten evolution{\"a}re Algorithmus als Inspiration f{\"u}r einen neuen Formalismus zur Beschreibung des Leistungs{\"u}bertrages zwischen einer optischen Antenne und einem Punkt-Dipol, welcher sich als "dreidimensionaler Moden{\"u}berlapp" beschreiben l{\"a}sst. Damit k{\"o}nnen erstmals intuitive Regeln f{\"u}r die Form einer optischen Antenne aufgestellt werden. Die G{\"u}ltigkeit der Theorie wird analytisch f{\"u}r den Fall eines Dipols nahe einer metallischen Nano-Kugel gezeigt. Das vollst{\"a}ndige Problem, Licht mittels einer optischen Antenne zu fokussieren, l{\"a}sst sich so auf die Erf{\"u}llung zweier Moden{\"u}berlapp-Bedingungen reduzieren -- mit dem Feld eines Punktdipols, sowie mit einer ebenen Welle. Damit lassen sich zwei Arten idealer Antennenmoden identifizieren, welche sich von der bekannten Dipol-Antennen-Mode grundlegend unterscheiden. Zum einen l{\"a}sst sich dadurch die Funktionalit{\"a}t der evolution{\"a}ren und Split-Ring-Antennen erkl{\"a}ren, zum lassen sich neuartige plasmonische Hohlraum-Antennen entwerfen, welche zu besserer Fokussierung von Licht f{\"u}hren. Dies wird numerisch im direkten Vergleich mit einer klassischen Dipolantennen-Geometrie gezeigt.}, subject = {Physik}, language = {en} } @phdthesis{Braun2016, author = {Braun, Tristan}, title = {Spektroskopie an positionierten III-V-Halbleiterquantenpunkten}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-146151}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {Viele Forschergruppen konzentrieren sich derzeit auf die Entwicklung von neuartigen Technologien, welche den Weg f{\"u}r die kommerzielle Nutzung einer Quantenkommunikation bereiten sollen. Erste Erfolge konnten dabei insbesondere auf dem Gebiet der Quantenschl{\"u}sselverteilung erzielt werden. In diesem Bereich nutzt man die Eigenschaft einzelner, ununterscheidbarer Photonen nicht kopiert werden zu k{\"o}nnen, um eine abh{\"o}rsichere {\"U}bertragung sensibler Daten zu realisieren. Als Lichtquellen daf{\"u}r eignen sich Halbleiter-Quantenpunkte. Diese Quantenpunkte lassen sich außerdem leicht in komplexe Halbleiter-Mikrostrukturen integrieren und sind somit besonders interessant f{\"u}r die Entwicklung solch fortschrittlicher Technologien, welche f{\"u}r eine abh{\"o}rischere Kommunikation notwendig sind. Basierend auf diesem Hintergrund wurden in der vorliegenden Arbeit Halbleiter-Quantenpunkte spektroskopisch hinsichtlich ihres Potentials als Quanten-Lichtquelle f{\"u}r die Quantenkommunikation untersucht. Dabei wurden die Quantenpunkte aus InAs/GaAs und InP/GaInP unter anderem in einem speziellen Verfahren deterministisch positioniert und letztendlich in eine photonische Mikrostruktur integriert, welche aus einer Goldscheibe und einem dielektrischen Spiegel besteht. Als Grundcharakterisierungsmittel kam haupts{\"a}chlich die Mikrophotolumineszenzspektroskopie zur Bestimmung der Emissionseigenschaften zum Einsatz. Weiterf{\"u}hrend wurden Photonen-Korrelationsmessungen zweiter Ordnung durchgef{\"u}hrt, um den Nachweis einer Quanten-Lichtquelle zu erbringen. Einfluss eines RTA-Prozesses auf die Emissionseigenschaften von InAs/GaAs-Quantenpunkten Zur Untersuchung des Einflusses eines Rapid-Thermal-Annealing-Prozesses auf die elektronischen Eigenschaften und die Oszillatorst{\"a}rke selbstorganisierter InAs/GaAs-Quantenpunkte wurden Mikrophotolumineszenzmessungen an verschiedenen Proben im externen Magnetfeld von bis zu 5 T durchgef{\"u}hrt. Die Quantenpunkte wurden dabei in einem besonderen Verfahren gewachsen, bei dem die nominelle Quantenpunkth{\"o}he durch eine bestimmte Bedeckungsschichtdicke vorgegeben wurde. Insgesamt wurden drei Proben mit Schichtdicken von 2 nm, 3 nm und 4 nm hergestellt, die jeweils nachtr{\"a}glich bei Temperaturen von 750° C bis 850° C f{\"u}r f{\"u}nf Minuten ausgeheilt wurden. Anhand polarisationsaufgel{\"o}ster Spektroskopie konnten aus den aufgenommenen Quantenpunktspektren die Zeemanaufspaltung und die diamagnetische Verschiebung extrahiert und damit der effektive Land{\´e} g-Faktor sowie der diamagnetische Koeffizient bestimmt werden. Die Auswertung der Zeemanaufspaltung zeigte, dass sowohl h{\"o}here Ausheiltemperaturen als auch dickere Bedeckungsschichten zu einer drastischen Abnahme der absoluten g-Faktoren sorgen. Dies l{\"a}sst darauf schließen, dass eine dickere Bedeckungsschicht zu einer st{\"a}rkeren Interdiffusion der Atome und einer steigenden Ausdehnung der Quantenpunkte f{\"u}r ex-situ Ausheilprozesse f{\"u}hrt. Im Gegensatz dazu steigen die diamagnetischen Koeffizienten der Quantenpunkte mit zunehmender Ausheiltemperatur, was auf eine Ausdehnung der Exzitonwellenfunktion hindeutet. Außerdem wurden mittels zeitaufgel{\"o}ster Mikrophotolumineszenzspektroskopie die Lebensdauern am Quantenpunktensemble bestimmt und eine Abnahme dieser mit steigender Temperatur festgestellt. Sowohl {\"u}ber die Untersuchungen des diamagnetischen Koeffizienten als auch {\"u}ber die Analyse der Lebensdauer konnte schließlich die Oszillatorst{\"a}rke der Quantenpunkte ermittelt werden. Beide Messverfahren lieferten innerhalb der Fehlergrenzen {\"a}hnliche Ergebnisse. Die h{\"o}chste Oszillatorst{\"a}rke \(f_{\chi}=34,7\pm 5,2\) konnte f{\"u}r eine Schichtdicke von d = 3 nm und einer Ausheiltemperatur von 850° C {\"u}ber den diamagnetischen Koeffizienten berechnet werden. Im Falle der Bestimmung {\"u}ber die Lebensdauer ergab sich ein maximaler Wert von \(f_{\tau}=25,7\pm 5,7\). Dies entspricht einer deutlichen Steigerung der Oszillatorst{\"a}rke im Vergleich zu den Referenzproben um einem Faktor gr{\"o}ßer als zwei. Des Weiteren konnte eine Ausdehnung der Schwerpunktswellenfunktion der Exzitonen um etwa 70\% festgestellt werden. Insgesamt betrachtet, l{\"a}sst sich durch ex-situ Rapid-Thermal-Annealing-Prozesse die Oszillatorst{\"a}rke nachtr{\"a}glich deutlich erh{\"o}hen, wodurch InAs/GaAs-Quantenpunkte noch interessanter f{\"u}r Untersuchungen im Regime der starken Kopplung werden. Temperatur- und Leistungsabh{\"a}ngigkeit der Emissionseigenschaften positionierter InAs/GaAs Quantenpunkte Um einen Einblick in den Ablauf des Zerfallsprozesses eines Exzitons in positionierten Quantenpunkten zu bekommen, wurden temperatur- und leistungsabh{\"a}ngige Messungen durchgef{\"u}hrt. Diese Quantenpunkte wurden in einem speziellen Verfahren deterministisch an vorher definierten Stellen gewachsen. Anhand der Temperaturserien konnten dann R{\"u}ckschl{\"u}sse auf die auftretenden Verlustkan{\"a}le in einem Quantenpunkt und dessen Emissionseigenschaften gezogen werden. Dabei wurden zwei dominante Prozesse als Ursache f{\"u}r den Intensit{\"a}tsabfall bei h{\"o}heren Temperaturen identifiziert. Die Anhebung der Elektronen im Grundzustand in die umgebende Barriere oder in delokalisierte Zust{\"a}nde in der Benetzungsschicht sorgt f{\"u}r die anf{\"a}ngliche Abnahme der Intensit{\"a}t bei niedrigeren Temperaturen. Der starke Abfall bei h{\"o}heren Temperaturen ist dagegen dem Aufbruch der exzitonischen Bindung und der thermischen Aktivierung der Ladungstr{\"a}ger in das umgebende Substratmaterial geschuldet. Hierbei lassen sich exemplarisch f{\"u}r zwei verschiedene Quantenpunkte die Aktivierungsenergien \(E_{2A}=(102,2\pm 0,4)\) meV und \(E_{2B}=(163,2\pm 1,3)\) meV bestimmen, welche in etwa den Lokalisierungsenergien der Exzitonen in dem jeweiligen Quantenpunkt von 100 meV bzw. 144 meV entsprechen. Weiterhin deckte die Auswertung des Intensit{\"a}tsprofils der Exzitonemission die Streuung der Exzitonen an akustischen und optischen Phononen als Hauptursache f{\"u}r die Zunahme der Linienbreite auf. F{\"u}r hohe Temperaturen dominierte die Wechselwirkung mit longitudinalen optischen Phononen den Verlauf und es konnten f{\"u}r das InAs/GaAs Materialsystem typische Phononenenergien von \(E_{LOA}=(30,9\pm 4,8)\) meV und \(E_{LOB}=(32,2\pm 0,8)\) meV bestimmt werden. In abschließenden Messungen der Leistungsabh{\"a}ngigkeit der Linienbreite wurde festgestellt, dass spektrale Diffusion die inh{\"a}rente Grenze f{\"u}r die Linienbreite bei niedrigen Temperaturen setzt. Optische Spektroskopie an positionierten InP/GaInP-Quantenpunkten Weiterhin wurden positionierte InP/GaInP-Quantenpunkte hinsichtlich der Nutzung als Quanten-Lichtquelle optisch spektroskopiert. Zun{\"a}chst wurden die Emissionseigenschaften der Quantenpunkte in grundlegenden Experimenten analysiert. Leistungs- und polarisationsabh{\"a}ngige Messungen ließen dabei die Vermutung sowohl auf exzitonische als auch biexzitonische Zerfallsprozesse zu. Weiterhin brachten die Untersuchungen der Polarisation einen ungew{\"o}hnlich hohen Polarisationsgrad der Quantenpunktemission hervor. Aufgrund von lokalen Ordnungsph{\"a}nomenen in der umgebenden GaInP-Matrix wurden im Mittel {\"u}ber 66 Quantenpunkte der Grad der Polarisation von Exziton und Biexziton zu \(p_{Mittel}=(93^{+7}_{-9})\)\% bestimmt. Des Weiteren wiesen die Quantenpunkte eine sehr hohe Feinstrukturaufspaltung von \(\Delta_{FSS}^{Mittel}=(300\pm 130)\) µeV auf, welche sich nur durch eine stark anisotrope Quantenpunktform erkl{\"a}ren l{\"a}sst. Durch Auto- und Kreuzkorrelationsmessungen zweiter Ordnung wurden dann sowohl der nicht-klassische Einzelphotonencharakter von Exziton und Biexziton als auch erstmalig f{\"u}r diese Strukturen der kaskadierte Zerfall der Biexziton-Exziton-Kaskade demonstriert. Hierbei wurden \(g^{(2)}(0)\)-Werte von bis 0,08 erreicht. Diese Ergebnisse zeigen das Potential von positionierten InP/GaInP-Quantenpunkten als Grundbausteine f{\"u}r Quanten-Lichtquellen, insbesondere in Bezug auf den Einsatz in der Quantenkommunikation. Realisierung einer Einzelphotonenquelle auf Basis einer Tamm-Plasmonen-Struktur Nachdem die vorangegangen Untersuchungen die Eignung der positionierten InP/GaInP-Quantenpunkte als Emitter einzelner Photonen demonstrierten, befasst sich dieser Teil nun mit der Integration dieser Quantenpunkte in eine Tamm-Plasmonen-Struktur zur Realisierung einer effizienten Einzelphotonenquelle. Diese Strukturen bestehen aus einem dielektrischen Spiegel aus 30,5 AlGaAs/AlAs-Schichtpaaren und einer einigen Zehn Nanometer dicken Goldschicht, zwischen denen die Quantenpunkte eingebettet sind. Anhand von Messungen an einer planaren Tamm-Plasmonen-Struktur wurde das Bauteil charakterisiert und neben der Exziton- und Biexzitonemission der Zerfall eines Trions beobachtet, was durch Polarisations- und Korrelationsmessungen nachgewiesen wurde. Um eine Verst{\"a}rkung der Einzelphotonenemission durch die Kopplung der Teilchen an eine lokalisierte Tamm-Plasmonen-Mode demonstrieren zu k{\"o}nnen, wurde ein Bereich der Probe mit mehreren Goldscheiben von Durchmessern von 3-6 µm abgerastert und die Lichtintensit{\"a}t aufgenommen. Unterhalb der untersuchten Goldscheiben konnte eine signifikante Erh{\"o}hung des Lumineszenzsignals festgestellt werden. Eine quantitative Analyse eines einzelnen Quantenpunktes mittels einer Temperaturserie lieferte dabei eine maximale Emissionsrate von \(\eta_{EPQ}^{Max}=(6,95\pm 0,76)\) MHz und damit eine Effizienz von \((6,95\pm 0,76)\)\% solch einer Einzelphotonenquelle unter gepulster Anregung bei 82 MHz. Dies entspricht einer deutlichen Verbesserung der Effizienz im Vergleich zu Quantenpunkten im Volumenmaterial und sogar zu denen in einer planaren DBR-Resonatorstruktur. Positionierte InP/GaInP-Quantenpunkte in einer Tamm-Plasmonen-Struktur bilden somit eine vielversprechende Basis f{\"u}r die Realisierung hocheffizienter Einzelphotonenquellen.}, subject = {Drei-F{\"u}nf-Halbleiter}, language = {de} } @phdthesis{Heindel2013, author = {Heindel, Tobias}, title = {Elektrisch gepumpte Quantenpunkt-Einzelphotonenquellen f{\"u}r die Quantenkommunikation}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-105778}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Als erste kommerziell verf{\"u}gbare Technologie der Quanteninformation erm{\"o}glicht die Quanten-Schl{\"u}sselverteilung eine sichere Daten{\"u}bertragung indem einzelne Photonen oder quantenmechanisch verschr{\"a}nkte Photonenpaare zur Erzeugung eines Schl{\"u}ssels verwendet werden. Die hierf{\"u}r ben{\"o}tigten nicht-klassischen Photonen-Zust{\"a}nde k{\"o}nnen durch Halbleiter-Quantenpunkte erzeugt werden. Im Gegensatz zu anderen Quanten-Emittern wie isolierten Atomen, organischen Molek{\"u}len oder Fehlstellen in Diamantnanokristallen bieten diese zudem den Vorteil, direkt in komplexe Halbleiter-Mikrostrukturen integriert werden zu k{\"o}nnen. Quantenpunkte sind somit pr{\"a}destiniert f{\"u}r die Entwicklung neuartiger optoelektronischer Bauelemente auf einer skalierbaren Technologieplattform. Vor diesem Hintergrund werden in der vorliegenden Arbeit die Eigenschaften elektrisch gepumpter Quantenpunkt-Mikrostrukturen untersucht. Als optisch aktives Medium dienen dabei selbstorganisierte InAs/GaAs-Quantenpunkte. Die Zielsetzung ist die Erzeugung nicht-klassischen Lichts f{\"u}r Anwendungen in der Quantenkommunikation, wobei ein besonderer Fokus auf dem elektrischen Betrieb der entsprechenden Quantenlichtquellen liegt. Dabei werden sowohl ausgepr{\"a}gte Resonatoreffekte im Regime der schwachen Licht-Materie-Wechselwirkung ausgenutzt, um helle Einzelphotonenquellen zu realisieren, als auch die Eigenschaften korrelierter Photonenpaare zweier spektral separierter Quantenpunkt-Zust{\"a}nde analysiert. Als Untersuchungsmethode wird in erster Linie die spektral und zeitlich hochaufl{\"o}sende Mikro-Lumineszenz-Spektroskopie bei kryogenen Temperaturen eingesetzt. Zudem erfolgen Experimente zur Photonenstatistik anhand von Messungen der Auto- sowie Kreuzkorrelationsfunktion zweiter Ordnung. Wie im Folgenden aufgef{\"u}hrt, gelingt dabei der Bogenschlag von grundlegenden Untersuchungen an Quantenpunkt-Mikrostrukturen bis hin zur erstmaligen Implementierung elektrisch getriggerter Quantenpunkt-Einzelphotonenquellen in realistischen Experimenten zur Quanten-Schl{\"u}sselverteilung außerhalb einer gesch{\"u}tzten Laborumgebung. Elektrisch getriggerte Einzelphotonenquellen: F{\"u}r die Erzeugung elektrisch getriggerter, einzelner Photonen wurden Quantenpunkte in Mikroresonatoren eingebettet. Diese basieren auf dotierten, zylindrischen Fabry-P{\´e}rot Mikros{\"a}ulenresonatoren, deren Design bez{\"u}glich der Photonen-Auskoppeleffizienz optimiert wurde. […] Anhand von Messungen zur Photonenstatistik konnte f{\"u}r diese spektral resonant gekoppelten Quantenpunkt-Mikroresonatorsysteme sowohl unter kontinuierlicher- als auch unter gepulst-elektrischer Anregung Einzelphotonen-Emission nachgewiesen werden. […] Anhand einer eingehenden Analyse der Emissionsraten sowie der elektrischen Injektionseffizienzen bei Anregungs-Repetitionsraten von bis zu 220 MHz konnte gezeigt werden, dass die untersuchten Mikroresonatoren zudem als {\"a}ußerst effiziente, elektrisch getriggerte Einzelphotonenquellen eingesetzt werden k{\"o}nnen. Sowohl bez{\"u}glich der Einzelphotonen-Emissionsraten von bis zu (47,0+/-6,9) MHz als auch der Gesamteffizienz der Bauteile bis hin zu (34+/-7) \% konnten dabei Rekordwerte erzielt werden. Korrelierte Photonenpaare elektrisch gepumpter Quantenpunkte: […] Quanten-Schl{\"u}sselverteilung mit elektrisch getriggerten Einzelphotonenquellen: Ausgehend von den grundlegenden Untersuchungen dieser Arbeit, erfolgte die erstmalige Implementierung elektrisch getriggerter Quantenpunkt-Einzelphotonenquellen in Experimenten zur Quanten-Schl{\"u}sselverteilung. Basierend auf den eingehend analysierten Quantenpunkt-Mikroresonatoren, wurden dabei zwei Experimente in Freistrahloptik mit unterschiedlichen {\"U}bertragungsdistanzen durchgef{\"u}hrt. In beiden F{\"a}llen wurde ein BB84-Protokoll nachgeahmt, indem auf die einzelnen Photonen eine feststehende Abfolge von vier unterschiedlichen Polarisationszust{\"a}nden aufmoduliert wurde. Das erste Experiment, durchgef{\"u}hrt im Labormaßstab in W{\"u}rzburg, basierte auf einem Quantenkanal mit einer L{\"a}nge von etwa 40 cm und arbeitete bei einer Taktrate von 183 MHz. Die h{\"o}chste dabei erzielte ausgesiebte Schl{\"u}sselrate (engl. sifted-key rate) betrug 35,4 kbit/s bei einem Quanten-Bitfehlerverh{\"a}ltnis (QBER) von 3,8 \%. Der Einzelphotonen-Charakter der Emission innerhalb des Quantenkanals konnte jeweils eindeutig nachgewiesen werden […]. Das zweite Experiment zur Quanten-Schl{\"u}sselverteilung wurde mittels zweier Teleskope {\"u}ber eine Distanz von 500 m in der M{\"u}nchner Innenstadt zwischen den D{\"a}chern zweier Geb{\"a}ude der Ludwig-Maximilians-Universit{\"a}t realisiert. […] Bei einer Taktrate von 125 MHz konnte mit diesem System im Einzelphotonen-Regime eine maximale sifted-key rate von 11,6 kbit/s bei einem QBER von 6,2 \% erzielt werden. Diese erstmalige Implementierung elektrisch betriebener, nicht-klassischer Lichtquellen in Experimenten zur Quanten-Schl{\"u}sselverteilung stellt einen wichtigen Schritt hinsichtlich der Realisierung effizienter und praktikabler Systeme f{\"u}r die Quantenkommunikation dar.}, subject = {Quantenpunkt}, language = {de} } @phdthesis{Thienel2015, author = {Thienel, Cornelius}, title = {Exploring the transport properties of the three-dimensional topological insulator material HgTe}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-122031}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {In der vorliegenden Dissertation werden die Transporteigenschaften von verspannten HgTe-Volumenkristallen untersucht. Verspanntes HgTe stellt einen dreidimensionalen topologischen Isolator dar und ist zur Erkundung von topologischen Oberfl{\"a}chenzust{\"a}nden von speziellem Interesse, da es mit Hilfe von Molekularstrahlepitaxie in hoher Kristallqualit{\"a}t gewachsen werden kann. Die niedrige Defektdichte f{\"u}hrt zu beachtlichen Ladungstr{\"a}gerbeweglichkeiten, die deutlich {\"u}ber denen anderer topologischer Isolatoren liegen. Verspanntes HgTe hat jedoch eine kleine Energiel{\"u}cke von ca. 20 meV. Deshalb ist es f{\"u}r eine m{\"o}gliche Verwendung des Materials ein wichtiger Aspekt, in welchem Parameterbereich Oberfl{\"a}chentransport stattfindet. Um dieser Frage nachzugehen, werden die HgTe-Proben bei tiefen Temperaturen (T < 100 mK) und unter dem Einfluss hoher Magnetfelder in verschiedenen Orientierungen untersucht. Der Einfluss von Gate-Elektroden ober- und unterhalb der Struktur sowie von Deckschichten, die die Oberfl{\"a}chen sch{\"u}tzen, wird diskutiert. Basierend auf einer Analyse des Quanten-Hall-Effekts wird gezeigt, dass der Transport in diesem Material von topologischen Oberfl{\"a}chenzust{\"a}nden dominiert ist. Die Abh{\"a}ngigkeit der topologischen Oberfl{\"a}chenzust{\"a}nde von der Gate-Spannung wird dargestellt. Durch diese Abh{\"a}ngigkeit ist es zum ersten Mal m{\"o}glich, eine ungerade ganzzahlige Quanten-Hall-Plateau Sequenz nachzuweisen, die von den Oberfl{\"a}chen senkrecht zum Magnetfeld stammt. Des Weiteren wird im Rahmen dieser Arbeit in Proben hoher Oberfl{\"a}chenqualit{\"a}t zum ersten Mal f{\"u}r einen 3D TI der p-Typ QHE der Oberfl{\"a}chenzust{\"a}nde beobachtet. Aus der Gate-Abh{\"a}ngigkeit der Messungen wird geschlossen, dass das Abschirmverhalten in 3D TIs nicht trivial ist. Die Transportdaten werden mit Hilfe von intuitiven theoretischen Modellen auf qualitative Weise analysiert.}, subject = {Topologischer Isolator}, language = {en} } @article{KoenigBaenningerGarciaetal.2013, author = {K{\"o}nig, Markus and Baenninger, Matthias and Garcia, Andrei G. F. and Harjee, Nahid and Pruitt, Beth L. and Ames, C. and Leubner, Philipp and Br{\"u}ne, Christoph and Buhmann, Hartmut and Molenkamp, Laurens W. and Goldhaber-Gordon, David}, title = {Spatially Resolved Study of Backscattering in the Quantum Spin Hall State}, series = {Physical Review X}, volume = {3}, journal = {Physical Review X}, number = {2}, issn = {2160-3308}, doi = {10.1103/PhysRevX.3.021003}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-127225}, pages = {21003}, year = {2013}, abstract = {The discovery of the quantum spin Hall (QSH) state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the quantum spin Hall state have confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples, and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using scanning gate microscopy to perturb the QSH edge states on a submicron scale, we identify well-localized scattering sites which likely limit the expected nondissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed, as expected for an ideal QSH system, and are found to be robust against weak induced potential fluctuations.}, language = {en} } @phdthesis{Lermer2013, author = {Lermer, Matthias}, title = {Wachstum und Charakterisierung von Quantenpunkt-Mikrot{\"u}rmchen mit adiabatischer Modenanpassung}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-127438}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Verschiedene Konzepte zur Realisierung einer geeigneten Umgebung f{\"u}r Licht- Materie-Wechselwirkung konkurrieren um Anerkennung und eine st{\"a}ndige Optimierung der Systemparameter findet statt. Das Konzept von Mikrot{\"u}rmchen scheint pr{\"a}destiniert, da es viele anwendungsfreundliche Eigenschaften in sich vereint. Allerdings stellt die drastische Abnahme des Q Faktors f{\"u}r kleiner werdende Durchmesser d einen wesentlichen Limitierungsfaktor dieser Strukturen dar. F{\"u}r viele Anwendungen resultiert daraus ein Kompromiss aus hohem Q Faktor und kleinem Modenvolumen der Strukturen, wodurch das volle Potential des Resonatorsystems nicht ausgesch{\"o}pft werden kann. Ziel dieser Arbeit war es, die drastische Abnahme des Q Faktors von Mikrot{\"u}rmchen mit Durchmessern um 1μm aufzuheben und dadurch Resonatoren mit d < 1μm f{\"u}r ausgepr{\"a}gte Licht-Materie-Wechselwirkung herzustellen. Dazu wurde erstmalig beabsichtigt eine Modenanpassung in Mikrot{\"u}rmchen vorgenommen. Mittels Molekularstrahlepitaxie konnte eine {\"U}bergangsregion, bestehend aus drei Segmenten, in diese Strukturen implementiert und so ein adiabatischer Moden{\"u}bergang zwischen der aktiven Mittelschicht und den Spiegelbereichen vorgenommen werden. Der positive Einfluss dadurch ergab sich in einer signifikanten Verbesserung des gemessenen Q Faktors f{\"u}r Durchmesser unter 1μm. F{\"u}r d = 0.85μm konnte ein Q Faktor von 14 400 bestimmt werden. Dies stellt damit den h{\"o}chsten je gemessenen Wert f{\"u}r Mikrot{\"u}rmchen im Submikrometerbereich dar. Dadurch wird ein Bereich mit Modenvolumina < 3 kubischen Wellenl{\"a}ngen erschlossen und ausgepr{\"a}gte Wechselwirkungseffekte im Mikrot{\"u}rmchensystem sind zu erwarten. Starke Quantenpunkt-Licht-Kopplung konnte in diesen Strukturen nachgewiesen werden. Die h{\"o}chste Vakuum-Rabiaufspaltung betrug 85μeV und die Visibilit{\"a}t wurde zu 0.41 bestimmt. Im Zuge der weiteren Optimierung der Systemparameter f{\"u}r die starke Kopplung wurde ein ex-situ Ausheilschritt auf die verwendete Quantenpunktsorte angewendet. In magnetooptischen Untersuchungen konnte damit eine Verdopplung der mittleren Oszillatorst{\"a}rke auf einen Wert von 12 abgesch{\"a}tzt werden. Weiter konnte in adiabatischen Mikrot{\"u}rmchen {\"u}ber einen großen Durchmesserbereich von 2.25 bis 0.95μm eindeutiger Laserbetrieb des Quantenpunktensembles nachgewiesen werden. Dabei konnte eine kontinuierliche Reduzierung der Laserschwelle von {\"u}ber zwei Gr{\"o}ßenordnungen f{\"u}r kleiner werdende Durchmesser beobachtet werden. F{\"u}r Durchmesser � < 1.6μm betrug der Beta-Faktor der Mikrolaser in etwa 0.5. Sie zeigten damit beinahe schwellenloses Verhalten. Zuletzt wurde der elektrische Betrieb von adiabatischen Mikrot{\"u}rmchen gezeigt. Daf{\"u}r wurde eine dotierte Struktur mit adiabatischem Design hergestellt. Im Vergleich zur undotierten Struktur fielen die gemessenen Q Faktoren in etwa um 5 000 geringer aus. Die spektralen Eigenschaften sowohl des Resonators als auch einzelner Quantenpunktlinien zeigten vernachl{\"a}ssigbare Abh{\"a}ngigkeit der Anregungsart (optisch oder elektrisch) und zeugen von einem erfolgreichen Konzept zum elektrischen Betrieb der Bauteile. Zeitaufgel{\"o}ste Messungen erlaubten die Beobachtung von interessanten Dynamiken der Rekombination von Ladungstr{\"a}gern in den Proben. Als Ursache daf{\"u}r wurde ein hohes intrinsisches Feld, welches auf Grund des Designs der Schichtstruktur entsteht, identifiziert. Weiter zeigte sich, dass sich das interne Feld durch Anregungsart und extern angelegte Spannungen manipulieren l{\"a}sst.}, subject = {Molekularstrahlepitaxie}, language = {de} } @article{AadAbbottAbdallahetal.2012, author = {Aad, G. and Abbott, B. and Abdallah, J. and Abdel Khalek, S. and Abdelalim, A. A.}, title = {Search for the Standard Model Higgs boson in the H→WW(⋆)→ℓνℓνH→WW(⋆)→ℓνℓν decay mode with 4.7 fb\(^{-1}\) of ATLAS data at \(\sqrt{s}\)=7 TeV}, series = {Physics Letters B}, volume = {761}, journal = {Physics Letters B}, number = {1}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2012.08.010}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-127307}, pages = {62-81}, year = {2012}, abstract = {A search for the Standard Model Higgs boson in the H→WW(⋆)→ℓνℓνH→WW(⋆)→ℓνℓν (ℓ=e,μℓ=e,μ) decay mode is presented. The search is performed using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb\(^{-1}\) at a centre-of-mass energy of 7 TeV collected during 2011 with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed. An upper bound is placed on the Higgs boson production cross section as a function of its mass. A Standard Model Higgs boson with mass in the range between 133 GeV and 261 GeV is excluded at 95\% confidence level, while the expected exclusion range is from 127 GeV to 233 GeV.}, language = {en} } @article{AadAbbottAbdallahetal.2012, author = {Aad, G. and Abbott, B. and Abdallah, J. and Abdelalim, A. A. and Abdesselam, A.}, title = {Electron performance measurements with the ATLAS detector using the 2010 LHC proton-proton collision data}, series = {The European Physical Journal C}, volume = {72}, journal = {The European Physical Journal C}, number = {1909}, doi = {10.1140/epjc/s10052-012-1909-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-127313}, year = {2012}, abstract = {Detailed measurements of the electron performance of the ATLAS detector at the LHC are reported, using decays of the Z, W and J/ψ particles. Data collected in 2010 at s√=7 TeV are used, corresponding to an integrated luminosity of almost 40 pb\(^{-1}\). The inter-alignment of the inner detector and the electromagnetic calorimeter, the determination of the electron energy scale and resolution, and the performance in terms of response uniformity and linearity are discussed. The electron identification, reconstruction and trigger efficiencies, as well as the charge misidentification probability, are also presented.}, language = {en} } @phdthesis{Fuchs2015, author = {Fuchs, Franziska}, title = {Optical spectroscopy on silicon vacancy defects in silicon carbide}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-124071}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {This work sheds light on different aspects of the silicon vacancy in SiC: (1) Defect creation via irradiation is shown both with electrons and neutrons. Optical properties have been determined: the excitation of the vacancy is most efficient at excitation wavelengths between 720nm and 800nm. The PL decay yields a characteristic excited state lifetime of (6.3±0.6)ns. (2) Defect engineering, meaning the controlled creation of vacancies in SiC with varying neutron fluence. The defect density could be engineered over eight orders of magnitude. On the one hand, in the sample with highest emitter density, the huge PL signal could even be enhanced by factor of five via annealing mechanisms. On the other hand, in the low defect density samples, single defects with photostable room temperature NIR emission were doubtlessly proven. Their lifetime of around 7ns confirmed the value of the transient measurement. (3) Also electrical excitation of the defects has been demonstrated in a SiC LED structure. (4) The investigations revealed for the first time that silicon vacancies can even exist SiC nanocrystals down to sizes of about 60 nm. The defects in the nanocrystals show stable PL emission in the NIR and even magnetic resonance in the 600nm fraction. In conclusion, this work ascertains on the one hand basic properties of the silicon vacancy in silicon carbide. On the other hand, proof-of-principle measurements test the potential for various defect-based applications of the vacancy in SiC, and confirm the feasibility of e.g. electrically driven single photon sources or nanosensing applications in the near future.}, subject = {Siliciumcarbid}, language = {en} } @article{MaierGoldForcheletal.2014, author = {Maier, Sebastian and Gold, Peter and Forchel, Alfred and Gregersen, Niels and Mork, Jesper and H{\"o}fling, Sven and Schneider, Christian and Kamp, Martin}, title = {Bright single photon source based on self-aligned quantum dot-cavity systems}, series = {Optics Express}, volume = {22}, journal = {Optics Express}, number = {7}, issn = {1094-4087}, doi = {10.1364/OE.22.008136}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119801}, pages = {8136-42}, year = {2014}, abstract = {We report on a quasi-planar quantum-dot-based single-photon source that shows an unprecedented high extraction efficiency of 42\% without complex photonic resonator geometries or post-growth nanofabrication. This very high efficiency originates from the coupling of the photons emitted by a quantum dot to a Gaussian shaped nanohill defect that naturally arises during epitaxial growth in a self-aligned manner. We investigate the morphology of these defects and characterize the photonic operation mechanism. Our results show that these naturally arising coupled quantum dot-defects provide a new avenue for efficient (up to 42\% demonstrated) and pure (g(2)(0) value of 0.023) single-photon emission.}, language = {en} } @article{OPUS4-13986, title = {Search for s-channel single top-quark production in proton-proton collisions at \(\sqrt{s}\)=8 TeV with the ATLAS detector}, series = {Physics Letters B}, volume = {740}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2014.11.042}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-139869}, pages = {118-136}, year = {2015}, abstract = {This Letter presents a search at the LHC for s-channel single top-quark production in proton-proton collisions at a centre-of-mass energy of 8 TeV. The analyzed data set was recorded by the ATLAS detector and corresponds to an integrated luminosity of 20.3 fb\(^{-1}\). Selected events contain one charged lepton, large missing transverse momentum and exactly two b-tagged jets. A multivariate event classifier based on boosted decision trees is developed to discriminate s-channel single top-quark events from the main background contributions. The signal extraction is based on a binned maximum-likelihood fit of the output classifier distribution. The analysis leads to an upper limit on the s-channel single top-quark production cross-section of 14.6 pb at the 95\% confidence level. The fit gives a cross-section of σs=5.0±4.3 pb, consistent with the Standard Model expectation.}, language = {en} } @article{OPUS4-13826, title = {Search for \(W′→t\overline {b}\) in the lepton plus jets final state in proton-proton collisions at a centre-of-mass energy of \(\sqrt {s}\)=8 TeV with the ATLAS detector}, series = {Physics Letters B}, volume = {743}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.02.051}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-138262}, pages = {235-255}, year = {2015}, abstract = {A search for new charged massive gauge bosons, called W′W′, is performed with the ATLAS detector at the LHC, in proton-proton collisions at a centre-of-mass energy of \(\sqrt {s}\)=8 TeV, using a dataset corresponding to an integrated luminosity of 20.3 fb\(^{-1}\). This analysis searches for W′W′ bosons in the \(W′→t\overline{b}\) decay channel in final states with electrons or muons, using a multivariate method based on boosted decision trees. The search covers masses between 0.5 and 3.0 TeV, for right-handed or left-handed W′W′ bosons. No significant deviation from the Standard Model expectation is observed and limits are set on the \(W′→t\overline{b}\) cross-section times branching ratio and on the W′W′-boson effective couplings as a function of the W′W′-boson mass using the CL\(_s\) procedure. For a left-handed (right-handed) W′W′ boson, masses below 1.70 (1.92) TeV are excluded at 95\% confidence level.}, language = {en} } @article{OPUS4-13827, title = {Search for the \(X_b\) and other hidden-beauty states in the \(π^+π^-ϒ\)(1S) channel at ATLAS}, series = {Physics Letters B}, volume = {740}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2014.11.055}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-138273}, pages = {199-217}, year = {2014}, abstract = {This Letter presents a search for a hidden-beauty counterpart of the X(3872) in the mass ranges of 10.05-10.31 GeV and 10.40-11.00 GeV, in the channel X\(_b\)→π\(^+\)π\(^-\)ϒ(1S)(→μ\(^+\)μ\(^-\)), using 16.2 fb\(^{-1}\) of s=8 TeV \(pp\) collision data collected by the ATLAS detector at the LHC. No evidence for new narrow states is found, and upper limits are set on the product of the X\(_b\) cross section and branching fraction, relative to those of the ϒ(2S), at the 95\% confidence level using the CLSCLS approach. These limits range from 0.8\% to 4.0\%, depending on mass. For masses above 10.1 GeV, the expected upper limits from this analysis are the most restrictive to date. Searches for production of the ϒ(1\(^3\)D\(_J\)), ϒ(10860), and ϒ(11020) states also reveal no significant signals.}, language = {en} } @article{YuNatarajanHorikirietal.2015, author = {Yu, Leo and Natarajan, Chandra M. and Horikiri, Tomoyuki and Langrock, Carsten and Pelc, Jason S. and Tanner, Michael G. and Abe, Eisuke and Maier, Sebastian and Schneider, Christian and H{\"o}fling, Sven and Kamp, Martin and Hadfield, Robert H. and Fejer, Martin M. and Yamamoto, Yoshihisa}, title = {Two-photon interference at telecom wavelengths for time-bin-encoded single photons from quantum-dot spin qubits}, series = {Nature Communications}, volume = {6}, journal = {Nature Communications}, doi = {10.1038/ncomms9955}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-138677}, pages = {8955}, year = {2015}, abstract = {Practical quantum communication between remote quantum memories rely on single photons at telecom wavelengths. Although spin-photon entanglement has been demonstrated in atomic and solid-state qubit systems, the produced single photons at short wavelengths and with polarization encoding are not suitable for long-distance communication, because they suffer from high propagation loss and depolarization in optical fibres. Establishing entanglement between remote quantum nodes would further require the photons generated from separate nodes to be indistinguishable. Here, we report the observation of correlations between a quantum-dot spin and a telecom single photon across a 2-km fibre channel based on time-bin encoding and background-free frequency downconversion. The downconverted photon at telecom wavelengths exhibits two-photon interference with another photon from an independent source, achieving a mean wavepacket overlap of greater than 0.89 despite their original wavelength mismatch (900 and 911 nm). The quantum-networking operations that we demonstrate will enable practical communication between solid-state spin qubits across long distances.}, language = {en} } @phdthesis{Muehlbauer2015, author = {M{\"u}hlbauer, Mathias Josef}, title = {Nanolithography on Mercury Telluride}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-137152}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {Topological insulators belong to a new quantum state of matter that is currently one of the most recognized research fields in condensed matter physics. Strained bulk HgTe and HgTe/HgCdTe quantum well structures are currently one of few topological insulator material systems suitable to be studied in transport experiments. In addition HgTe quantum wells provide excellent requirements for the conduction of spintronic experiments. A fundamental requirement for most experiments, however, is to reliably pattern these heterostructures into advanced nano-devices. Nano-lithography on this material system proves to be challenging because of inherent temperature limitations, its high reactivity with various metals and due to its properties as a topological insulator. The current work gives an insight into why many established semiconductor lithography processes cannot be easily transferred to HgTe while providing alternative solutions. The presented developments include novel ohmic contacts, the prevention of metal sidewalls and redeposition fences in combination with low temperature (80 °C) lithography and an adapted hardmask lithography process utilizing a sacrificial layer. In addition we demonstrate high resolution low energy (2.5 kV) electron beam lithography and present an alternative airbridge gating technique. The feasibility of nano-structures on HgTe quantum wells is exemplarily verified in two separate transport experiments. We are first to realize physically etched quantum point contacts in HgTe/HgCdTe high mobility 2DEGs and to prove their controllability via external top-gate electrodes. So far quantum point contacts have not been reported in TI materials. However, these constrictions are part of many proposals to probe the nature of the helical quantum spin Hall edge channels and are suggested as injector and detector devices for spin polarized currents. To confirm their functionality we performed four-terminal measurements of the point contact conductance as a function of external gate voltage. Our measurements clearly exhibit quantized conductance steps in 2e2/h, which is a fundamental characteristic of quantum point contacts. Furthermore we conducted measurements on the formation and control of collimated electron beams, a key feature to realize an all electrical spin-optic device. In a second study several of the newly developed lithography techniques were implemented to produce arrays of nano-wires on inverted and non-inverted HgTe quantum well samples. These devices were used in order to probe and compare the weak antilocalization (WAL) in these structures as a function of magnetic field and temperature. Our measurements reveal that the WAL is almost an order of magnitude larger in inverted samples. This observation is attributed to the Dirac-like dispersion of the energy bands in HgTe quantum wells. The described lithography has already been successfully implemented and adapted in several published studies. All processes have been optimized to guarantee a minimum effect on the heterostructure's properties and the sample surface, which is especially important for probing the topological surface states of strained HgTe bulk layers. Our developments therefore serve as a base for continuous progress to further establish HgTe as a topological insulator and give access to new experiments.}, subject = {Topologischer Isolator}, language = {en} } @article{OPUS4-13609, title = {Search for pair-produced long-lived neutral particles decaying to jets in the ATLAS hadronic calorimeter in pp collisions at \(\sqrt{2}\)=8 TeV}, series = {Physics Letters B}, volume = {743}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.02.015}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-136099}, pages = {15-34}, year = {2015}, abstract = {The ATLAS detector at the Large Hadron Collider at CERN is used to search for the decay of a scalar boson to a pair of long-lived particles, neutral under the Standard Model gauge group, in 20.3 fb-120.3 fb\(^{-1}\) of data collected in proton-proton collisions at \(\sqrt{2}\)=8 TeV. This search is sensitive to long-lived particles that decay to Standard Model particles producing jets at the outer edge of the ATLAS electromagnetic calorimeter or inside the hadronic calorimeter. No significant excess of events is observed. Limits are reported on the product of the scalar boson production cross section times branching ratio into long-lived neutral particles as a function of the proper lifetime of the particles. Limits are reported for boson masses from 100 GeV to 900 GeV, and a long-lived neutral particle mass from 10 GeV to 150 GeV.}, language = {en} } @article{OPUS4-13610, title = {Search for H→γγH→γγ produced in association with top quarks and constraints on the Yukawa coupling between the top quark and the Higgs boson using data taken at 7 TeV and 8 TeV with the ATLAS detector}, series = {Physics Letters B}, volume = {740}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2014.11.049}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-136100}, pages = {222-242}, year = {2015}, abstract = {A search is performed for Higgs bosons produced in association with top quarks using the diphoton decay mode of the Higgs boson. Selection requirements are optimized separately for leptonic and fully hadronic final states from the top quark decays. The dataset used corresponds to an integrated luminosity of 4.5 fb\(^{-1}\) of proton-proton collisions at a center-of-mass energy of 7 TeV and 20.3 fb-120.3 fb\(^{-1}\) at 8 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess over the background prediction is observed and upper limits are set on the \(t\overline{t}H\) production cross section. The observed exclusion upper limit at 95\% confidence level is 6.7 times the predicted Standard Model cross section value. In addition, limits are set on the strength of the Yukawa coupling between the top quark and the Higgs boson, taking into account the dependence of the \(t\overline{t}H\) and tH   cross sections as well as the H→γγH→γγ branching fraction on the Yukawa coupling. Lower and upper limits at 95\% confidence level are set at -1.3 and +8.0 times the Yukawa coupling strength in the Standard Model.}, language = {en} } @phdthesis{Weber2016, author = {Weber, Stefan}, title = {Simulation Studies on the New Small Wheel Shielding of the ATLAS Experiment and Design and Construction of a Test Facility for Gaseous Detectors}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-133084}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {In this thesis two main projects are presented, both aiming at the overall goal of particle detector development. In the first part of the thesis detailed shielding studies are discussed, focused on the shielding section of the planned New Small Wheel as part of the ATLAS detector upgrade. Those studies supported the discussions within the upgrade community and decisions made on the final design of the New Small Wheel. The second part of the thesis covers the design, construction and functional demonstration of a test facility for gaseous detectors at the University of W{\"u}rzburg. Additional studies on the trigger system of the facility are presented. Especially the precision and reliability of reference timing signals were investigated.}, subject = {Teilchendetektor}, language = {en} } @article{OPUS4-13153, title = {Measurement of hard double-parton interactions in W(→ℓν) + 2-jet events at \(\sqrt{s}=7\) TeV with the ATLAS detector}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {033038}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/3/033038}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131531}, year = {2013}, abstract = {The production of W bosons in association with two jets in proton-proton collisions at a centre-of-mass energy of \(\sqrt{s}=7\),TeV has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of \(36 pb^{-1}\), collected with the ATLAS detector at the Large Hadron Collider. The fraction of events arising from double-parton interactions, \(f^D_{DP}\), has been measured through the pT balance between the two jets and amounts to \(f^D_{DP}\) = 0.08 ± 0.01 (stat.) ± 0.02 (sys.) for jets with transverse momentum \(p_T\) > 20 GeV and rapidity |y| < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of \(\sigma_{eff} = 15 ± 3 (stat.)^{+5}_{-3} (sys.)\) mb.}, language = {en} } @article{OPUS4-13154, title = {Search for extra dimensions in diphoton events from proton-proton collisions at \(\sqrt {s} = 7\)  TeV in the ATLAS detector at the LHC}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/4/043007}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131548}, pages = {043007}, year = {2013}, abstract = {The large difference between the Planck scale and the electroweak scale, known as the hierarchy problem, is addressed in certain models through the postulate of extra spatial dimensions. A search for evidence of extra spatial dimensions in the diphoton channel has been performed using the full set of proton-proton collisions at \(\sqrt {s} = 7\) TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. This dataset corresponds to an integrated luminosity of \(4.9 fb^{-1}\). The diphoton invariant mass spectrum is observed to be in good agreement with the Standard Model expectation. In the context of the model proposed by Arkani-Hamed, Dimopoulos and Dvali, 95\% confidence level lower limits of between 2.52 and 3.92 TeV are set on the ultraviolet cutoff scale MS depending on the number of extra dimensions and the theoretical formalism used. In the context of the Randall-Sundrum model, a lower limit of 2.06 (1.00) TeV at 95\% confidence level is set on the mass of the lightest graviton for couplings of \(k/\overline {M}_{Pl} = 0.1 (0.01)\). Combining with the ATLAS dilepton searches based on the 2011 data, the 95\% confidence level lower limit on the Randall-Sundrum graviton mass is further tightened to 2.23 (1.03) TeV for \(k/\overline {M}_{Pl} = 0.1 (0.01)\).}, language = {en} } @article{OPUS4-13155, title = {Search for excited electrons and muons in \(\sqrt {s}=8\) TeV proton-proton collisions with the ATLAS detector}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {093011}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/9/093011}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131557}, year = {2013}, abstract = {The ATLAS detector at the Large Hadron Collider is used to search for excited electrons and excited muons in the channel \(pp →ℓℓ^\ast→ℓℓ\gamma \), assuming that excited leptons are produced via contact interactions. The analysis is based on \(13 fb^{-1}\) of pp collisions at a centre-of-mass energy of 8 TeV. No evidence for excited leptons is found, and a limit is set at the 95\% credibility level on the cross section times branching ratio as a function of the excitedlepton mass \(m_{ℓ^\ast}\) . For \(m_{ℓ^\ast}\) ≽0.8 TeV, the respective upper limits on \(\sigma B(ℓ→ ℓ\gamma)\) are 0.75 and 0.90 fb for the \(e^\ast\) and \(μ^\ast\) searches. Limits on \(\sigma B\) are converted into lower bounds on the compositeness scale \(\Lambda\). In the special case where \(\Lambda = m_{ℓ^\ast}\), excited-electron and excited-muon masses below 2.2 TeV are excluded.}, language = {en} } @article{OPUS4-13156, title = {Search for WH production with a light Higgs boson decaying to prompt electron-jets in proton-proton collisions at \(\sqrt {s}=7\)  TeV with the ATLAS detector}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {043009}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/4/043009}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131565}, year = {2013}, abstract = {A search is performed for WH production with a light Higgs boson decaying to hidden-sector particles resulting in clusters of collimated electrons, known as electron-jets. The search is performed with \(2.04 fb^{-1}\) of data collected in 2011 with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at \(\sqrt {s}=7 TeV\) . One event satisfying the signal selection criteria is observed, which is consistent with the expected background rate. Limits on the product of the WH production cross section and the branching ratio of a Higgs boson decaying to prompt electron-jets are calculated as a function of a Higgs boson mass in the range from 100 to 140 GeV.}, language = {en} } @phdthesis{Reis2022, author = {Reis, Felix}, title = {Realization and Spectroscopy of the Quantum Spin Hall Insulator Bismuthene on Silicon Carbide}, doi = {10.25972/OPUS-25825}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-258250}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Topological matter is one of the most vibrant research fields of contemporary solid state physics since the theoretical prediction of the quantum spin Hall effect in graphene in 2005. Quantum spin Hall insulators possess a vanishing bulk conductivity but symmetry-protected, helical edge states that give rise to dissipationless charge transport. The experimental verification of this exotic state of matter in 2007 lead to a boost of research activity in this field, inspired by possible ground-breaking future applications. However, the use of the quantum spin Hall materials available to date is limited to cryogenic temperatures owing to their comparably small bulk band gaps. In this thesis, we follow a novel approach to realize a quantum spin Hall material with a large energy gap and epitaxially grow bismuthene, i.e., Bi atoms adopting a honeycomb lattice, in a \((\sqrt{3}\times\sqrt{3})\) reconstruction on the semiconductor SiC(0001). In this way, we profit both from the honeycomb symmetry as well as the large spin-orbit coupling of Bi, which, in combination, give rise to a topologically non-trivial band gap on the order of one electronvolt. An in-depth theoretical analysis demonstrates that the covalent bond between the Si and Bi atoms is not only stabilizing the Bi film but is pivotal to attain the quantum spin Hall phase. The preparation of high-quality, unreconstructed SiC(0001) substrates sets the basis for the formation of bismuthene and requires an extensive procedure in ultra-pure dry H\(_2\) gas. Scanning tunneling microscopy measurements unveil the (\(1\times1\)) surface periodicity and smooth terrace planes, which are suitable for the growth of single Bi layers by means of molecular beam epitaxy. The chemical configuration of the resulting Bi film and its oxidation upon exposure to ambient atmosphere are inspected with X-ray photoelectron spectroscopy. Angle-resolved photoelectron spectroscopy reveals the excellent agreement of probed and calculated band structure. In particular, it evidences a characteristic Rashba-splitting of the valence bands at the K point. Scanning tunneling spectroscopy probes signatures of this splitting, as well, and allows to determine the full band gap with a magnitude of \(E_\text{gap}\approx0.8\,\text{eV}\). Constant-current images and local-density-of-state maps confirm the presence of a planar honeycomb lattice, which forms several domains due to different, yet equivalent, nucleation sites of the (\(\sqrt{3}\times\sqrt{3}\))-Bi reconstruction. Differential conductivity measurements demonstrate that bismuthene edge states evolve at atomic steps of the SiC substrate. The probed, metallic local density of states is in agreement with the density of states expected from the edge state's energy dispersion found in density functional theory calculations - besides a pronounced dip at the Fermi level. By means of temperature- and energy-dependent tunneling spectroscopy it is shown that the spectral properties of this suppressed density of states are successfully captured in the framework of the Tomonaga-Luttinger liquid theory and most likely originate from enhanced electronic correlations in the edge channel.}, subject = {Zweidimensionales Material}, language = {en} } @phdthesis{Proppert2014, author = {Proppert, Sven Martin}, title = {Design, implementation and characterization of a microscope capable of three-dimensional two color super-resolution fluorescence imaging}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-107905}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {This thesis reviews the fundamentals of three-dimensional super-resolution localization imaging. In order to infer the axial coordinate of the emission of single fluorophores, the point spread function is engineered following a technique usually referred to as astigmatic imaging by the introduction of a cylindrical lens to the detection path of a microscope. After giving a short introduction to optics and localization microscopy, I outline sources of aberrations as frequently encountered in 3D-localization microscopy and will discuss their respective impact on the precision and accuracy of the localization process. With the knowledge from these considerations, experiments were designed and conducted to verify the validity of the conclusions and to demonstrate the abilities of the proposed microscope to resolve biological structures in the three spatial dimensions. Additionally, it is demonstrated that measurements of huge volumes with virtually no aberrations is in principle feasible. During the course of this thesis, a new method was introduced for inferring axial coordinates. This interpolation method based on cubic B-splines shows superior performance in the calibration of a microscope and the evaluation of subsequent measurement and will therefore be used and explained in this work. Finally, this work is also meant to give future students some guidance for entering the field of 3D localization microscopy and therefore, detailed protocols are provided covering the specific aspects of two color 3D localization imaging.}, subject = {Dimension 3}, language = {en} } @phdthesis{Iff2022, author = {Iff, Oliver}, title = {Implementierung und Charakterisierung von Einzelphotonenquellen in zweidimensionalen Übergangsmetall-Dichalkogeniden und deren Kopplung an optische Resonatoren}, doi = {10.25972/OPUS-28140}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-281404}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Schon heute bilden Einzelphotonenquellen einen wichtigen Baustein in der Photonik und Quanteninformation. Der Fokus der Forschung liegt entsprechend auf dem Finden und Charakterisieren daf{\"u}r geeigneter Materialsysteme. Konkret beschäftigt sich die vorliegende Arbeit vorwiegend mit dem Übergangsmetall-Dichalkogenid (TMDC1 ) Wolframdiselenid und seinen Eigenschaften. Diese Wahl ist durch den direkte Zugang zu Einzelphotonenquellen begr{\"u}ndet, die sich in dessen Monolagen ausbilden können. Diese Lichtquellen können {\"u}ber eine Modulation der Verspannung der Monolage gezielt aktiviert werden. Durch die, verglichen mit ihrem Volumen, riesige Kontaktfläche lassen sich Monolagen zudem mit Hilfe des Substrats, auf das sie transferiert wurden, wesentlich beeinflussen. Im Rahmen dieser Arbeit wurden Monolagen von WSe2 in unterschiedlichen Bauteilen wie zirkulare Bragg-Gittern oder vorstrukturierten, metallischen Oberflächen implementiert und die Photolumineszenz des TMDCs untersucht. Diese Arbeit belegt die Möglichkeit, Einzelphotonenquellen basierend aufWSe2 -Monolagen auf verschiedenste Weise modulieren zu können. Dank ihrer zwei- dimensionalen Geometrie lassen sie sich einfach in bestehende Strukturen integrieren oder auch in der Zukunft mit weiteren 2D-Materialien kombinieren.}, subject = {Einzelphotonenemission}, language = {de} } @article{OPUS4-17238, title = {Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at \( \sqrt{s}=13 \) TeV using the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {2017}, journal = {Journal of High Energy Physics}, number = {191}, organization = {The ATLAS Collaboration}, doi = {10.1007/JHEP11(2017)191}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172386}, year = {2017}, abstract = {Measurements of differential cross-sections of top-quark pair production in fiducial phase-spaces are presented as a function of top-quark and \(t\overline{t}\) system kinematic observables in proton-proton collisions at a centre-of-mass energy of \(\sqrt{s}\) = 13 TeV. The data set corresponds to an integrated luminosity of 3.2 fb\(^{-1}\), recorded in 2015 with the ATLAS detector at the CERN Large Hadron Collider. Events with exactly one electron or muon and at least two jets in the final state are used for the measurement. Two separate selections are applied that each focus on different top-quark momentum regions, referred to as resolved and boosted topologies of the \(t\overline{t}\) final state. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations by means of calculated \(χ^2\) and \(p\)-values.}, language = {en} } @article{OPUS4-15016, title = {Search for production of WW/WZ resonances decaying to a lepton, neutrino and jets in pp collisions at \(\sqrt {s}\)= 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {5}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3425-6}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150164}, pages = {209}, year = {2015}, abstract = {A search is presented for narrow diboson resonances decaying to WW or WZ in the final state where one W boson decays leptonically (to an electron or a muon plus a neutrino) and the other W/Z boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb\(^{-1}\) of pp collisions at \(\sqrt {s}\) = 8 TeV collected by the ATLAS detector at the large hadron collider. No evidence for resonant diboson production is observed, and resonance masses below 700 and 1490 GeV are excluded at 95 \% confidence level for the spin-2 Randall-Sundrum bulk graviton G\(^{*}\) with coupling constant of 1.0 and the extended gauge model W′ boson respectively.}, language = {en} } @article{OPUS4-14990, title = {Measurement of the top-quark mass in the fully hadronic decay channel from ATLAS data at \(\sqrt {s}\)=7TeV}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {4}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3373-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149905}, pages = {158}, year = {2015}, abstract = {The mass of the top quark is measured in a data set corresponding to 4.6  fb\(^{-1}\) of proton-proton collisions with centre-of-mass energy \(\sqrt {s}\)=7 TeV collected by the ATLAS detector at the LHC. Events consistent with hadronic decays of top-antitop quark pairs with at least six jets in the final state are selected. The substantial background from multijet production is modelled with data-driven methods that utilise the number of identified b-quark jets and the transverse momentum of the sixth leading jet, which have minimal correlation. The top-quark mass is obtained from template fits to the ratio of three-jet to dijet mass. The three-jet mass is calculated from the three jets produced in a top-quark decay. Using these three jets the dijet mass is obtained from the two jets produced in the W boson decay. The top-quark mass obtained from this fit is thus less sensitive to the uncertainty in the energy measurement of the jets. A binned likelihood fit yields a top-quark mass of m\(_{t}\)= 175.1 ± 1.4 (stat.) ± 1.2 (syst.) GeV.}, language = {en} } @article{LeeSongHanetal.2015, author = {Lee, Eun-Hye and Song, Jin-Dong and Han, Il-Ki and Chang, Soo-Kyung and Langer, Fabian and H{\"o}fling, Sven and Forchel, Alfred and Kamp, Martin and Kim, Jong-Su}, title = {Structural and optical properties of position-retrievable low-density GaAs droplet epitaxial quantum dots for application to single photon sources with plasmonic optical coupling}, series = {Nanoscale Research Letters}, volume = {10}, journal = {Nanoscale Research Letters}, number = {114}, doi = {10.1186/s11671-015-0826-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143692}, year = {2015}, abstract = {The position of a single GaAs quantum dot (QD), which is optically active, grown by low-density droplet epitaxy (DE) (approximately 4 QDs/μm\(^{2}\)), was directly observed on the surface of a 45-nm-thick Al\(_{0.3}\)Ga\(_{0.7}\)As capping layer. The thin thickness of AlGaAs capping layer is useful for single photon sources with plasmonic optical coupling. A micro-photoluminescence for GaAs DE QDs has shown exciton/biexciton behavior in the range of 1.654 to 1.657 eV. The direct observation of positions of low-density GaAs DE QDs would be advantageous for mass fabrication of devices that use a single QD, such as single photon sources.}, language = {en} } @phdthesis{Bauernfeind2023, author = {Bauernfeind, Maximilian Josef Xaver}, title = {Epitaxy and Spectroscopy of Two-Dimensional Adatom Systems: the Elemental Topological Insulator Indenene on SiC}, doi = {10.25972/OPUS-31166}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-311662}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {Two-dimensional (2D) topological insulators are a new class of materials with properties that are promising for potential future applications in quantum computers. For example, stanene represents a possible candidate for a topological insulator made of Sn atoms arranged in a hexagonal lattice. However, it has a relatively fragile low-energy spectrum and sensitive topology. Therefore, to experimentally realize stanene in the topologically non-trivial phase, a suitable substrate that accommodates stanene without compromising these topological properties must be found. A heterostructure consisting of a SiC substrate with a buffer layer of adsorbed group-III elements constitutes a possible solution for this problem. In this work, 2D adatom systems of Al and In were grown epitaxially on SiC(0001) and then investigated structurally and spectroscopically by scanning tunneling microscopy (STM) and photoelectron spectroscopy. Al films in the high coverage regime \( (\Theta_{ML}\approx2\) ML\( ) \) exhibit unusually large, triangular- and rectangular-shaped surface unit cells. Here, the low-energy electron diffraction (LEED) pattern is brought into accordance with the surface topography derived from STM. Another Al reconstruction, the quasi-one-dimensional (1D) Al phase, exhibits a striped surface corrugation, which could be the result of the strain imprinted by the overlayer-substrate lattice mismatch. It is suggested that Al atoms in different surface areas can occupy hexagonal close-packed and face-centered cubic lattice sites, respectively, which in turn lead to close-packed transition regions forming the stripe-like corrugations. On the basis of the well-known herringbone reconstruction from Au(111), a first structural model is proposed, which fits well to the structural data from STM. Ultimately, however, thermal treatments of the sample could not generate lower coverage phases, i.e. in particular, a buffer layer structure. Strong metallic signatures are found for In high coverage films \( (\Theta_{ML}\approx3\) to \(2\) ML\() \) by scanning tunneling spectroscopy (STS) and angle-resolved photoelectron spectroscopy (ARPES), which form a \( (7\times7) \), \( (6\times4\sqrt{3}) \), and \( (4\sqrt{3}\times4\sqrt{3}) \) surface reconstruction. In all these In phases electrons follow the nearly-free electron model. Similar to the Al films, thermal treatments could not obtain the buffer layer system. Surprisingly, in the course of this investigation a triangular In lattice featuring a \( (1\times1) \) periodicity is observed to host massive Dirac-like bands at \( K/K^{\prime} \) in ARPES. Based on this strong electronic similarity with graphene at the Brillouin zone boundary, this new structure is referred to as \textit{indenene}. An extensive theoretical analysis uncovers the emergence of an electronic honeycomb network based on triangularly arranged In \textit{p} orbitals. Due to strong atomic spin-orbit coupling and a comparably small substrate-induced in-plane inversion symmetry breaking this material system is rendered topologically non-trivial. In indenene, the topology is intimately linked to a bulk observable, i.e., the energy-dependent charge accumulation sequence within the surface unit cell, which is experimentally exploited in STS to confirm the non-trivial topological character. The band gap at \( K/K^{\prime} \), a signature of massive Dirac fermions, is estimated by ARPES to approximately 125 meV. Further investigations by X-ray standing wave, STM, and LEED confirm the structural properties of indenene. Thus, this thesis presents the growth and characterization of the novel quantum spin Hall insulator material indenene.}, subject = {Dreiecksgitter}, language = {en} } @article{OPUS4-14992, title = {Measurements of the W production cross sections in association with jets with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {2}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3262-7}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149925}, pages = {82}, year = {2015}, abstract = {This paper presents cross sections for the production of a W boson in association with jets, measured in proton-proton collisions at \(\sqrt {s}\) = 7 TeV with the ATLAS experiment at the large hadron collider. With an integrated luminosity of 4.6 fb\(^{-1}\), this data set allows for an exploration of a large kinematic range, including jet production up to a transverse momentum of 1 TeV and multiplicities up to seven associated jets. The production cross sections for W bosons are measured in both the electron and muon decay channels. Differential cross sections for many observables are also presented including measurements of the jet observables such as the rapidities and the transverse momenta as well as measurements of event observables such as the scalar sums of the transverse momenta of the jets. The measurements are compared to numerous QCD predictions including next-to-leading-order perturbative calculations, resummation calculations and Monte Carlo generators.}, language = {en} } @article{OPUS4-14961, title = {Constraints on the off-shell Higgs boson signal strength in the high-mass ZZ and WW final states with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3542-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149615}, pages = {335}, year = {2015}, abstract = {Measurements of the ZZ and WW final states in the mass range above the 2m\(_Z\) and 2m\(_W\) thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the ZZ→4ℓ, ZZ→2ℓ2ν and WW→eνμν final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb\(^{-1}\) at a collision energy of \(\sqrt {s}\)=8 TeV. Using the CL\(_S\) method, the observed 95 \% confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1-8.6, with an expected range of 6.7-11.0. In each case the range is determined by varying the unknown gg→ZZ and gg→WW background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95 \% CL upper limit on Γ\(_H\)/Γ\(^{SM}_{H}\) in the range 4.5-7.5 (6.5-11.2) using the same variations of the background K-factor. Assuming that the unknown gg→VV background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95 \% CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.}, language = {en} }