@article{OPUS4-31268, title = {Search for new phenomena in events with same-charge leptons and b-jets in pp collisions at √\(s\) = 13 TeV with the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {12}, journal = {Journal of High Energy Physics}, number = {39}, organization = {The ATLAS Collaboration}, doi = {10.1007/JHEP12(2018)039}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-312681}, pages = {1-55}, year = {2018}, abstract = {A search for new phenomena in events with two same- charge leptons or three leptons and jets identi fi ed as originating from b - quarks in a data sample of 36.1 fb of pp collisions at ps = 13TeV recorded by the ATLAS detector at the Large Hadron Collider is reported. No signi fi cant excess is found and limits are set on vector- like quark, fourtop- quark, and same- sign top- quark pair production. The observed ( expected) 95\% CL mass limits for a vector- like T - and B - quark singlet are mT > 0 : 98 ( 0 : 99) TeV and mB > 1 : 00 ( 1 : 01) TeV respectively. Limits on the production of the vector- like T5=3 - quark are also derived considering both pair and single production; in the former case the lower limit on the mass of the T5=3 - quark is ( expected to be) 1.19 ( 1.21) TeV. The Standard Model fourtop- quark production cross- section upper limit is ( expected to be) 69 ( 29) fb. Constraints are also set on exotic four- top- quark production models. Finally, limits are set on samesign top- quark pair production. The upper limit on uu ! tt production is ( expected to be) 89 ( 59) fb for a mediator mass of 1TeV, and a dark- matter interpretation is also derived, excluding a mediator of 3TeV with a dark- sector coupling of 1.0 and a coupling to ordinary matter above 0.31.}, language = {en} } @phdthesis{Baumann2024, author = {Baumann, Johannes}, title = {Induced Superconductivity in HgTe Quantum Point Contacts}, doi = {10.25972/OPUS-36940}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-369405}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2024}, abstract = {In this thesis, the Josephson effect in mercury telluride based superconducting quantum point contacts (SQPCs) is studied. Implementing such confined structures into topological superconductors has been proposed as a means to detect and braid Majorana fermions. For the successful realization of such experiments though, coherent transport across the constriction is essential. By demonstrating the Josephson effect in a confined topological system, the presented experiments lay the foundation for future quantum devices that can be used for quantum computation. In addition, the experiments also provide valuable insights into the behavior of the Josephson effect in the low-channel limit (N<20). Due to the confinement of the weak link, we can also study the Josephson effect in a topological insulator, where the edge modes interact. In conclusion, this thesis discusses the fabrication of, and low-temperature measurements on mercury telluride quantum point contacts embedded within Josephson junctions. We find that the merging of the currently used fabrication methods for mercury telluride quantum point contacts and Josephson junctions does not yield a good enough device quality to resolve subbands of the quantum point contact as quantization effects in the transport properties. As we attribute this to the long dry etching time that is necessary for a top-contact, the fabrication process was adapted to reduce the defect density at the superconductor-semiconductor interface. Employing a technique that involves side contacting the mercury telluride quantum well and reducing the size of the mercury telluride mesa to sub-micrometer dimensions yields a quantized supercurrent across the junction. The observed supercurrent per mode is in good agreement with theoretical predictions for ballistic, one-dimensional modes that are longer than the Josephson penetration depth. Moreover, we find that oscillatory features superimpose the plateaus of the supercurrent and the conductance. The strength of these oscillatory features are sample-dependent and complicate the identification of plateaus. We suggest that the oscillatory features originate mainly from local defects and the short gate electrode. Additionally, resonances are promoted within the weak link if the transparency of the superconductor-HgTe interface differs from one. Furthermore, the research explores the regimes of the quantum spin Hall effect and the 0.5 anomaly. Notably, a small yet finite supercurrent is detected in the QSH regime. In samples fabricated from thick mercury telluride quantum wells, the supercurrent appears to vanish when the quantum point contact is tuned into the regime of the 0.5 anomaly. For samples fabricated from thin mercury telluride quantum wells, the conductance as well as the supercurrent vanish for strong depopulation. In these samples though, the supercurrent remains detectable even for conductance values significantly below 2 e²/h. Numerical calculation reproduce the transport behavior of the superconducting quantum point contacts. Additionally, the topological nature of the weak link is thoroughly investigated using the supercurrent diffraction pattern and the absorption of radio frequency photons. The diffraction pattern reveals a gate independent, monotonous decay of \$I_\text{sw}(B)\$, which is associated with the quantum interference of Andreev bound states funneled through the quantum point contact. Interestingly, the current distribution in the weak link appears unaffected as the quantum point contact is depleted. In the RF measurements, indications of a 4π periodic supercurrent are observed as a suppression of odd Shapiro steps. The ratio of the 4π periodic current to the 2π periodic current appears to decrease for smaller supercurrents, as odd Shapiro steps are exclusively suppressed for large supercurrents. Additionally, considering the observation that the supercurrent is small when the bulk modes in the quantum point contact are fully depleted, we suggest that the re-emerging of odd Shapiro steps is a consequence of the group velocity of the edge modes being significantly suppressed when the bulk modes are absent. Consequently, the topological nature of the superconducting quantum point contact is only noticeable in the transport properties when bulk modes are transmitted through the superconducting quantum point contact. The shown experiments are the first demonstration of mercury telluride superconducting quantum point contacts that exhibit signatures of quantization effects in the conductance as well as the supercurrent. Moreover, the experiments suggest that the regime of interacting topological edge channels is also accessible in mercury telluride superconducting quantum point contacts. This is potentially relevant for the realization of Majorana fermions and their application in the field of quantum computation.}, subject = {Topologischer Isolator}, language = {en} } @article{KreinbergGrbešićStraussetal.2018, author = {Kreinberg, S{\"o}ren and Grbešić, Tomislav and Strauß, Max and Carmele, Alexander and Emmerling, Monika and Schneider, Christian and H{\"o}fling, Sven and Porte, Xavier and Reitzenstein, Stephan}, title = {Quantum-optical spectroscopy of a two-level system using an electrically driven micropillar laser as a resonant excitation source}, series = {Light: Science \& Applications}, volume = {7}, journal = {Light: Science \& Applications}, doi = {10.1038/s41377-018-0045-6}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-229802}, year = {2018}, abstract = {Two-level emitters are the main building blocks of photonic quantum technologies and are model systems for the exploration of quantum optics in the solid state. Most interesting is the strict resonant excitation of such emitters to control their occupation coherently and to generate close to ideal quantum light, which is of utmost importance for applications in photonic quantum technology. To date, the approaches and experiments in this field have been performed exclusively using bulky lasers, which hinders the application of resonantly driven two-level emitters in compact photonic quantum systems. Here we address this issue and present a concept for a compact resonantly driven single-photon source by performing quantum-optical spectroscopy of a two-level system using a compact high-β microlaser as the excitation source. The two-level system is based on a semiconductor quantum dot (QD), which is excited resonantly by a fiber-coupled electrically driven micropillar laser. We dress the excitonic state of the QD under continuous wave excitation, and trigger the emission of single photons with strong multi-photon suppression (g\(^{(2)}\)(0)=0.02) and high photon indistinguishability (V = 57±9\%) via pulsed resonant excitation at 156 MHz. These results clearly demonstrate the high potential of our resonant excitation scheme, which can pave the way for compact electrically driven quantum light sources with excellent quantum properties to enable the implementation of advanced quantum communication protocols.}, language = {en} } @article{WaldherrLundtKlaasetal.2018, author = {Waldherr, Max and Lundt, Nils and Klaas, Martin and Betzold, Simon and Wurdack, Matthias and Baumann, Vasilij and Estrecho, Eliezer and Nalitov, Anton and Cherotchenko, Evgenia and Cai, Hui and Ostrovskaya, Elena A. and Kavokin, Alexey V. and Tongay, Sefaattin and Klembt, Sebastian and H{\"o}fling, Sven and Schneider, Christian}, title = {Observation of bosonic condensation in a hybrid monolayer MoSe2-GaAs microcavity}, series = {Nature Communications}, volume = {9}, journal = {Nature Communications}, doi = {10.1038/s41467-018-05532-7}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-233280}, year = {2018}, abstract = {Bosonic condensation belongs to the most intriguing phenomena in physics, and was mostly reserved for experiments with ultra-cold quantum gases. More recently, it became accessible in exciton-based solid-state systems at elevated temperatures. Here, we demonstrate bosonic condensation driven by excitons hosted in an atomically thin layer of MoSe2, strongly coupled to light in a solid-state resonator. The structure is operated in the regime of collective strong coupling between a Tamm-plasmon resonance, GaAs quantum well excitons, and two-dimensional excitons confined in the monolayer crystal. Polariton condensation in a monolayer crystal manifests by a superlinear increase of emission intensity from the hybrid polariton mode, its density-dependent blueshift, and a dramatic collapse of the emission linewidth, a hallmark of temporal coherence. Importantly, we observe a significant spin-polarization in the injected polariton condensate, a fingerprint for spin-valley locking in monolayer excitons. Our results pave the way towards highly nonlinear, coherent valleytronic devices and light sources.}, language = {en} } @article{TanEloPuskaetal.2018, author = {Tan, Z. B. and Elo, T. and Puska, A. and Sarkar, J. and L{\"a}hteenm{\"a}ki, P. and Duerr, F. and Gould, C. and Molenkamp, L. W. and Nagaev, K. E. and Hakonen, P. J.}, title = {Hanbury-Brown and Twiss exchange and non-equilibrium-induced correlations in disordered, four-terminal graphene-ribbon conductor}, series = {Scientific Reports}, volume = {8}, journal = {Scientific Reports}, doi = {10.1038/s41598-018-32777-5}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-240348}, year = {2018}, abstract = {We have investigated current-current correlations in a cross-shaped conductor made of graphene. The mean free path of charge carriers is on the order of the ribbon width which leads to a hybrid conductor where there is diffusive transport in the device arms while the central connection region displays near ballistic transport. Our data on auto and cross correlations deviate from the predictions of Landauer-B{\"u}ttiker theory, and agreement can be obtained only by taking into account contributions from non-thermal electron distributions at the inlets to the semiballistic center, in which the partition noise becomes strongly modified. The experimental results display distinct Hanbury - Brown and Twiss (HBT) exchange correlations, the strength of which is boosted by the non-equilibrium occupation-number fluctuations internal to this hybrid conductor. Our work demonstrates that variation in electron coherence along atomically-thin, two-dimensional conductors has significant implications on their noise and cross correlation properties.}, language = {en} } @article{SoltamovKasperPoshakinskiyetal.2019, author = {Soltamov, V. A. and Kasper, C. and Poshakinskiy, A. V. and Anisimov, A. N. and Mokhov, E. N. and Sperlich, A. and Tarasenko, S. A. and Baranov, P. G. and Astakhov, G. V. and Dyakonov, V.}, title = {Excitation and coherent control of spin qudit modes in silicon carbide at room temperature}, series = {Nature Communications}, volume = {10}, journal = {Nature Communications}, doi = {10.1038/s41467-019-09429-x}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-239149}, year = {2019}, abstract = {One of the challenges in the field of quantum sensing and information processing is to selectively address and coherently manipulate highly homogeneous qubits subject to external perturbations. Here, we present room-temperature coherent control of high-dimensional quantum bits, the so-called qudits, associated with vacancy-related spins in silicon carbide enriched with nuclear spin-free isotopes. In addition to the excitation of a spectrally narrow qudit mode at the pump frequency, several other modes are excited in the electron spin resonance spectra whose relative positions depend on the external magnetic field. We develop a theory of multipole spin dynamics and demonstrate selective quantum control of homogeneous spin packets with sub-MHz spectral resolution. Furthermore, we perform two-frequency Ramsey interferometry to demonstrate absolute dc magnetometry, which is immune to thermal noise and strain inhomogeneity.}, language = {en} } @article{SchneiderGlazovKornetal.2018, author = {Schneider, Christian and Glazov, Mikhail M. and Korn, Tobias and H{\"o}fling, Sven and Urbaszek, Bernhard}, title = {Two-dimensional semiconductors in the regime of strong light-matter coupling}, series = {Nature Communications}, volume = {9}, journal = {Nature Communications}, doi = {10.1038/s41467-018-04866-6}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-231295}, year = {2018}, abstract = {The optical properties of transition metal dichalcogenide monolayers are widely dominated by excitons, Coulomb-bound electron-hole pairs. These quasi-particles exhibit giant oscillator strength and give rise to narrow-band, well-pronounced optical transitions, which can be brought into resonance with electromagnetic fields in microcavities and plasmonic nanostructures. Due to the atomic thinness and robustness of the monolayers, their integration in van der Waals heterostructures provides unique opportunities for engineering strong light-matter coupling. We review first results in this emerging field and outline future opportunities and challenges.}, language = {en} } @article{SchmittMorasBihlmayeretal.2019, author = {Schmitt, Martin and Moras, Paolo and Bihlmayer, Gustav and Cotsakis, Ryan and Vogt, Matthias and Kemmer, Jeannette and Belabbes, Abderrezak and Sheverdyaeva, Polina M. and Kundu, Asish K. and Carbone, Carlo and Bl{\"u}gel, Stefan and Bode, Matthias}, title = {Indirect chiral magnetic exchange through Dzyaloshinskii-Moriya-enhanced RKKY interactions in manganese oxide chains on Ir(100)}, series = {Nature Communications}, volume = {10}, journal = {Nature Communications}, doi = {10.1038/s41467-019-10515-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-230986}, year = {2019}, abstract = {Localized electron spins can couple magnetically via the Ruderman-Kittel-Kasuya-Yosida interaction even if their wave functions lack direct overlap. Theory predicts that spin-orbit scattering leads to a Dzyaloshinskii-Moriya type enhancement of this indirect exchange interaction, giving rise to chiral exchange terms. Here we present a combined spin-polarized scanning tunneling microscopy, angle-resolved photoemission, and density functional theory study of MnO2 chains on Ir(100). Whereas we find antiferromagnetic Mn-Mn coupling along the chain, the inter-chain coupling across the non-magnetic Ir substrate turns out to be chiral with a 120° rotation between adjacent MnO2 chains. Calculations reveal that the Dzyaloshinskii-Moriya interaction results in spin spirals with a periodicity in agreement with experiment. Our findings confirm the existence of indirect chiral magnetic exchange, potentially giving rise to exotic phenomena, such as chiral spin-liquid states in spin ice systems or the emergence of new quasiparticles.}, language = {en} } @article{LeeImhofBergeretal.2018, author = {Lee, Ching Hua and Imhof, Stefan and Berger, Christian and Bayer, Florian and Brehm, Johannes and Molenkamp, Laurens W. and Kiessling, Tobias and Thomale, Ronny}, title = {Topolectrical Circuits}, series = {Communications Physics}, volume = {1}, journal = {Communications Physics}, doi = {10.1038/s42005-018-0035-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-236422}, year = {2018}, abstract = {Invented by Alessandro Volta and F{\´e}lix Savary in the early 19th century, circuits consisting of resistor, inductor and capacitor (RLC) components are omnipresent in modern technology. The behavior of an RLC circuit is governed by its circuit Laplacian, which is analogous to the Hamiltonian describing the energetics of a physical system. Here we show that topological insulating and semimetallic states can be realized in a periodic RLC circuit. Topological boundary resonances (TBRs) appear in the impedance read-out of a topolectrical circuit, providing a robust signal for the presence of topological admittance bands. For experimental illustration, we build the Su-Schrieffer-Heeger circuit, where our impedance measurement detects the TBR midgap state. Topolectrical circuits establish a bridge between electrical engineering and topological states of matter, where the accessibility, scalability, and operability of electronics synergizes with the intricate boundary properties of topological phases.}, language = {en} } @article{KuegelKarolakKroenleinetal.2018, author = {K{\"u}gel, Jens and Karolak, Michael and Kr{\"o}nlein, Andreas and Serrate, David and Bode, Matthias and Sangiovanni, Giorgio}, title = {Reversible magnetic switching of high-spin molecules on a giant Rashba surface}, series = {npj Quantum Materials}, volume = {3}, journal = {npj Quantum Materials}, doi = {10.1038/s41535-018-0126-z}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-230866}, year = {2018}, abstract = {The quantum mechanical screening of a spin via conduction electrons depends sensitively on the environment seen by the magnetic impurity. A high degree of responsiveness can be obtained with metal complexes, as the embedding of a metal ion into an organic molecule prevents intercalation or alloying and allows for a good control by an appropriate choice of the ligands. There are therefore hopes to reach an "on demand" control of the spin state of single molecules adsorbed on substrates. Hitherto one route was to rely on "switchable" molecules with intrinsic bistabilities triggered by external stimuli, such as temperature or light, or on the controlled dosing of chemicals to form reversible bonds. However, these methods constrain the functionality to switchable molecules or depend on access to atoms or molecules. Here, we present a way to induce bistability also in a planar molecule by making use of the environment. We found that the particular "habitat" offered by an antiphase boundary of the Rashba system BiAg2 stabilizes a second structure for manganese phthalocyanine molecules, in which the central Mn ion moves out of the molecular plane. This corresponds to the formation of a large magnetic moment and a concomitant change of the ground state with respect to the conventional adsorption site. The reversible spin switch found here shows how we can not only rearrange electronic levels or lift orbital degeneracies via the substrate, but even sway the effects of many-body interactions in single molecules by acting on their surrounding.}, language = {en} }