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In this work fluorescence-based single molecule detection at low concetration is investigated, with an emphasis on the usage of active transport and waveguides.
Active transport allows to overcome the limits of diffusion-based systems in terms of the lowest detectable threshold of concentration.
The effect of flow in single molecule experiments is investigated and a theoretical model is derived for laminar flow.
Waveguides on the other hand promise compact detection schemes and show great potential for their possible integration into lab-on-a-chip applications. Their properties in single molecule experiments are analyzed with help of a method based on the reciprocity theorem of electromagnetic theory.
This thesis reports a successful fabrication and characterisation of ferromagnetic/superconductor junction (F/S) on graphene. The thesis preposes a fabrication method to produce F/S junctions on graphene which make use of ALD grown Al2O3 as the tunnel barrier for the ferromagnetic contacts. Measurements done on F/G/S/G/F suggests that by injecting spin polarised current into the superconductor, a spin imbalance is created in the quasiparticle density of states of the superconductor which then diffuses through the graphene channel. The observed characteristic curves are similar to the ones which are already reported on metallic ferromagnet/superconductor junctions where the spin imbalance is created using Zeeman splitting. Further measurements also show that the curves loose their characteristic shapes when the temperature is increased above the critical temperature (Tc) or when the external magnetic field is higher then the critical field (Hc) of the superconducting contact. But to prove conclusively and doubtlessly the existence of spin imbalance in ferromagnet/superconductor junctions on graphene, more devices have to be made and characterised preferably in a dilution refrigerator.
The top-quark mass is measured in the all-hadronic top-antitop quark decay channel using proton-proton collisions at a centre-of-mass energy of \(\sqrt{s}=8\) TeV with the ATLAS detector at the CERN Large Hadron Collider. The data set used in the analysis corresponds to an integrated luminosity of 20.2 fb\(^{−1}\). The large multi-jet background is modelled using a data-driven method. The top-quark mass is obtained from template fits to the ratio of the three-jet to the dijet mass. The three-jet mass is obtained from the three jets assigned to the top quark decay. From these three jets the dijet mass is obtained using the two jets assigned to the W boson decay. The top-quark mass is measured to be 173.72 ± 0.55 (stat.) ± 1.01 (syst.) GeV.
The issue of quantum mechanical coupling between a semiconductor quantum dot and a quantum well is studied in two families of GaAs- and InP- based structures at cryogenic temperatures. It is shown that by tuning the quantum well parameters one can strongly disturb the 0D-character of the coupled system ground state, initially located in a dot. The out-coupling of either an electron or a hole state from the quantum dot confining potential is viewed by a significant elongation of the photoluminescence decay time constant. Band structure calculations show that in the GaAs-based coupled system at its ground state a hole remains isolated in the dot, whereas an electron gets delocalized towards the quantum well. The opposite picture is built for the ground state of a coupled system based on InP.
The subject of this thesis is the control of strain in HgTe thin-film crystals. Such systems are members of the new class of topological insulator materials and therefore of special research interest. A major task was the experimental control of the strain in the HgTe films. This was achieved by a new epitaxial approach and confirmed by cristallographic analysis and magneto-transport measurements.
In this work, strain was induced in thin films by means of coherent epitaxy on substrate crystals. This means that the film adopts the lattice constant of the substrate in the plane of the substrate-epilayer interface. The level of strain is determined by the difference between the strain-free lattice constants of the substrate and epilayer material (the so-called lattice mismatch). The film responds to an in-plane strain with a change of its lattice constant perpendicular to the interface. This relationship is crucial for both the correct interpretation of high resolution X-ray diffraction (HRXRD) measurements, and the precise determination of the band dispersion. The lattice constant of HgTe is smaller than the lattice constant of CdTe. Therefore, strain in HgTe is tensile if it is grown on a CdTe substrate. In principle, compressive strain can be achieved by using an appropriate \(\text{Cd}_{1-x}\text{Zn}_{x}\text{Te}\) substrate. This concept was modified and applied in this work.
Epilayers have been fabricated by molecular-beam epitaxy (MBE). The growth of thick buffer layers of CdTe on GaAs:Si was established as an alternative to commercial CdTe and \(text{Cd}_{0.96}\text{Zn}_{0.04}\text{Te}\) substrates. The growth conditions have been optimized by an analysis of atomic force microscopy and HRXRD studies. HRXRD measurements reveal a power-law increase of the crystal quality with increasing thickness. Residual strain was found in the buffer layers, and was attributed to a combination of finite layer thickness and mismatch of the thermal expansion coefficients of CdTe and GaAs. In order to control the strain in HgTe epilayers, we have developed a new type of substrate with freely adjustable lattice constant.
CdTe-\(\text{Cd}_{0.5}\text{Zn}_{0.5}\text{Te}\) strained-layer-superlattices have been grown by a combination of MBE and atomic-layer epitaxy (ALE), and have been analyzed by HRXRD. ALE of the \(\text{Cd}_{0.5}\text{Zn}_{0.5}\text{Te}\) layer is self-limiting to one monolayer, and the effective lattice constant can be controlled reproducibly and straightforward by adjusting the CdTe layer thickness. The crystal quality has been found to degrade with increasing Zn-fraction. However, the effect is less drastic compared to single layer \(\text{Cd}_{1-x}\text{Zn}_{x}\text{Te}\) solid solutions. HgTe quantum wells (QWs) sandwiched in between CdHgTe barriers have been fabricated in a similar fashion on superlattices and conventional CdTe and \(\text{Cd}_{0.96}\text{Zn}_{0.04}\text{Te}\) substrates. The lower critical thickness of the CdHgTe barrier material grown on superlattice substrates had to be considered regarding the sample design. The electronic properties of the QWs depend on the strain and thickness of the QW. We have determined the QW thickness with an accuracy of \(\pm\)0.5 nm by an analysis of the beating patterns in the thickness fringes of HRXRD measurements and X-ray reflectometry measurements. We have, for the first time, induced compressive strain in HgTe QWs by an epitaxial technique (i.e. the effective lattice constant of the superlattice is lower compared to the lattice constant of HgTe). The problem of the lattice mismatch between superlattice and barriers has been circumvented by using CdHgTe-ZnHgTe superlattices instead of CdHgTe as a barrier material. Furthermore, the growth of compressively strained HgTe bulk layers (with a thickness of at least 50 nm) was demonstrated as well.
The control of the state of strain adds a new degree of freedom to the design of HgTe epilayers, which has a major influence on the band structure of QWs and bulk layers. Strain in bulk layers lifts the degeneracy of the \(\Gamma_8\) bands at \(\mathbf{k}=0\). Tensile strain opens an energy gap, compressive strain shifts the touching points of the valence- and conduction band to positions in the Brillouin zone with finite \(\mathbf{k}\). Such a situation has been realized for the first time in the course of this work. For QWs in the inverted regime, it is demonstrated that compressive strain can be used to significantly enhance the thermal energy gap of the two-dimensional electron gas (2DEG). In addition, semi-metallic and semiconducting behavior is expected in wide QWs, depending on the state of strain. An examination of the temperature dependence of the subband ordering in QWs revealed that the band gap is only temperature-stable for appropriate sample parameters and temperature regimes. The band inversion is always lifted for sufficiently high temperatures.
A large number of models investigate the influence of the band gap on the stability of the quantum-spin-Hall (QSH) effect. An enhancement of the stability of QSH edge state conductance is expected for enlarged band gaps. Furthermore, experimental studies on the temperature dependence of the QSH conductance are in contradiction to theoretical predictions. Systematic studies of these aspects have become feasible based on the new flexibility of the sample design.
Detailed low-temperature magnetotransport studies have been carried out on QWs and bulk layers. For this purpose, devices have been fabricated lithographically, which consist of two Hall-bar geometries with different dimensions. This allows to discriminate between conductance at the plane of the 2DEG and the edge of the sample. The Fermi energy in the 2DEG has been adjusted by means of a top gate electrode. The strain-induced transition from semi-metallic to semiconducting characteristics in wide QWs was shown. The magnitude of the semi-metallic overlap of valence- and conduction band was determined by an analysis of the two-carrier conductance and is in agreement with band structure calculations. The band gap of the semiconducting sample was determined by measurements of the temperature dependence of the conductance at the charge-neutrality point. Agreement with the value expected from theory has been achieved for the first time in this work. The influence of the band gap on the stability of QSH edge state conductance has been investigated on a set of six samples. The band gap of the set spans a range of 10 to 55 meV. The latter value has been achieved in a highly compressively strained QW, has been confirmed by temperature-dependent conductance measurements, and is the highest ever reported in the inverted regime. Studies of the carrier mobility reveal a degradation of the sample quality with increasing Zn-fraction in the superlattice, in agreement with HRXRD observations. The enhanced band gap does not suppress scattering mechanisms in QSH edge channels, but lowers the conductance in the plane of the 2DEG. Hence, edge state conductance is the dominant conducting process even at elevated temperatures. An increase in conductance with increasing temperature has been found, in agreement with reports from other groups. The increase follows a power-law dependency, the underlying physical mechanism remains open. A cause for the lack of an increase of the QSH edge state conductance with increasing energy gap has been discussed. Possibly, the sample remains insulating even at finite carrier densities, due to localization effects. The measurement does not probe the QSH edge state conductance at the situation where the Fermi energy is located in the center of the energy gap, but in the regime of maximized puddle-driven scattering. In a first set of measurements, it has been shown that the QSH edge state conductance can be influenced by hysteretic charging effects of trapped states in the insulating dielectric. A maximized conductance of \(1.6\ \text{e}^2/\text{h}\) was obtained in a \(58\ \mu\text{m}\) edge channel. Finally, measurements on three dimensional samples have been discussed. Recent theoretical works assign compressively strained HgTe bulk layers to the Weyl semi-metal class of materials. Such layers have been synthesized and studied in magnetotransport experiments for the first time. Pronounced quantum-Hall- and Shubnikov-de-Haas features in the Hall- and longitudinal resistance indicate two-dimensional conductance on the sample surface. However, this conductance cannot be assigned definitely to Weyl surface states, due to the inversion of \(\Gamma_6\) and \(\Gamma_8\) bands. If a magnetic field is aligned parallel to the current in the device, a decrease in the longitudinal resistance is observed with increasing magnetic field. This is a signature of the chiral anomaly, which is expected in Weyl semi-metals.
Spin- and angle-resolved photoelectron spectroscopy is the prime method to investigate
spin polarized electronic states at solid state surfaces. In how far the spin polarization
of an emitted photoelectron reflects the intrinsic spin character of an electronic state is
the main question in the work at hand. It turns out that the measured spin polarization
is strongly influenced by experimental conditions, namely by the polarization of the
incoming radiation and the excitation energy. The photoemission process thus plays a
non-negligible role in a spin-sensitive measurement. This work is dedicated to unravel
the relation between the result of a spin-resolved measurement and the spin character
in the ground state and, therefore, to gain a deep understanding of the spin-dependent
photoemission process.
Materials that exhibit significant spin-splittings in their electronic structure,
owing to a strong spin-orbit coupling, serve as model systems for the investigations in
this work. Therefore, systems with large Rashba-type spin-splittings as BiTeI(0001)
and the surface alloys BiAg2/Ag(111) and PbAg2/Ag(111) are investigated. Likewise,
the surface electronic structure of the topological insulators Bi2Te2Se(0001) and
Bi2Te3(0001) are analyzed.
Light polarization dependent photoemission experiments serve as a probe of the
orbital composition of electronic states. The knowledge of the orbital structure helps
to disentangle the spin-orbital texture inherent to the different surface states, when
in addition the spin-polarization is probed. It turns out that the topological surface
state of Bi2Te2Se(0001) as well as the Rashba-type surface state of BiTeI(0001) exhibit
chiral spin-textures associated with the p-like in-plane orbitals. In particular, opposite
chiralities are coupled to either tangentially or radially aligned p-like orbitals,
respectively. The results presented here are thus evidence that a coupling between
spin- and orbital part of the wave function occurs under the influence of spin-orbit
coupling, independent of the materials topology.
Systematic photon energy dependent measurements of the out-of-plane spin polarization
of the topological surface state of Bi2Te3(0001) reveal a strong dependence and
even a reversal of the sign of the photoelectron spin polarization with photon energy.
Similarly, the measured spin component perpendicular to the wave vector of the surface
state of BiAg2/Ag(111) shows strong modulations and sign reversals when the photon energy is changed. In BiAg2/Ag(111) the variations in the photoelectron spin
polarization are accompanied by significant changes and even a complete suppression
of the photoemission intensity from the surface state, indicating that the variations of
the spin polarization are strongly related to the photoemission cross section.
This relation is finally analyzed in detail by employing a simple model, which is
based on an evaluation of the transition matrix elements that describe the presented
experiments. The model shows that the underlying cause for the observed photoelectron
spin reversals can be found in the coupling of the spin structure to the spatial part
of the initial state wave function, revealing the crucial role of spin-orbit interaction
in the initial state wave function. The model is supported by ab initio photoemission
calculations, which show strong agreement with the experimental results.
Soft x-ray spectroscopic study of methanol and glycine peptides in different physical environments
(2017)
Ion-specific effects occur in a huge variety of aqueous solutions of electrolytes and larger molecules like peptides, altering properties such as viscosity, enzyme activity, protein stability, and salting-in and salting-out behavior of proteins. Typically, these type of effects are rationalized in terms of the Hofmeister series, which originally orders cations and anions according to their ability to enhance or suppress the solubility of proteins in water. This empirical order, however, is still not understood yet. Quite some effort was made to gain a molecular level understanding of this phenomenon, yet no consensus has been found about the underlying mechanisms and the determination and localization of the interaction sites.
Resonant inelastic soft x-ray scattering (RIXS) combines x-ray emission (XES) and absorption spectroscopies (XAS), probing the partial local density of states of both occupied and unoccupied electronic states and is thus a promising candidate to shed more light onto the issue. The studies presented in this work are directed towards an improved understanding of the interaction between salts and peptides. In order to address this topic, the impact of different physical environments on the electronic structure of small molecules (i.e., methanol and glycine derived peptides) is investigated systematically using soft x-ray spectroscopic methods, corroborated with density functional theory (DFT) calculations.
In a first step, molecules without any interactions to the surrounding are investigated, using gas-phase methanol as a model system. Thereby, the local and element specific character of RIXS is demonstrated and used to separately probe the local electronic structure of methanol’s hydroxyl and methyl group, respectively. The attribution of the observed emission features to distinct molecular orbitals is confirmed by DFT calculations, which also quantitatively explain the different relative intensities of the emission features. For resonant excitation of the O K pre-edge absorption resonance, strong isotope effects are found that are explained by dynamical processes at the hydroxyl group. This serves as an excellent example for possible consequences of a local change in the geometric structure or symmetry of a molecule on its electronic structure.
In the following, the sample system is expanded to the amino acid glycine and its smallest derived peptides diglycine and triglycine. As a first step, they are studied in their crystalline form in solid state. Again, a comprehensive picture of the electronic structure is developed by measuring RIXS maps at the oxygen and nitrogen K absorption edge, corroborated by DFT calculations. Similar to the case of methanol, dynamic processes at the protonated amino group of the molecules after exciting the nitrogen atom have a strong influence on the emission spectra. Furthermore, it is shown that RIXS can be used to selectively excite the peptide nitrogen to probe the electronic structure around it. A simple building block approach for XES spectra is applied to separate the contribution of the emission attributed to transitions into core holes at the peptide and the amino nitrogen, respectively.
In the aqueous solution, the surrounding water molecules slightly change the electronic structure, probably via interactions with the charged functional groups. The effects on the x-ray emission spectra, however, are rather small. Much bigger changes are observed when manipulating the protonation state of the functional groups by adjusting the pH value of the solution. A protonation of the carboxyl group at low pH values, as well as a deprotonation of the amino group at high pH values lead to striking changes in the shape of the RIXS maps. In a comprehensive study of glycine’s XES spectra at varying pH values, changes in the local electronic structure are not only observed in the immediate surrounding of the manipulated functional groups but also in more distant moieties of the molecule.
Finally, the study is extended to mixed aqueous solutions of diglycine and a variety of different salts as examples for systems where Hofmeister effects are observed. To investigate the influence of different cations and anions on the electronic structure of diglycine, two series of chlorine and potassium salts are used. Ion-specific effects are identified for both cases. Some of the changes in the x-ray emission spectra of diglycine in the mixed solutions qualitatively follow the Hofmeister series as a function of the used salt. The observed trends thereby indicate an increased interaction between the electron density around the peptide oxygen with the cations, whereas anions seem to interact with the amino group of the peptide.
We theoretically investigate the propagation of heat currents in a three-terminal quantum dot engine. Electron–electron interactions introduce state-dependent processes which can be resolved by energy-dependent tunneling rates. We identify the relevant transitions which define the operation of the system as a thermal transistor or a thermal diode. In the former case, thermal-induced charge fluctuations in the gate dot modify the thermal currents in the conductor with suppressed heat injection, resulting in huge amplification factors and the possible gating with arbitrarily low energy cost. In the latter case, enhanced correlations of the state-selective tunneling transitions redistribute heat flows giving high rectification coefficients and the unexpected cooling of one conductor terminal by heating the other one. We propose quantum dot arrays as a possible way to achieve the extreme tunneling asymmetries required for the different operations.
The Micromegas technology is one of the most successful modern gaseous detector concepts and widely utilized in nuclear and particle physics experiments. Twenty years of R & D rendered the technology sufficiently mature to be selected as precision tracking detector for the New Small Wheel (NSW) upgrade of the ATLAS Muon spectrometer. This will be the first large scale application of Micromegas in one of the major LHC experiments. However, many of the fundamental microscopic processes in these gaseous detectors are still not fully understood and studies on several detector aspects, like the micromesh geometry, have never been addressed systematically.
The studies on signal formation in Micromegas, presented in the first part of this thesis, focuses on the microscopic signal electron loss mechanisms and the amplification processes in electron gas interaction. Based on a detailed model of detector parameter dependencies, these processes are scrutinized in an iterating comparison between exper- imental results, theory prediction of the macroscopic observables and process simulation on the microscopic level. Utilizing the specialized detectors developed in the scope of this thesis as well as refined simulation algorithms, an unprecedented level of accuracy in the description of the microscopic processes is reached, deepening the understanding of the fundamental process in gaseous detectors.
The second part is dedicated to the challenges arising with the large scale Micro- megas production for the ATLAS NSW. A selection of technological choices, partially influenced or determined by the herein presented studies, are discussed alongside a final report on two production related tasks addressing the detectors’ core components: For the industrial production of resistive anode PCBs a detailed quality control (QC) and quality assurance (QA) scheme as well as the therefore required testing tools have been developed. In parallel the study on micromesh parameter optimization and production feasibility resulted in the selection of the proposed mesh by the NSW community and its full scale industrial manufacturing. The successful completion of both tasks were im- portant milestones towards the construction of large size Micromegas detectors clearing the path for NSW series production.
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.