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The interplay between disorder and interactions can dramatically influence the physical properties of thin-film superconductors. In the most extreme case, strong disorder is able to suppress superconductivity as an insulating phase emerges. Due to the known pair-breaking potential of magnetic disorder on superconductors, the research focus is on the influence of non-magnetic disorder. Here we provide direct evidence that magnetic disorder is also present at the surface of amorphous superconducting films. This magnetic disorder is present even in the absence of magnetic impurity atoms and is intimately related to the surface termination itself. While bulk superconductivity survives in sufficiently thick films, we suggest that magnetic disorder may crucially affect the superconductor-to-insulator transition in the thin-film limit.

The most important direction in the development of fundamental and applied physics is the study of the properties of optical systems at nanoscales for creating optical and quantum computers, biosensors, single-photon sources for quantum informatics, DNA sequencing devices, detectors of various fields, etc. In all these cases, nanosize light sources such as dye molecules, quantum dots (epitaxial or colloidal), color centers in crystals, and nanocontacts in metals are of utmost importance. In the nanoenvironment, the characteristics of these elementary quantum systems—pumping rates, radiative and nonradiative decay rates, the local density of states, lifetimes, level shifts—experience changes, which can be used to create nanosize light sources with the desired properties. Modern theoretical and experimental works on controlling the emission of elementary quantum systems with the help of plasmonic and dielectric nanostructures, metamaterials, and metamaterial nanoparticles are analyzed.

We employ the first-principles calculation with non-equilibrium Green's function method to comprehensively investigate the crucial role of interfacial geometry in spin transport properties of Co/1,4-benzenediamine (BDA)/Co single-molecule magnetic junctions (SMMJs). Two bonding mechanisms are proposed for the hard-hard Co-N coupling: (1) the covalent bonding between the H-dissociated amine linker and spin-polarized Co apex atoms and (2) the dative interaction between the H-non-dissociated (denoted by +H) amine linker and Co apex atoms. The former covalent contact dominates the π-resonance interfacial spin selection that can be well preserved in H-dissociated cases regardless of the choice of top, bridge, and hollow contact sites. From our detailed analyses of spin-polarized transmission spectra, local density of states, and molecular density of states, the underlying mechanism is that the strong hybridization between Co-d, N-p y , and the π-orbital of the phenyl ring in dissociated cases renders the 2-fold HOMO (4-fold LUMO) of the central molecule closer to the Fermi energy. In contrast, the enlarged Co-N bond length of the latter dative contact in the H-non-dissociated case not only destroys the spinterface coupling but also blocks the spin injection. This theoretical work may provide vital and practical insights to illustrate the spin transport property in real amine-ended SMMJs since the contact geometries and interfacial bond mechanisms remain unclear during the breaking junction technique.

The transport property of a four-terminal MoS2NR/V7(Bz)8 device is computed within the framework of density functional theory combined with the nonequilibrium Green’s function (NEGF) technique. This device is constructed by a MoS2 nanoribbon (MoS2NR) as the source-to-drain channel and a spin-polarized V7(Bz)8 nanowire grafted on the MoS2NR surface as the double gate channel, where the four terminals are all connected to a semi-infinite one-dimensional (1D) Au lead. The transport characteristic is explored by investigating the conductance, currents, local density of states (LDOS), and scattering states. The currents of different leads are dissimilar due to the complex interplay between the four terminals that is otherwise not present in a two-terminal setup. The most interesting feature we articulate is that the induced promising properties including negative differential resistance (NDR) behavior, input/output current switching, as well as spin-polarized lead currents can be fine-tuned by the magnitude of either source bias or gate bias. These features can be utilized in designing multi-terminal nanoelectronic devices.

Usually, the cavity is considered an intrinsic part of laser design to enable coherent emission. For different types of cavities, it is assumed that the light coherence is achieved by different ways. We show that regardless of the type of cavity, the lasing condition is universal and is determined by the ratio of the width of the atomic spectrum to the product of the number of atoms and the spontaneous radiation rate in the laser structure. We demonstrate that cavity does not play a crucial role in lasing since it merely decreases the threshold by increasing the photon emission rate thanks to the Purcell effect. A threshold reduction can be achieved in a cavity-free structure by tuning the local density of states of the electromagnetic field. This paves the way for the design of laser devices based on cavity-free systems.

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V–V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron-electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when ±1, ±2 and ±3 electrons occupy each moire unit cell, and lead to the formation of various correlated phases1-4. Although some phases have been shown to have a non-zero Chern number5,6, the local microscopic properties and topological character of many other phases have not yet been determined. Here we introduce a set of techniques that use scanning tunnelling microscopy to map the topological phases that emerge in MATBG in a finite magnetic field. By following the evolution of the local density of states at the Fermi level with electrostatic doping and magnetic field, we create a local Landau fan diagram that enables us to assign Chern numbers directly to all observed phases. We uncover the existence of six topological phases that arise from integer fillings in finite fields and that originate from a cascade of symmetry-breaking transitions driven by correlations7,8. These topological phases can form only for a small range of twist angles around the magic angle, which further differentiates them from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions, exhibits prominent electron-hole asymmetry, and features an unexpectedly large splitting between zero Landau levels (about 3 to 5 millielectronvolts). Our results show how strong electronic interactions affect the MATBG band structure and lead to correlation-enabled topological phases.

Optical nanoantennas are well-known for the confinement of light into nanoscale hot spots, suitable for emission enhancement and sensing applications. Here, we show how control of the antenna dimensions allows tuning the local optical phase, hence turning a hot spot into a cold spot. We manipulate the local intensity exploiting the interference between driving and scattered field. Using single molecules as local detectors, we experimentally show the creation of subwavelength pockets with full suppression of the driving field. Remarkably, together with the cold excitation spots, we observe inhibition of emission by the phase-tuned nanoantenna. The fluorescence lifetime of a molecule scanned in such volumes becomes longer, showing slow down of spontaneous decay. In conclusion, the spatial phase of a nanoantenna is a powerful knob to tune between enhancement and inhibition in a 3-dimensional subwavelength volume.

We studied the electronic band structure and corresponding local density of states of low-index fcc Ag surfaces (100), (110), and (111) by using the empirical tight-binding method in the framework of the Surface Green’s Function Matching formalism. The energy values for different surface and resonance states are reported and a comparison with the available experimental and theoretical data is also done.

Dissertation, RWTH Aachen University, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2022). = Dissertation, RWTH Aachen University, 2022, The last decades have witnessed the rise of topology, which became one of the dominating paradigms in condensed matter physics. From a theoretical perspective, the focus often lies on topological invariants which, through the bulk-boundary correspondence can be used to predict the presence of edge states at the surfaces of insulating materials. These edge states are believed to be integral in the advancement of future technologies, such as quantum computation, microelectronics and spintronics. This thesis contributes to the field of topological phases of matter by reporting research results concerning the characterization of the latter using two complementary approaches: an energy-resolved, local density of states approach and an energy-unresolved, boundary charge approach. With respect to the first approach, we demonstrate how the local density of states, as computed using the Keldysh Green���s function formalism, can be used to model trans-port experiments in tight-binding models and how to identify topological edge states in the resulting data. As a testbed for this approach we study models of one-dimensional, periodically driven (Floquet) topological insulators and two-dimensional higher order topological superconductors. The findings in this regard are twofold. Firstly, we show how, in the case of the Floquet model, contrary to the local density of states approach, the topological invariants are not able to distinguish between degenerate edge states with vastly different localization lengths, rendering the invariant practically meaning-less. Secondly, while the simulations generally agree with the predictions made using topological considerations, we demonstrate how bulk effects can skew the picture and should therefore be kept in mind when interpreting experimental results. We identify the origin of the aforementioned bulk effects with nontopological edge states, argue for the universality of these states and analytically examine them in the context of the Floquet model and a generalized Kitaev chain, which we identify as the simplest model to feature said states. The second approach to the characterization of topological phases of matter that we discuss is based on the concept of the boundary charge. This experimentally accessible quantity is defined as the charge that accumulates at the surface of an insulator. At zero temperature, it can be thought of as the integrated (up to the chemical potential) excess electronic density, and thus lacks the energy resolution which is essential in detecting the subtle bulk effects mentioned above. Since it generally contains contributions from both localized edge states and extended bulk states, its analysis with respect to the topology of the phase in question is highly nontrivial. We show how the boundary charge can be divided into contributions stemming from bulk states, edge states and from the polarization of the unit cell respectively. We propose novel, boundary charge based invariants, which are given in terms of winding numbers of bulk Green���s functions and which are applicable to all insulating systems, irrespective of symmetry constraints. We demonstrate their validity using randomly generated one-dimensional models. The scope of this thesis lies in the demonstration that the topological invariant approach yields an incomplete picture in certain circumstances, necessitating the use of further theoretical tools, such as those discussed in the following., Published by RWTH Aachen University, Aachen

Metamaterials are composites that are engineered purposefully for realizing electromagnetic characteristics that do not occur naturally in mineral or organic form. These characteristics are realized by regular arrangements of meta atoms, the building blocks of metamaterials, that mimic the atoms in an element. The electric and magnetic responses of these building blocks are engineered in such a way that the effective permittivity and permeability of metamaterials can be tuned. Similarly, the anisotropy of constitutive electromagnetic parameters can also be controlled efficiently. This dissertation focuses on the enhancement of the interaction between light and matter using bulk anisotropic metamaterials and magnetic meta atoms, in order to enhance, control and/or isolate electric and magnetic nature of emitters. This is achieved first through utilization of hyperbolic metamaterials (HMs) which are a subcategory of uniaxially anistotropic materials exhibiting opposite signs of permittivity or permeability along and orthogonal the axis of anisotropy. HMs host a wide spatial spectrum of propagating waves, i.e., high gradient field features can be transferred in HMs owing to the propagating waves in ideally indefinitely large spatial spectrum. Optical HMs are mainly constructed by periodic alternating layers of plasmonic metals and dielectrics. The power emitted by point sources (and arrays of point sources) in the vicinity of HMs is highly boosted compared to regular dielectric media. Moreover, most of the power is absorbed by the HM. Similar characteristics can be realized using graphene layers instead of metals in the infrared regime, where the chemical potential can be an effective means of controlling emission enhancement. Thereafter, a reactively loaded transmission line grid is presented as an example of a two-dimensional HMs, where the canalization of large spectral waves leads to transferring high resolution features. HMs can also be molded into resonators that provide high-quality resonances even in subwavelength dimensions. These extraordinary resonances are demonstrated to boost radiative emission of dipolar emissions. Another exotic property of anisotropic metamaterials is investigated with near-zero permittivity conditions where huge electric field enhancements are achieved in larger intensities than those demonstrated using isotropic near-zero permittivity materials. Lastly, a circular cluster of plasmonic nanospheres under azimuthally polarized vector beams are studied as a way of boosting local magnetic field and isolating it from electric field, which is promising for studying weak magnetic transitions in high frequency range.

Characterizing the many-body localization (MBL) transition in strongly disordered and interacting quantum systems is an important issue in the field of condensed matter physics. We study the single particle Green's functions for a disordered interacting system in one dimension using exact diagnonalization in the infinite temperature limit and provide strong evidence that single particle excitations carry signatures of delocalization to MBL transition. In the delocalized phase, the typical values of the local density of states and the scattering rate are finite while in the MBL phase, the typical values for both the quantities become vanishingly small. The probability distribution functions of the local density of states and the scattering rate are broad log-normal distributions in the delocalized phase while the distributions become very narrow and sharply peaked close to zero in the MBL phase. We also study the eigenstate Green's function for all the many-body eigenstates and demonstrate that both, the energy resolved typical scattering rate and the typical local density of states, can track the many-body mobility edges., Comment: A few changes to the text to improve clarity. No change in results. Close to the published version

We derive a general expression for the electric and magnetic part of the near-field energy density of $N$ dipoles of temperatures ${T}_{1},...,{T}_{N}$ immersed in a background field having a different temperature ${T}_{b}$. In contrast to former expressions this inclusion of the background field allows for determining the energy density of heated or cooled isotropic dipolar objects within an arbitrary environment which is thermalized at a different temperature. Furthermore, we show how the energy density is related to the local density of states. We use this general expression to study the near-field enhanced energy density at the edges and corners of one-dimensional (1D) Su-Schrieffer-Heeger chains and 2D Su-Schrieffer-Heeger lattices of plasmonic InSb nanoparticles when the phase transition from a topological trivial to a topological nontrivial state is made. We discuss the robustness of these modes when adding defects and the possibility to measure the topological edge and corner modes.

Three dimensional photonic crystals offer the possibility to completely inhibit the local density of states (LDOS) for emitters inside the crystal [1] allowing the control of the LDOS which is crucial for emission control, quantum information science, photovoltaics and many more. However, total inhibition is completely true only for crystals of infinite size, whereas real devices are finite. Therefore, it is important to investigate the role of thr position and the dipole orientation of sources inside finite three-dimensional photonic crystals.

The concept of the local density of optical states (widely referred to as "LDOS") is ubiquitous in nanophotonics as it serves to explain a wide range of electromagnetic phenomena, varying from antenna physics [1] , fluorescence [2] - [4] and emission control [5] , [6] , inter-molecular energy transfer [7] , [8] , and strong coupling [9] . Recently, we explored the fundamentals of this concept and its consequences in a broad didactic overview [10] . Here, we focus in greater detail on one of the wide-ranging implications from our study, namely our newly gained insights of how one can view the local density of states in several different ways. Each of these different approaches offers an alternative conceptual insight into the local density of optical states. Importantly, by making a detailed comparison between the different approaches we show the equivalence of rather different research fields, namely electrical engineering, quantum optics, and nanophotonics viewpoints.

The direct measurement of a single emitter decay rate and the simultaneous knowledge of their position is a powerful tool for the study of light-matter interaction at the nanometer scale. In particular, the decay rate is directly related to the local density of states (LDOS) which measures the number of modes of the electromagnetic environment available for the decay of an emitter.

In this study, we use first-principles calculations to investigate the electronic and structural properties of $\text{Mo}{X}_{2}$ $(X=\text{S}, \mathrm{Se}, \mathrm{Te})$ monolayers doped with substitutional Sb atoms, with a central focus on the Sb(Mo) substitution. In ${\mathrm{MoS}}_{2}$, we observe that this substitution is energetically favored under S-rich conditions, where the ${\mathrm{S}}_{2}$ gaseous phase is likely to be present. This result is compatible with a recent experimental observation in Sb-doped ${\mathrm{MoS}}_{2}$ nanosheets grown by chemical vapor deposition. A similar behavior is found in ${\mathrm{MoSe}}_{2}$, but in ${\mathrm{MoTe}}_{2}$ the Sb(Mo) substitution is less likely to occur due to the possible absence of gaseous Te phases in experimental setups. In all cases, several impurity-induced states are found inside the band gap, with energies that span the entire gap. The Fermi energy is pinned a few tenths of eV above the top of the valence band, suggesting a predominant $p$-type behavior, and gap energies are slightly increased in comparison to the pristine systems. The orbital nature of these states is further investigated with projected and local density of states calculations, which reveal similarities to defect states induced by single Mo vacancies as well as their rehybridization with the $5s$ orbital from Sb. Additionally, we find that the band gap of the doped systems is increased in comparison with the pristine materials, in contrast with a previous calculation in Sb-doped ${\mathrm{MoS}}_{2}$ that predicts a gap reduction with a different assignment of valence band and impurity levels. We discuss the similarities, discrepancies, and the limitations of both calculations. We also speculate possible reasons for the experimentally observed redshifts of the $A$ and $B$ excitons in the presence of the Sb dopants in ${\mathrm{MoS}}_{2}$. We hope that these results spark future investigations on other aspects of the problem, particularly those concerning the effects of disorder and electron-hole interaction, and continue to reveal the potential of doped transition-metal dichalcogenides for applications in optoelectronic devices.

The properties of semiconductors can be crucially impacted by midgap states induced by dopants, which can be native or intentionally incorporated in the crystal lattice. For Bernal-stacked bilayer graphene (BLG), which has a tunable bandgap, the existence of midgap states induced by dopants has been conjectured, but never confirmed experimentally. Here, we report scanning tunneling microscopy and spectroscopy results, supported by tight-binding calculations, that demonstrate the existence of midgap states in BLG. We show that the midgap state in BLG -- for which we demonstrate gate-tunability -- appears when the dopant is hosted on the non-dimer sublattice sites. We further evidence the presence of narrow resonances at the onset of the high energy bands (valence or conduction, depending on the dopant type) when the dopants lie on the dimer sublattice sites. These results suggest that dopants/defects can play an important role in the transport and optical properties of multilayer graphene samples, especially at energies close to the band extrema., Includes supplementary material

We theoretically study the structural and electronic properties of a molecular conductor, α-(BEDT-TTF)2I3, using first-principles density-functional theory calculations, especially in its low-temperature charge-ordered state at ambient pressure. We apply a hybrid functional approach and compare the results with a conventional exchange-correlation functional within the generalized gradient approximation. By performing structural optimization, we found a stable charge-ordered solution for the former, in contrast to the latter approach where the magnitude of the charge imbalance becomes considerably small compared to that when the experimental structure is adopted. The electronic band structure near the Fermi level, with and without structural optimization, as well as the molecule-dependent local density of states of the charge-ordered state are discussed.

International audience; Rock-salt lead selenide nanocrystals can be used as building blocks for large scale square superlattices via two-dimensional assembly of nanocrystals at a liquid-air interface followed by oriented attachment. Here we report Scanning Tunneling Spectroscopy (STS) measurements of the local density of states of an atomically coherent superlattice with square geometry made from PbSe nanocrystals. Controlled annealing of the sample permits the imaging of a clean structure and to reproducibly probe the band gap and the valence hole and conduction electron states. The measured band gap and peak positions are compared to the results of optical spectroscopy and atomistic tight-binding calculations of the square superlattice band Electronic properties of atomically coherent square PbSe nanocrystal superlattice resolved by Scanning Tunne structure. In spite of the crystalline connections between nanocrystals that induce significant electronic couplings, the electronic structure of the superlattices remains very strongly influenced by the effects of disorder and variability.

The Hamiltonian size reduction method or the mode space method applicable to general heterogeneous structures is developed in this work. The effectiveness and accuracy of the method are demonstrated for four example devices, such as GaSb/InAs tunnel field effect transistors (FETs), MoTe 2/SnS 2 bilayer vertical FETs, InAs nanowire FETs with a defect, and Si nanowire FETs with rough surfaces. The Hamiltonian size is reduced to around 5% of the original full-Hamiltonian size without losing the accuracy of the calculated transmission and local density of states in a practical sense. The method developed in this work can be used with any type of Hamiltonian and can be applied virtually to any hetero-structure, and so it has the potential to become an enabling technology for efficient simulations of hetero-structures.

We use scanning gate microscopy to study electron transport through an open, gate-defined resonator in a Ga(Al)As heterostructure. Raster-scanning the voltage-biased metallic tip above the resonator, we observe distinct conductance modulations as a function of the tip position and voltage. Quantum-mechanical simulations reproduce these conductance modulations and reveal their relation to the partial local density of states in the resonator. Our measurements illustrate the current frontier between possibilities and limitations in imaging the local density of states in buried electron systems using scanning gate microscopy., Physical Review Research, 3 (3), ISSN:2643-1564

Simulations based on solving the Kohn-Sham (KS) equation of density functional theory (DFT) have become a vital component of modern materials and chemical sciences research and development portfolios. Despite its versatility, routine DFT calculations are usually limited to a few hundred atoms due to the computational bottleneck posed by the KS equation. Here we introduce a machine-learning-based scheme to efficiently assimilate the function of the KS equation, and by-pass it to directly, rapidly, and accurately predict the electronic structure of a material or a molecule, given just its atomic configuration. A new rotationally invariant representation is utilized to map the atomic environment around a grid-point to the electron density and local density of states at that grid-point. This mapping is learned using a neural network trained on previously generated reference DFT results at millions of grid-points. The proposed paradigm allows for the high-fidelity emulation of KS DFT, but orders of magnitude faster than the direct solution. Moreover, the machine learning prediction scheme is strictly linear-scaling with system size.