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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

This work presents an investigation of carrier transport in $\mathrm{Ga}\mathrm{N}$-based light-emitting diodes in the subthreshold forward-bias regime where tunneling processes are relevant. A quantum kinetic theory of trap-assisted tunneling is developed within the framework of the nonequilibrium Green's function formalism. Based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure, the model provides access to spectral quantities, such as the local density of states and the current density, which are essential to understand the nature of the tunneling process. The quantum nonradiative recombination rates can be reproduced by the conventional Shockley-Read-Hall theory, provided that the classical charge is replaced with the correct quantum charge, which means that trap-assisted tunneling can be described with drift-diffusion solvers complemented with appropriate quantum corrections for the calculation of the local density of states. The subthreshold $I$-$V$ characteristics and ideality factors predicted by the quantum kinetic model are in agreement with measurements.

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.

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.

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

The spatial dependence of the local density of states at the Fermi level of a stepped Au(111) surface is studied by thermovoltage scanning tunneling microscopy. The periodicity of the standing waves is not given by ${\ensuremath{\lambda}}_{\text{F}}/2=1.8$ nm, as expected based on the band diagram of Au(111), but rather varies between 1.5 and 2.1 nm depending on the exact width of the terraces. This counterintuitive result can be understood by considering the superposition of incident and reflected electron waves, which have a periodicity of ${\ensuremath{\lambda}}_{\text{F}}/2$. The change in periodicity is a direct consequence of the decaying nature of the Friedel oscillations.

In a Type II superconductor, the vortex core behaves like a normal metal. Consequently, the single-particle density of states in the vortex core of a conventional Type II superconductor remains either flat or (for very clean single crystals) exhibits a peak at zero bias due to the formation of Caroli-de Gennes-Matricon bound state inside the core. Here we report an unusual observation from scanning tunneling spectroscopy measurements in a weakly pinned thin film of the conventional s-wave superconductor a-MoGe, namely, that a soft gap in the local density of states continues to exist even at the center of the vortex core. We ascribe this observation to rapid fluctuation of vortices about their mean position that blurs the boundary between the gapless normal core and the gapped superconducting region outside. Analyzing the data as a function of magnetic field we show that the variation of fluctuation amplitude as a function of magnetic field is consistent with quantum zero-point motion of vortices., arXiv admin note: substantial text overlap with arXiv:1905.01045

In this work we investigate the electromagnetic local density of states (LDOS) near a twisted bilayer graphene (TBG) deposited over a general isotropic substrate. The band structure of the TBG is calculated within a tight-binding framework, and then used to determine the TBG's conductivity. The latter presents a nontrivial dependence upon the angle of twist, which shows up in the LDOS, allowing for a moir\'e pattern-dependent quantum emission. For some specific twist angles we show that it is possible to either enhance or decrease the LDOS by an order of magnitude at selected frequencies when compared to the monolayer. This impressive variation is explained in terms of the presence/absence of well defined surface plasmon polaritons (SPPs). Altogether our findings demonstrate that TBG is an alternative, versatile material platform for controlling spontaneous emission and enhancing light-matter interactions, and pave the way for further studies and applications of quantum emission in the emerging class of two-dimensional moir\'e materials.

We test the valley-filtering capabilities of a quantum dot inscribed by locally straining an $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathcal{T}}_{3}$ lattice. Specifically, we consider an out-of-plane Gaussian bump in the center of a four-terminal configuration and calculate the generated pseudomagnetic field having an opposite direction for electrons originating from different valleys, the resulting valley-polarized currents, and the conductance between the injector and collector situated opposite one another. Depending on the quantum dot's width and width-to-height ratio, we detect different transport regimes with and without valley filtering for both the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathcal{T}}_{3}$ and dice lattice structures. In addition, we analyze the essence of the conductance resonances with a high valley polarization in terms of related (pseudo-) Landau levels, the spatial distribution of the local density of states, and the local current densities. The observed local charge and current density patterns reflect the local inversion symmetry breaking by the strain, besides the global inversion symmetry breaking due to the scaling parameter $\ensuremath{\alpha}$. By this way we can also filter out different sublattices.

There is immense interest in how the local environment influences the electronic structure of materials at the single-layer limit. We characterize moir\'e induced spatial variations in the electronic structure of in situ grown monolayer ${\mathrm{V}}_{2}{\mathrm{S}}_{3}$ on Au(111) by means of low-temperature scanning tunneling microscopy and spectroscopy. We observe a long-range modulation of the integrated local density of states (LDOS), and quantify this modulation with respect to the moir\'e superstructure for multiple orientations of the monolayer with respect to the substrate. Scanning tunneling spectroscopy reveals a prominent peak in the LDOS, which is shifted in energy at different points of the moir\'e superstructure. Comparing ab initio calculations with angle-resolved photoemission, we are able to attribute this peak to bands that exhibit a large out of plane $d$-orbital character. This suggests that the moir\'e driven variations in the measured density of states are driven by a periodic modulation of the monolayer-substrate hybridization.

Spontaneous orbital magnetism observed in twisted bilayer graphene (tBG) on nearly aligned hexagonal boron nitride (BN) substrate builds on top of the electronic structure resulting from combined G/G and G/BN double moire interfaces. Here we show that tBG/BN commensurate double moire patterns can be classified into two types, each favoring the narrowing of either the conduction or valence bands on average, and obtain the evolution of the bands as a function of the interlayer sliding vectors and electric fields. Finite valley Chern numbers $\pm 1$ are found in a wide range of parameter space when the moire bands are isolated through gaps, while the local density of states associated to the flat bands are weakly affected by the BN substrate invariably concentrating around the AA-stacked regions of tBG. We illustrate the impact of the BN substrate for a particularly pronounced electron-hole asymmetric band structure by calculating the optical conductivities of twisted bilayer graphene near the magic angle as a function of carrier density. The band structures corresponding to other $N$-multiple commensurate moire period ratios indicate it is possible to achieve narrow width $W \lesssim 30$ meV isolated folded band bundles for tBG angles $\theta \lesssim 1^{\circ}$., Comment: 17 pages, 9 figures

We report a microscopic electronic mechanism for nanoscale skyrmion formation and topological metalicity originating from Rashba and double-exchange physics. The results are based on hybrid simulations in a model that explicitly retains itinerant electronic degrees of freedom. A simple physical picture is provided via an effective short-range spin model. We identify hexagonal and square lattice arrangements of skyrmions in two different regimes of the parameter space. Sparse skyrmions emerge at finite temperatures as excitations of the ferromagnetic phase. The skyrmion states are characterized as topological metals via explicit calculations of the Bott index and the Hall conductivity. Oscillations in local density of states are shown to arise from a combination of confinement effects and emergent gauge-fields. We also emphasize the importance of a consistent treatment of spin-orbit coupling for calculating electronic properties of metals hosting unconventional magnetic textures such as skyrmions.

We describe the formation of charge- and spin-density patterns induced by spin-selective photoexcitations of interacting fermionic systems in the presence of a microstructure. As an example, we consider a one-dimensional Hubbard-like system with a periodic magnetic microstructure, which has a uniform charge distribution in its ground state, and in which a long-lived charge-density pattern is induced by the spin-selective photoexcitation. Using tensor-network methods, we study the full quantum dynamics in the presence of electron-electron interactions and identify doublons as the main decay channel for the induced charge pattern. Our setup is compared to the optically induced spin transfer (OISTR) mechanism, in which ultrafast optically induced spin transfer in Heusler and magnetic compounds is associated to the difference of the local density of states of the different elements in the alloys. We find that applying a spin-selective excitation there induces spatially periodic patterns in local observables. Implications for pump-probe experiments on correlated materials and experiments with ultracold gases on optical lattices are discussed.

We calculate and resolve with unprecedented detail the local density of states (DOS) and momentum-dependent spectral functions at zero temperature of one of the key models for strongly correlated electron materials, the degenerate two-orbital Kanamori-Hubbard model, by means of a highly optimized Dynamical Mean Field Theory which uses the Density Matrix Renormalization Group as the impurity solver. When the system is hole doped, and in the presence of a finite interorbital Coulomb interaction we find the emergence of a novel holon-doublon in-gap subband which is split by the Hund's coupling. We also observe new interesting features in the DOS like the splitting of the lower Hubbard band into a coherent narrowly dispersing peak around the Fermi energy, and another subband which evolves with the chemical potential. We characterize the main transitions giving rise to each subband by calculating the response functions of specific projected operators and comparing with the energies in the atomic limit, obtaining excellent agreement. The detailed results for the spectral functions found in this work pave the way to study with great precision the microscopic quantum behavior in correlated materials., Replaced with updated results

Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate DBCO. We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film. It is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CuO2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one corresponding to the superconductivity and the other one to the pseudogap. While the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.

In this work, we investigate models for bulk, bi- and multilayers containing half-metallic ferromagnets (HMFs), at zero and at finite temperature, in order to elucidate the effects of strong electronic correlations on the spectral properties (density of states). Our focus is on the evolution of the finite-temperature many-body induced tails in the half-metallic gap. To this end, the dynamical mean-field theory (DMFT) is employed. For the bulk, a Bethe lattice model is solved using a matrix product states based impurity solver at zero temperature and a continuous-time quantum Monte Carlo (CT-QMC) solver at finite temperature. We demonstrate numerically, in agreement with the analytical result, that the tails vanish at the Fermi level at zero temperature. In order to study multilayers, taken to be square lattices within the layers, we use the real-space DMFT extension with the CT-QMC impurity solver. For bilayers formed by the HMF with a band or correlated insulator, we find that charge fluctuations between the layers enhance the finite temperature tails. In addition, in the presence of inter-layer hopping, a coherent quasiparticle peak forms in the otherwise correlated insulator. In the multilayer heterostructure setup, we find that by suitably choosing the model parameters, the tails at the HMF/Mott insulator interface can be reduced significantly, and that a high spin polarization is conceivable, even in the presence of long-ranged electrostatic interactions., Comment: to be published in Physical Review Research

Hydrogenation of nitrogen (N) doped GaAs allows for reversible tuning of the band gap and the creation of site controlled quantum dots through the manipulation of N−nH complexes, N−nH complexes, wherein a nitrogen atom is surrounded by n hydrogen (H) atoms. Here we employ cross-sectional scanning tunneling microscopy (X-STM) to study these complexes in the GaAs (110) surface at the atomic scale. In addition to that we performed density functional theory (DFT) calculations to determine the atomic properties of the N−nH complexes. We argue that at or near the (110) GaAs surface two H atoms from N−nH complexes dissociate as an H2 molecule. We observe multiple features related to the hydrogenation process, of which a subset is classified as N-1H complexes. These N-1H related features show an apparent reduction of the local density of states (LDOS), characteristic to N atoms in the GaAs (110) surface with an additional apparent localized enhancement of the LDOS located in one of three crystal directions. N−nH features can be manipulated with the STM tip. Showing in one case a switching behavior between two mirror-symmetric states and in another case a removal of the localized enhancement of the LDOS. The disappearance of the bright contrast is most likely a signature of the removal of an H atom from the N−nH complex.

Nonconservative current-induced forces (NCIFs) in nanowires have attracted attention both as a failure mechanism and as a driver for atomic-scale motors. Here we investigate the view that the nonconservative character of the electron wind force is an intrinsic property of the electron flow. To this end we consider the curl of the force on a noninvasive test scatterer immersed in the electron current. Using a free-electron Hamiltonian, we find nonzero curls at the opening of conductance channels due to variations in the local density of states. Current vortices are observed in these simulations, and one would naively expect them to give rise to large curls. However, this is not always the case. We find two types, low- and high-curl current vortices. Bouncy patterns, the ‘snaking’ of the current flow through the wire, are a prominent effect that gives rise to large curls. Ultimately NCIFs reduce the stability of nanowires and therefore it is crucial to study their origin to manage their effects. This approach enhances our understanding of NCIFs by going beyond forces on individual nuclei.

Based on reliable ab initio computations and the numerical renormalization group method, systematic studies on a two-dimensional GaSe monolayer with Co adatoms have been carried out. It is shown that the stable lowest-energy configuration of the system involves the Co adatom located over a Ga atom. For such a configuration, it is demonstrated that the electronic and magnetic properties of the system can be effectively controlled by means of external factors, such as magnetic field, gate voltage, or temperature. Moreover, if properly tuned, the GaSe-Co system can also exhibit the Kondo effect. The development of the Kondo phenomenon is revealed in the local density of states of the Co adatom, its magnetic field dependence, which presents the splitting of the Kondo peak, as well as in the temperature dependence of the conductance, which exhibits scaling typical of the spin-$\frac{1}{2}$ Kondo effect.

Andreev bound states (ABSs), as a close resemblance and a prototype of Majorana bound states, attracted particular interest in semiconductor-superconductor hybrids. So far, ABSs were explored as an equilibrium behavior employing tunneling spectroscopy. Under nonequilibrium, theorists did predict that the local density of states (DOS) of the ABSs split and broaden. We investigated an InAsSb quantum dot coupled to two superconductors to explore the nonequilibrium dynamics of ABSs and their interplay with Kondo effect. Through tunneling spectroscopy, we demonstrated a full picture of the nonequilibrium dynamic splitting and broadening of ABSs, which interplays with the nonequilibrium Kondo DOS. Since the observed features might mimic those in the normal superconductor-topological superconductor or Majorana junctions, our discovery could provide a relevant comparison to benchmark the Majorana signal in future experiments.

We study, both experimentally and theoretically, the modification of ${\mathrm{Er}}^{3+}$ photoluminescence properties in $\mathrm{Si}$ dielectric nanoslots. The ultrathin nanoslot (down to 5-nm thickness), filled with $\mathrm{Er}$ in ${\mathrm{Si}\mathrm{O}}_{2}$, boosts the electric and magnetic local density of states via coherent near-field interaction. We report an experimental 20-fold enhancement of the radiative decay rate with negligible losses. Moreover, via modifying the geometry of the all-dielectric nanoslot, the outcoupling of the emitted radiation to the far field can be strongly improved, without affecting the strong decay-rate enhancement given by the nanoslot structure. Indeed, for a periodic square array of slotted nanopillars an almost one-order-of-magnitude-higher ${\mathrm{Er}}^{3+}$ PL intensity is measured with respect to the unpatterned structures. This has a direct impact on the design of more efficient CMOS-compatible light sources operating at telecom wavelengths.

We theoretically investigate the application of topological plasmon polaritons (TPPs) to temperature sensing for the first time. Based on an analogy of the topological edge states in the Su-Schrieffer-Heeger model, TPPs are realized in a one-dimensional intrinsic indium antimonide (InSb) microsphere chain. The existence of TPPs is demonstrated by analyzing the topology of the photonic band structures and the eigenmode distribution. By exploiting the temperature dependence of the permittivity of InSb in the terahertz range, the resonance frequency of the TPPs can be largely tuned by the temperature. Moreover, it is shown that the temperature sensitivity of the TPP resonance frequency can be as high as $0.0264~\mathrm{THz/K}$ at room temperature, leading to a figure of merit over 150. By calculating the LDOS near the chain, we further demonstrate that the temperature sensitivity of TPPs is experimentally detectable via near-field probing techniques. This sensitivity is robust since TPPs are highly protected modes immune to disorder and can achieve a strong confinement of radiation. We envisage these TPPs can be utilized as promising candidates for robust and enhanced temperature sensing., 6 pages, 4 figures. comments are welcome

We report on the formation and comprehensive study of atomic and electronic structures of a new Tl-Mg two-dimensional compound reconstruction with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ periodicity on a Si(111) substrate. Owing to properties of its constituent elements and crystal structure with a breathing kagome lattice of Mg and a dense Tl layer (4/3 monolayer), this double-layer system displays promising features, including spin-split metallic bands, nearly flat bands, and a saddle point with corresponding divergences in the local density of states. These features indicate the possibility that strongly correlated electronic states exist in this structure after tuning the Fermi level position. In accordance with the ${C}_{3}$ symmetry, we observe spin-orbit-coupling-driven Rashba-type spin splitting with spin degeneracy at the $\overline{M}$ point and Zeeman-like splitting at the $\overline{K}$ point. In contrast to the previously reported cases, splitting at the $\overline{K}$ point is significant to produce fully out-of-plane spin-polarized cones. Due to opposite Berry curvatures at the $\overline{K}$ and ${\overline{K}}^{\ensuremath{'}}$ points of the hexagonal surface Brillouin zone, spins at these valleys have opposite signs, which ensure effective spin-pseudospin coupling.

Previous works on deformed graphene predict the existence of valley-polarized states, however, optimal conditions for their detection remain challenging. We show that in the quantum Hall regime, edgelike states in strained regions can be isolated in energy within Landau gaps. We identify precise conditions for conducting edgelike states to be valley polarized. By the appropriate design of strain profiles these states can be positioned at chosen locations in the sample. A map of the local density of states as a function of energy and position reveals a unique braid pattern that serves as a fingerprint to identify valley polarization.

As the surface Andreev bound state (ABS) forms at the open ($1,1$) edge of a $d_{x^2-y^2}$-wave superconductor, the local density of states (LDOS) increases. Therefore, a strong electron correlation and drastic phenomena may occur. However, a theoretical study on the effects of the ABS on the electron correlation has not been performed yet. To understand these effects, we study large cluster Hubbard model with an open ($1,1$) edge in the presence of a bulk $d$-wave gap. We calculate the site-dependent spin susceptibility by performing random-phase-approximation (RPA) and modified fluctuation-exchange (FLEX) approximation in the real space. We find that near the ($1,1$) edge, drastic ferromagnetic (FM) fluctuations occur owing to the ABS. In addition, as the temperature decreases, the system rapidly approaches a magnetic-order phase slightly below the transition temperature of the bulk $d$-wave superconductivity (SC). In this case, the FM fluctuations are expected to induce interesting phenomena such as edge-induced triplet SC and quantum critical phenomena., 9 pages, 15 figures

In this paper, we investigate the relation between odd-frequency pairing and proximity effect in non-uniform Kitaev chain systems with a particular interest in the topological critical point. First, we correlate the odd-frequency pairing and Majorana fermion in a semi-infinite Kitaev chain, where we find that the spatial dependence of the odd-frequency pair amplitude coincides with that of the local density of states at low frequencies. Second, we demonstrate that, contrary to the standard view, the odd-frequency pair amplitude spreads into the bulk of a semi-infinite Kitaev chain at the topological critical point. Finally, we show that odd-frequency Cooper pairs cause the proximity effect in a normal metal/diffusive normal metal/ Kitaev chain junction even at the topological critical point. Our results hold relevance to the investigation of odd-frequency pairing and topological superconductivity in more complicated systems that involve Rashba nanowire with magnetic fields., 15 pages, 10 figures

Subband occupancy (i.e. the number of occupied subbands or energy levels in the semiconductor) is a key physical parameter characterizing the topological properties of superconductor-semiconductor hybrid systems in the context of the search for non-Abelian Majorana zero modes. We theoretically study the subband occupation of semiconductor nanowire devices as function of the applied gate potential, the semiconductor-superconductor (SM-SC) work function difference, and the surface charge density by solving self-consistently the Schr\"{o}dinger-Poisson equations for the conduction electrons of the semiconductor nanowire. Realistic surface charge densities, which are responsible for band bending, are shown to significantly increase the number of occupied subbands, making it difficult or impossible to reach a regime where only a few subbands are occupied. We also show that the energy separation between subbands is significantly reduced in the regime of many occupied subbands, with highly detrimental consequences for the realization and observation of robust Majorana zero modes. As a consequence, the requirements for the realization of robust topological superconductivity and Majorana zero modes should include a low value of the chemical potential, consistent with the occupation of only a few subbands. Finally, we show that the local density of states on the exposed nanowire facets provides a powerful tool for identifying a regime with many occupied subbands and is capable of providing additional critical information regarding the feasibility of Majorana physics in semiconductor-superconductor devices. In our work, we address both InAs/Al and InSb/Al superconductor-nanowire hybrid systems of current experimental interest., Comment: 11 pages, 8 figures

We study the energy spectrum of Weyl fermions in a topological semimetal in the presence of a central charged impurity. When the strength of the charge exceeds a critical value, an infinite family of quasibound states appear that show characteristic signatures of discrete scale invariance. The invariance is broken by the presence of screening or a finite mass. If these terms are small enough the quasibound states still exist, but there can be perturbations of the shallowest quasibound states. These special quasibound states manifest themselves in the local density of states as well as the relative phase, which can be observed by scanning tunneling microscopy. Such effects that are usually associated with the relativistic quantum electrodynamics of strong fields become observable in topological materials.

We study two charge-ordered states in a $t\ensuremath{-}J\ensuremath{-}U$ model with the method of renormalized mean-field theory. Compared with the $t\ensuremath{-}J$ model, which excludes the existence of one site occupied by two electrons, finite Coulomb interaction $U$ will allow the appearance of doublons and produce more charge fluctuations. The charge density wave states accompanied by Cooper pair density modulations have similar features with the states found in the $t\ensuremath{-}J$ model. The difference is that the modulation of doublons exists, and it has the same wavelength and phase as holes. Charge-ordered states disappear at the $U/t\ensuremath{\le}9$ region when the system does not correspond to the strongly correlated resonating valence-bond state. This feature indicates that local Coulomb interactions are the origin of charge-ordered states observed in cuprates. In addition, we calculate the strength of superconducting coherence and the gap depth of the nodal pair-density-wave state based on the local density of states. These two quantities also vary with lattice sites periodically. The oscillations have same wave vector and phase as the modulation of holes. These results are consistent with recent experiments.

We consider the Kondo effect arising from a hydrogen impurity in graphene. As a first approximation, the strong covalent bond to a carbon atom removes that carbon atom without breaking the $C_{3}$ rotation symmetry, and we only retain the Hubbard interaction on the three nearest neighbors of the removed carbon atom which then behave as magnetic impurities. These three impurity spins are coupled to three conduction channels with definite helicity, two of which support a diverging local density of states (LDOS) $\propto 1/ [ | \omega \ | \ln ^{2}( \Lambda /| \omega \ | \ ) \ ] $ near the Dirac point $\omega \rightarrow 0$ even though the bulk density of states vanishes linearly. We study the resulting 3-impurity multi-channel Kondo model using the numerical renormalization group method. For weak potential scattering, the ground state of the Kondo model is a particle-hole symmetric spin-$1/2$ doublet, with ferromagnetic coupling between the three impurity spins; for moderate potential scattering, the ground state becomes a particle-hole asymmetric spin singlet, with antiferromagnetic coupling between the three impurity spins. This behavior is inherited by the Anderson model containing the hydrogen impurity and all four carbon atoms in its vicinity., Comment: 22+ pages, 16 figures; additional information on ferromagnetic p-h asymmetric strong-coupling fixed point

Free electrons have a uniform kinetic energy density (KED), which evolves into a spatially varying quantity as the electrons respond to the gradual imposition of an external potential. In this paper and a companion paper, we examine two sets of functionals for describing the local, non-negative KED that emerges after such a perturbation. In this paper, we emphasize potential functionals, deriving the first- and second-order deviations from the free-electron KED as functionals of the perturbing potential, also reconsidering the analogous functionals for the local density of states and the electron density. (In the second paper, we use these results to re-express the KED response in terms of functionals of the induced electron density.) We develop reciprocal-space formulations of the response kernels to complement previously known real-space forms. The first-order function is straightforward to obtain, but the second-order function requires considerable effort. To manage the derivations, we relate the KED response to that of the one-electron Green function, and then examine the latter in detail. Finally, we provide extensive validation of the derived response functions based on asymptotic analysis of an integral representation, numerical integration of the same generating integral, and application to the linear potential model.

Weyl semimetals with time-reversal symmetry breaking are expected to show various fascinating physical behaviors, such as intrinsic giant anomalous Hall effect, chiral anomaly effect in the bulks, and Fermi arcs on the surfaces. Here we report a scanning tunneling microscopy study on the magnetic Weyl semimetal candidate ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$. According to the morphology and local density of states of the surface, we provide assignments to different surface terminations. The measured local density of states reveals a semimetal gap of $\ensuremath{\sim}300$ mV, which is further verified as the gap in spin-minority bands using spin-resolved tunneling spectra. Additionally, signature for the nontrivial surface states around 50 mV is proposed. This is further confirmed by the observations of standing waves around a step edge of the sample. Our observations and their comparison with band-structure calculations provide direct yet timely evidence for the bulk and surface band structures of the magnetic Weyl semimetal candidate ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$.

We investigate atomic collapse in pseudospin-1 Dirac material systems whose energy band structure constitutes a pair of Dirac cones and a flat band through the conic intersecting point. We obtain analytic solutions of the Dirac-Weyl equation for the three-component spinor in the presence of a Coulomb impurity and derive a general criterion for the occurrence of atomic collapse in terms of the normalized strength of Coulomb interaction and the angular momentum quantum number. In particular, for the lowest angular momentum state, the solution coincides with that for pseudospin-1/2 systems, but with a reduction in the density of resonance peaks. For higher angular momentum states, the underlying pseudospin-1 wave functions exhibit a singularity at the point of zero kinetic energy. Divergence of the local density of states associated with the flat band leads to an inverse square type of singularity in the conductivity. These results provide insights into the physics of the two-body problem for relativistic quantum pseudospin-1 quasiparticle systems.

Kondo physics in doped monolayer graphene is predicted to exhibit unusual features due to the linear vanishing of the pristine material's density of states at the Dirac point. Despite several attempts, conclusive experimental observation of the phenomenon remains elusive. One likely obstacle to identification is a very small Kondo temperature scale $T_K$ in situations where the chemical potential lies near the Dirac point. We propose tailored mechanical deformations of monolayer graphene as a means of revealing unique fingerprints of the Kondo effect. Inhomogeneous strains are known to produce specific alternating changes in the local density of states (LDOS) away from the Dirac point that signal sublattice symmetry breaking effects. Small LDOS changes can be amplified in an exponential increase or decrease of $T_K$ for magnetic impurities attached at different locations. We illustrate this behavior in two deformation geometries: a circular 'bubble' and a long fold, both described by Gaussian displacement profiles. We calculate the LDOS changes for modest strains and analyze the relevant Anderson impurity model describing a magnetic atom adsorbed in either a 'top-site' or a 'hollow-site' configuration. Numerical renormalization-group solutions of the impurity model suggest that higher expected $T_K$ values, combined with distinctive spatial patterns under variation of the point of graphene attachment, make the top-site configuration the more promising for experimental observation of signatures of the Kondo effect. The strong strain sensitivity of $T_K$ may lift top-site Kondo physics into the range experimentally accessible using local probes such as scanning tunneling microscopy., 19 pages, 7 figures (added Figs. 6 and 7 to version 1)

The bound states that can occur in a superfluid vortex have recently called for attention owing to the capability of detecting them experimentally. However, a detailed theoretical account for the presence of these vortex bound states is still lacking, for all temperatures in the superfluid phase and couplings along the BCS-BEC crossover. Here, we fill this gap and present a systematic theoretical study based on the Bogoliubov-de~Gennes equations for the bound states that occur over the two characteristic (inner and outer) spatial ranges in which the extension of a superfluid vortex can be partitioned. It is found that the total number of bound states decreases from the BCS (weak-coupling) side of the crossover toward the intermediate-coupling region where they are still present, whereas the bound states disappear upon entering the BEC (strong-coupling) side. A scaling relation is also obtained that connects the number of bound states in the inner spatial range of the vortex to the depth and width of the vortex itself. A criterion is finally provided in terms of the local density of states, to distinguish where a given fermionic superfluid is located in the coupling-temperature phase diagram of the BCS-BEC crossover., 10 pages, 7 figures

We develop a rigorous theory for nanophotonic cooperative spontaneous emission of an ensemble of quantum emitters near matter, which also incorporates absorption. Such theory is highly requested since one should expect that in the field of nanophotonics the close-by emitters will emit cooperatively simultaneously with strong interaction with the exotic metamaterial complex dielectric function. The closed-form model developed here considers the buildup both of interemitter correlation over time through emitter transitions via the mutual field and of geometry-induced spatial correlation. The presence of matter alters the local density of states (LDOS), resulting in modified decay rate and emission intensity for the entire ensemble relative to values of spontaneous emission in free space. The superradiant total emission is separable into a quantum-mechanical time-dependent part and a classical space-dependent part, both being functions of the LDOS. Two distinct radiative emitter--far-field coupling mechanisms, which together account for the superradiant emission, are obtained---the coupling of every individual emitter with the far field and the coupling of emitter pairs to the far field. The calculations for superradiating emitters near silver and near-zero epsilon (NZE) materials were performed with the following results: the silver sphere augments the superradiant far-field intensity and rate by up to 400 and 1000%, respectively, while the NZE sphere enhances the far-field intensity and rate of the superradiance by approximately 1400 and 400%, respectively.

We show how Floquet Majorana fermions may be experimentally realized by periodic driving of a solid-state platform. The system comprises a planar Josephson junction made of a proximitized heterostructure containing a 2D electron gas (2DEG) with Rashba spin-orbit coupling and Zeeman field. We map the sub-gap Andreev bound states of the junction to an effective one-dimensional Kitaev model with long range hopping and pairing terms. Using this effective model, we study the response of the system to periodic driving of the chemical potential, as applied via microwaves through a top-gate. We use the bulk Floquet topological invariants to characterize the system in terms of the number of zero and $\pi$ Majorana modes for experimentally realistic parameters. We present signatures of these modes on differential conductance and local density of states. A notable feature is subharmonic response to the drive when both Majorana zero and $\pi$ modes are present, thus opening up the possibility of using this feature to clearly identify Majorana modes. We highlight the robustness of these features to disorder, and show that despite hybridization due to long localization lengths, the main features of the subharmonic response are still identifiable in an experimental system that is readily accessible., Comment: 15 pages, 12 figures

We study the equilibrium properties of a ferromagnetic insulator/superconductor structure near a magnetic domain wall. We show how the domain wall size is affected by the superconductivity in such structures. Moreover, we calculate several physical quantities altered due to the magnetic domain wall, such as the spin current density and local density of states, as well as the resulting tunneling conductance into a structure with a magnetic domain wall., 12 pages, 7 figures

In this work, using the finite-U slave boson mean-field approximation to solve the single-impurity Anderson model, the authors apply two different strategies to study the Kondo cloud, by analyzing quantities that are dependent on the distance to the magnetic impurity and then finding a universal distance scale ${\ensuremath{\xi}}_{K}$ through the collapse of the results into an universal function. The first method is based on the analysis of the local density of states of the conduction electrons (denoted as ${\ensuremath{\xi}}_{K}^{L})$, while the second relies on the analysis of spin correlations $({\ensuremath{\xi}}_{K}^{\mathrm{\ensuremath{\Sigma}}})$. Our calculations show that there is exact quantitative agreement, in the way ${\ensuremath{\xi}}_{K}$ depends on $U/\mathrm{\ensuremath{\Gamma}}$, between ${\ensuremath{\xi}}_{K}^{\mathrm{\ensuremath{\Sigma}}}$ and the results obtained through the heuristic expression ${\ensuremath{\xi}}_{K}\ensuremath{\propto}{v}_{F}/{T}_{K}$, while there is very close quantitative agreement between ${\ensuremath{\xi}}_{K}^{\mathrm{\ensuremath{\Sigma}}}$ and ${\ensuremath{\xi}}_{K}^{L}$. The use of the slave boson technique to calculate the spin correlations, which eliminates finite size effects, allowed us to study very large Kondo clouds, something that is very difficult using other techniques, like the density matrix renormalization Group method, for example. In addition, the very smooth curves obtained for the spin correlations allowed us to qualitatively identify a region in the Kondo cloud, adjacent to the impurity, that had been connected to the free orbital fixed point in previous numerical renormalization group calculations [Phys. Rev. B 84, 115120 (2011)].

One of the typical features of Majorana zero mode (MZM) at the end of topological superconductor is a zero-bias peak in the tunneling spectroscopy of the normal lead-superconductor junction. In this paper we study on a model with one phonon mode coupling to the superconductor lead of the normal lead-superconductor junction, which can be viewed as an electron-lead/phonon-coupled-MZM/hole-lead structure. The phonon-coupled MZM acts as a series of channels in which electron can turn into hole by absorbing and emitting phonons. These channels present in the local density of states (LDOS) as a series of stripes, generating the corresponding peaks in the tunneling spectroscopy. In LDOS, the electron-phonon interaction narrows and redistributes the weight among stripes. In the tunneling spectroscopy, the heights of peaks present a feature of the multi-phonon process. With these investigations, our work illuminates the mechanism of phonon-assisted Andreev reflection at a Majorana zero mode., 11 pages, 8 figures

We report investigation of the geometry and electronic structure of iridium atoms adsorbed onto graphene through a combined experimental and theoretical study. Ir atoms were deposited onto a flake of graphene on a Pt(111) surface and found to form clusters even at low temperatures. The areal density of the observed clusters on the graphene flake suggests the clusters are most likely pairs of Ir atoms. Theoretical ab initio density functional (DFT) calculations indicate that these Ir dimers are oriented horizontally, near neighboring >bridge> sites of the graphene lattice, as this configuration has the strongest adsorption energy of all high-symmetry configurations for the Ir dimer. A large peak in the local density of states (LDOS) at the Dirac point energy was measured via scanning tunneling spectroscopy, and this result is reproduced by a DFT calculation of the LDOS. The peak at the Dirac point energy is found to be from the Ir s and p states. The LDOS in the monomer case was also calculated, and is found to significantly differ from the experimentally determined data, further supporting the hypothesis of low-temperature clustering., This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05-CH11231, within the sp2-Bonded Materials Program (KC2207), (DFT simulated electronic structure and STM spectroscopy). Support was also provided by National Science Foundation Grants No. DMR-1508412 (diffusivity analysis) and No. DMR-1807233 (sample preparation). Computational resources have been provided by the DOE at Lawrence Berkeley National Laboratory’s NERSC facility.

We investigate charge densities around the vortex cores of an $s$-wave and a chiral $p$-wave superconductor (SC) in two dimensions within quasiclassical theory. We consider contributions of particle-hole asymmetry through gradient expansions in first order of a quasiclassical parameter using augmented quasiclassical theory. The chiral $p$-wave SC has two inequivalent vortices: One is charged and the other is uncharged on the basis of Bogoliubov-de Gennes (BdG) equation. We explain this qualitative difference in vortex charges also by the augmented quasiclassical theory. In addition, we find that much larger charge is induced for the charged vortex of the chiral $p$-wave SC compared with the vortex of the $s$-wave SC in the clean system. We also confirm this enhancement through quantitative comparison with results based on the BdG theory. We also study effects of Born-type impurities using self energy formalism and find that the larger charge density is rather suppressed in contrast to the $s$-wave's one. Furthermore we study the local density of states as another equilibrium property of particle-hole asymmetry.

Adsorbates can drastically alter physical properties of graphene. Particularly important are adatoms and admolecules that induce resonances at the Dirac point. Such resonances limit electron mobilities and spin relaxation times. We present a systematic tight-binding as well as analytical modeling to investigate the properties of resonant states in the presence of a quantizing magnetic field. Landau levels are strongly influenced by the resonances, especially close to the Dirac point. Here the cyclotron motion of electrons around a defect leads to the formation of circulating local currents which are manifested by the appearance of side peaks around the zero-energy Landau level. Our study is based on realistic parameters for H, F, and Cu adatoms, each exhibiting distinct spectral features in the magnetic field. We also show that by observing a local density of states around an adatom in the presence of Landau levels useful microscopic model parameters can be extracted., Comment: 8 pages, 5 figures

Impurity scattering is found to lead to quasi-one-dimensional nanoscale modulation of the local density of states in iron pnictides and chalcogenides. This ``quasiparticle interference'' feature is remarkably similar across a wide variety of pnictide and chalcogenide phases, suggesting a common origin. We show that a unified understanding of the experiments can be obtained by simply invoking a fourfold symmetry breaking ${d}_{xz}\text{\ensuremath{-}}{d}_{yz}$ orbital splitting, of a magnitude already suggested by the experiments. This can explain the one-dimensional characteristics in the local density of states observed in the orthorhombic nematic, tetragonal paramagnetic, as well as the spin-density-wave and superconducting states in these materials.

The electronic structure of $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$ showing a metal-insulator transition and a sequence of different charge density wave (CDW) transformations was discussed in the frame of variable temperature angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. For the commensurate charge density wave phase (CCDW) the Mott gap was estimated to be 0.4 eV and energy gaps ${\mathrm{\ensuremath{\Delta}}}_{\text{CCDW},1},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{\text{CCDW},2},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{B3\ensuremath{-}\mathit{HHB}},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{B4\ensuremath{-}B3}$ were observed. For the nearly commensurate charge density wave phase (NCCDW), the reminiscent of higher and lower Hubbard bands and a very pronounced electronic state associated with the parabolic band at the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point in the Brillouin zone were identified. The incommensurate charge density wave phase (ICCDW) showed a high value of local density of states at the Fermi level and a very pronounced edge of the metallic surface state located in the range of 0.15--0.20 eV above the Fermi level. The obtained STS and ARPES results were consistent with our theoretical calculations performed within DFT formalism including spin-orbit coupling.

We study the superconducting pairing symmetry in twisted bilayer graphene by solving the Bogoliubov-de Gennes equation for all electrons in moir\'{e} supercells. With increasing the pairing potential, the system evolves from the mixed nontopological $d+id$ and $p+ip$ phase to the $s+p+d$ phase via the first-order phase transition. In the time-reversal symmetry breaking $d+id$ and $p+ip$ phase, vortex and antivortex lattices accompanying spontaneous supercurrent are induced by the twist. The superconducting order parameter is nonuniform in the moir\'{e} unit cell. Nevertheless, the superconducting gap in the local density of states is identical in the unit cell. The twist-induced vortices and nontopological nature of the mixed $d+id$ and $p+ip$ phase are not captured by the existing effective models. Our results suggest the importance of long-range pairing interaction for effective models., Comment: 9 pages, 9 figures

Scanning tunneling experiments on single crystals of superconducting ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ have recently provided evidence for discrete energy levels inside vortices. Although predicted long ago, such levels are seldom resolved due to extrinsic (temperature, instrumentation) and intrinsic (quasiparticle scattering) limitations. We study a microscopic multiband model with parameters appropriate for ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$. We confirm the existence of well-separated bound states and show that the chemical disorder due to random occupation of the chalcogen site does not affect significantly the vortex-core electronic structure. We further analyze the vortex bound states by projecting the local density of states on angular-momentum eigenstates. A rather complex pattern of bound states emerges from the multiband and mixed electron-hole nature of the normal-state carriers. The character of the vortex states changes from hole-like with negative angular momentum at low energy to electron-like with positive angular momentum at higher energy within the superconducting gap. We show that disorder in the arrangement of vortices most likely explains the differences found experimentally when comparing different vortices.

Using ab initio methods we successfully reproduce an unusual enhancement of magnetization experimentally observed in ferromagnetic ${\mathrm{SrRuO}}_{3}$ thin films deposited on the spin-triplet superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. We found that this enhancement is due to additional distortions of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ induced by the film. These distortions---rotation of the ${\mathrm{RuO}}_{6}$ octahedra---lead to a gradual reconstruction of electronic structure of the substrate, such that the Fermi level turns out to be exactly at the peak of the local density of states, corresponding to the Ru ions close to the interface. This results in the magnetic instability. The distortions propagate deep into the substrate and favor the formation of magnetic moments on the Ru ions in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ near the interface. Our estimates show that these distortions may penetrate into ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ substrate up to 40 nm in depth, which agrees with experimental observations.

Atomistic calculations of passivated nanostructures remain difficult due to the computational demands related to the high number of atoms to be typically considered. The empirical pseudopotential method (EPM) offers a good alternative in this sense, but finding trustable pseudopotentials for passivants in this method is still elusive. Following the idea of extracting nonspherically symmetric potentials from density functional theory (DFT) calculations, hydrogen pseudopotentials for silicon passivation are derived here and used to calculate the electronic states of low-dimensional structures within the EPM scheme. The single-particle Schr\"odinger equation is solved with the ensuing pseudopotentials for slabs with surfaces on the (111), (110), and (100) planes, as well as for passivated quantum dots and wires of different size. In all cases, the band gap is traced as a function of the sample size, showing good convergence towards the bulk value. For the slabs, the surface local density of states is also calculated and compared successfully to experiments. The derivation of the nonspherical pseudopotentials is based on an analytic formulation of the crystal potential and its connection to a series of DFT calculations, resulting in reliable, highly transferable and first-principles based passivant pseudopotentials to be used with the EPM.

We investigate in-gap states emerging when a single $3d$ transition metal impurity is embedded in topological insulators (${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$). We use a combined approach relying on first-principles calculations and an Anderson impurity model. By computing the local density of states of Cr, Mn, Fe, and Co embedded not only in surfaces of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ but also in their bulk phases, we demonstrate that in-gap states originate from the hybridization of the electronic states of the impurity with bulk bands and not with the topological surface states as is usually assumed. This finding is analyzed using a simplified Anderson impurity model. These observations are in contradiction with the prevailing models used to investigate the magnetic doping of topological insulators [R. R. Biswas et al., Phys. Rev. B 81, 233405 (2010)], which attribute the origin of the in-gap states to the hybridization with the topological surface states.