arXiv subjects

Ilya Vekhter

Publications and source records attributed to Ilya Vekhter.

At least 19 recordsLinked to original sources

arXiv2026

Microwave Kerr/Faraday Resonance in Two-dimensional Chiral Superconductors

We investigate the polar Kerr and Faraday effects in two-dimensional multiband chiral superconductors. We show that the clapping modes--the relative phase and amplitude oscillations between two chiral components of the superconducting order parameter--lie well within the quasiparticle excitation gap in multiband systems and dominate these magneto-optical responses in the microwave regime. The Kerr and Faraday rotation angles exhibit the resonant enhancement with sign reversals in the microwave regime as a function of the light frequency, reaching peak values on the order of 100 nrad--10 $μ$rad in thin films of candidate chiral superconductors. These resonances are accessible in superconducting atomic layer materials and provide a generic probe of chiral superconductivity in two-dimensional systems.

cond-mat.supr-con↗

arXiv2025

Intrinsic spin Nernst effect in spin-triplet superconductors

We theoretically investigate the intrinsic (impurity-independent) spin Nernst effect (SNE), a spin current generation perpendicular to temperature gradients, in spin-triplet superconductors. We show that, in these systems, the SNE consists of two distinct contributions: a direct quasiparticle contribution and an indirect supercurrent contribution. The quasiparticle contribution originates from the momentum space Berry curvature generated by spin-triplet Cooper pairs. The indirect contribution arises from a compensating supercurrent that cancels the bulk thermoelectric charge current. While this contribution vanishes when the condensate has no spin-polarization in momentum space, it can be comparable in magnitude to the quasiparticle contribution in nonunitary superconductors. These results demonstrate that thermoelectric spin supercurrent must be explicitly accounted for when evaluating the SNE in nonunitary superconductors.

cond-mat.supr-con↗

arXiv2025

Defect bound states in the continuum of bilayer electronic materials without symmetry protection

We analyze a class of bound defect states in the continuum electronic spectrum of bilayer materials, which emerge independent of symmetry protection or additional degrees of freedom. Taking graphene as a prototypical example, our comparative analysis of AA- and AB-stacked bilayer graphene demonstrates that these states originate from the intrinsic algebraic structure of the tight-binding Hamiltonian when trigonal warping is neglected rather than any underlying symmetry. Inclusion of trigonal warping and higher-order hoppings broaden the bound states into long-lived resonances. This discovery provides a pathway to previously unexplored approaches in defect and band-structure engineering. We conclude with a proposed protocol for observing these states in scanning tunneling microscopy experiments.

cond-mat.mes-hall↗

arXiv2025

Tuning Quantum States at Chirality-Reversed Planar Interface in Weyl Semimetals using an Interstitial Layer

The electronic band structure of Weyl semimetals possesses pairs of linear band crossings, called Weyl nodes, characterized by opposite chirality charges associated with each node. The momentum space position of the nodes can reverse across a planar interface and these host Fermi-arc-like bound states, in addition to scattering states. We show that a magnetic interstitial layer can tune these states in three distinct ways. The electrostatic potential and one of the in-plane magnetic potential components control the shape of the bound state Fermi-arcs. For moderate values of the same in-plane magnetic potential electrons are spin-filtered across the interface, while both the in-plane magnetic components and the electrostatic potential control the transmission of electrons. The ratio of in-plane to out-of-plane magnetic components can be used to turn on or turn off the magnetic potential effects, since the latter does not affect the interface states. The tunability arises from spin-momentum locking and chirality reversal at the interface. Thus, the effects can mix or interchange depending on the specific material but the states will remain tunable.

cond-mat.mes-hall↗

arXiv2025

Spin caloritronics as a probe of nonunitary superconductors

Superconducting spintronics explores the interplay between superconductivity and magnetism, sparking significant interest in nonunitary superconductors as a platform for novel magneto-superconducting phenomena. However, identifying nonunitary superconductors remains challenging. We demonstrate that spin current driven by thermal gradients sensitively probes the nature of the condensate in nonunitary superconductors. Spin polarization of the condensate in momentum space induces the superconducting spin Seebeck effect, where a spin current is generated along thermal gradients without a thermoelectric charge current. Notably, the nonvanishing superconducting spin Seebeck effect provides a smoking gun evidence of nonunitary superconductivity because it reflects the spin polarization of the condensate in momentum space, irrespective of whether the net pair spin magnetization vanishes. At the same time, the spin-chirality of the condensate induces the spin-Nernst effect, where a spin current is generated perpendicular to thermal gradients in nonunitary superconductors. These spin caloritronics phenomena offer a definitive probe of nonunitary superconductors.

cond-mat.supr-con↗

arXiv2024

Impurity bands, line-nodes, and anomalous thermal Hall effect in Weyl superconductors

We investigate the anomalous thermal Hall effect (ATHE) in Weyl superconductors realized by the $E_{1u}$ ($p$-wave and $f$-wave) chiral superconducting order for the point group $D_{6h}$. Using the quasiclassical transport theory, we analyze the influence of the impurity scatterings and the line-nodal excitations on the ATHE. We compare the extrinsic (impurity-induced) ATHE with the intrinsic (topological) ATHE to identify the dominant contribution. Because the transverse response is sensitive to the slope in the density of states (DOS) at the Fermi energy, the extrinsic ATHE vanishes in both the Born (weak impurity potential) and unitarity (strong impurity potential) limits. The amplitude of the impurity contribution to the thermal Hall conductivity (THC) reaches maximum between these limits when the slope of the DOS becomes large due to impurity bands near the Fermi energy. In such situations, the extrinsic ATHE dominates the intrinsic ATHE even at low temperatures. The extrinsic ATHE is sensitive to line-nodal excitations, whereas the intrinsic ATHE is insensitive to bulk excitations. When line nodes involve the sign change of the order parameter, the impurity contribution to the THC is suppressed even though the phase space for low-energy excitation is large. In contrast, if line nodes are not accompanied by such sign changes, the extrinsic ATHE is significantly enhanced. Our results form a basis for the comprehensive analysis of anomalous thermal transport in Weyl superconductors.

cond-mat.supr-con↗

arXiv2024

Trends in hot carrier distribution for disordered noble-transition metal alloys

We developed and tested an approach for predicting trends for efficient hot carrier generation among disordered metal alloys. We provide a simple argument for the importance of indirect transitions in the presence of disorder, thus justifying the use of Joint Density of States (JDOS)-like quantities for exploring these trends. We introduce a new JDOS-like quantity, JDOSK, which heuristically accounts for longer lifetimes of quasiparticles close to the Fermi energy. To demonstrate the efficacy of this new quantity, we apply it to the study of Cu50X50 where X = Ag, Au, Pd and Y50Pd50 where Y = Au, Ni. We predict that Ni50Pd50 produces the most hot carriers among the alloys considered. The improvement in the density of excited photocarriers over the base alloy used, Cu50Ag50, is 3.4 times for 800 nm and 19 times for 1550 nm light. This boost in hot-carrier generation is consequence of the ferromagnetic nature of the Ni alloy. We argue that our method allows efficient material-specific predictions for low bias photoconductivity of alloys.

cond-mat.mtrl-sci↗

arXiv2023

Bound states and controllable currents on Topological Insulator surfaces with extended magnetic defects

We show that a magnetic line defect on the surface of a topological insulator generically supports two distinct branches of spin-polarized and current carrying one-dimensional bound states. We identify the components of magnetic scattering that lead to the bound states. The velocity, and hence spin texture, of each of those branches can be independently tuned by a magnetic field rotated in the plane of the surface. We compute the local net and spin-resolved density of states as well as spin accumulation and charge currents. The net spin polarization and current due to both bound and scattering states vary stepwise as a function of the electrostatic and magnetic components of the scattering potential, and can be tuned by an applied field. We discuss stability of the bound states with respect to impurity scattering.

cond-mat.mes-hall↗

arXiv2022

Anomalous acoustoelectric effect induced by clapping modes in chiral superconductors

Clapping modes, which are relative amplitude and phase modes between two chiral components of Cooper pairs, are bosonic collective modes inherent to chiral superconductors. These modes behave as long-lived bosons with masses smaller than the threshold energy, $2|Δ|$, for decay into unbound fermion pairs. Here, we clarify that the real/imaginary clapping modes in chiral superconductors directly couple to acoustic wave propagation when the weak particle-hole asymmetry of the normal state quasiparticle dispersion is taken into account. The clapping modes driven by an acoustic wave generate an alternating electric current, that is, the acoustoelectric effect in superconductors. Significantly, the clapping modes give rise to a transverse electric current. When the sound velocity is comparable to the Fermi velocity, as in heavy fermion compounds, the transverse current is resonantly enhanced at energy below the threshold for continuum excitations. This resonance provides a smoking-gun evidence of chiral superconductivity.

cond-mat.supr-con↗

arXiv2022

Inter-quintuple layer coupling and topological phase transitions in the chalcogenide topological insulators

Driving quantum phase transitions in the 3D topological insulators offers pathways to tuning the topological states and their properties. We use DFT-based calculations to systematically investigate topological phase transitions in Bi$_2$Se$_3$, Sb$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$ by varying the $c/a$ ratio of lattice constants. This ensures no net hydrostatic pressure under anisotropic stress and strain and allows a clear identification of the physics leading to the transition. As a function of $c/a$, all of these materials exhibit structural and electronic stability of the quintuple layers (QLs), and quasi-linear behavior of both the inter-quintuple layer distance and the energy gap near the topological transition. Our results show that the transition is predominantly controlled by the inter-QL physics, namely by competing Coulomb and van der Waals interactions between the outer atomic sheets in neighboring quintuple layers. We discuss the implications of our results for topological tuning by alloying.

cond-mat.mes-hall↗

arXiv2022

Spin-Nernst Effect in Time-Reversal-Invariant Topological Superconductors

We investigate the spin-Nernst effect in time-reversal invariant topological superconductors, and show that it provides a smoking-gun evidence for helical Cooper pairs. The spin-Nernst effect stems from asymmetric, in spin space, scattering of quasiparticles at nonmagnetic impurities, and generates a transverse spin current by the temperature gradient. Both the sign and the magnitude of the effect sensitively depend on the scattering phase shift at impurity sites. Therefore the spin-Nernst effect is uniquely suitable for identifying time-reversal invariant topological superconducting orders.

cond-mat.supr-con↗

arXiv2021

Partial Charge Transfer and Absence of Induced Magnetization in EuS(111)/Bi$_2$Se$_3$ Heterostructures

Heterostructures made from topological and magnetic insulators promise to form excellent platforms for new electronic and spintronic functionalities mediated by interfacial effects. We report the results of a first-principles density functional theory study of the geometric, electronic structure, and magnetic properties of EuS(111)/Bi$_2$Se$_3$ interface, including van der Waals and relativistic spin-orbit effects. In contrast to previous theoretical studies, we find no appreciable magnetic anisotropy in such a heterostructure. We also do not see additional induced magnetization at the interface or the magnetic proximity effect on the topological states. This is due to the localized nature of Eu moments, and because of a partial charge transfer of $\sim$0.5 electron from Eu to Se. The formation of the surface dipole shifts the Dirac cone about 0.4~eV below the chemical potential, and the associated electrostatic screening moves the topological state from the first to the second quintuple layer of Bi$_2$Se$_3$.

cond-mat.mtrl-sci↗

arXiv2021

Andreev spectroscopy of nonhelical spin textures in topological insulators

We study how nonhelical spin textures affect the proximity-induced superconductivity of topological insulator (TI)-superconductor (SC) interface states. In particular we calculate the conductance of lateral heterojunctions which comprise a TI surface that is only partially covered by a superconducting material. Interface potentials at the TI-SC interface may lead to a Fermi velocity and spin texture mismatch between the two regions of the lateral heterojunction. By enforcing the hermiticity of the total Hamiltonian, we derive the boundary conditions and calculate the conductance of the structure in both the normal and superconducting state. The total Andreev conductance is calculated for both $s$-wave and spin-triplet parent SCs, and for several examples of nonhelical spin textures which lead to different Fermi surface mismatches between the two planar regions of the heterojunction. We find that for spin-triplet SCs, nonzero conductance signatures only appear for certain combinations of nonhelical spin textures and parent superconducting material.

cond-mat.supr-con↗

arXiv2021

Annihilation and Control of Chiral Domain Walls with Magnetic Fields

The control of domain walls is central to nearly all magnetic technologies, particularly for information storage and spintronics. Creative attempts to increase storage density need to overcome volatility due to thermal fluctuations of nanoscopic domains and heating limitations. Topological defects, such as solitons, skyrmions, and merons, may be much less susceptible to fluctuations, owing to topological constraints, while also being controllable with low current densities. Here, we present the first evidence for soliton/soliton and soliton/antisoliton domain walls in the hexagonal chiral magnet Mn1/3NbS2 that respond asymmetrically to magnetic fields and exhibit pair-annihilation. This is important because it suggests the possibility of controlling the occurrence of soliton pairs and the use of small fields or small currents to control nanoscopic magnetic domains. Specifically, our data suggest that either soliton/soliton or soliton/antisoliton pairs can be stabilized by tuning the balance between intrinsic exchange interactions and long-range magnetostatics in restricted geometries

cond-mat.mes-hall↗

arXiv2020

Conductance of gated junctions as a probe of topological interface states

Energy dispersion and spin orientation of the protected states at interfaces between topological insulators (TIs) and non-topological materials depend on the charge redistribution, strain, and atomic displacement at the interface. Knowledge of these properties is essential for applications of topological compounds, but direct access to them in the interface geometry is difficult. We show that conductance of a gated double junction at the surface of a topological insulator exhibits oscillations and a quasi-linear decay as a function of gate voltage in different regimes. These give the values for the quasiparticle velocities along and normal to the junction in the interface region, and determine the symmetry of the topological interface states. The results are insensitive to the boundary conditions at the junction.

cond-mat.mes-hall↗

arXiv2020

Observation of sixfold degenerate fermions in PdSb$_2$

Three types of fermions have been extensively studied in topological quantum materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions in high energy physics, exotic fermions are allowed in condensed matter systems residing in three-, six- or eightfold degenerate band crossings. Here, we use angle-resolved photoemission spectroscopy to directly visualize three-doubly-degenerate bands in PdSb$_2$. The ultrahigh energy resolution we are able to achieve allows for the confirmation of all the sixfold degenerate bands at the R point, in remarkable consistency with first-principles calculations. Moreover, we find that this sixfold degenerate crossing has quadratic dispersion as predicted by theory. Finally, we compare sixfold degenerate fermions with previously confirmed fermions to demonstrate the importance of this work: our study indicates a topological fermion beyond the constraints of high energy physics.

cond-mat.mes-hall↗

arXiv2018

Proximity-Induced Superconductivity at Non-Helical Topological Insulator Interfaces

We study how non-helical spin textures at the boundary between a topological insulator (TI) and a superconductor (SC) affect the proximity-induced superconductivity of the TI interface state. We consider TIs coupled to both spin-singlet and spin-triplet SCs, and show that for the spin-triplet parent SCs the resulting order parameter induced onto the interface state sensitively depends on the symmetries which are broken at the TI-SC boundary. For chiral spin-triplet parent SCs, we find that nodal proximity-induced superconductivity emerges when there is broken twofold rotational symmetry which forces the spins of the non-helical topological states to tilt away from the interface plane. We furthermore show that the Andreev conductance of lateral heterostructures joining TI-vacuum and TI-SC interfaces yields experimental signatures of the reduced symmetries of the interface states.

cond-mat.supr-con↗

arXiv2018

Topological phases of topological insulator thin films

We study the properties of a thin film of topological insulator material. We treat the coupling between helical states at opposite surfaces of the film in the properly-adapted tunneling approximation, and show that the tunneling matrix element oscillates as function of both the film thickness and the momentum in the plane of the film for Bi$_2$Se$_3$ and Bi$_2$Te$_3$. As a result, while the magnitude of the matrix element at the center of the surface Brillouin Zone gives the gap in the energy spectrum, the sign of the matrix element uniquely determines the topological properties of the film, as demonstrated by explicitly computing the pseudospin textures and the Chern number. We find a sequence of transitions between topological and non-topological phases, separated by semimetallic states, as the film thickness varies. In the topological phase the edge states of the film always exist but only carry a spin current if the edge potentials break particle-hole symmetry. The edge states decay very slowly away from the boundary in Bi$_2$Se$_3$, making Bi$_{2}$Te$_{3}$, where this scale is shorter, a more promising candidate for the observation of these states. Our results hold for free-standing films as well as heterostructures with large-gap insulators.

cond-mat.mes-hall↗