arXiv subjects

Vladyslav Kozii

Publications and source records attributed to Vladyslav Kozii.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear Hall effect in topological Dirac semimetals in parallel magnetic field

We compute the second-harmonic response of two-dimensional topological Dirac semimetals subjected to an external in-plane magnetic field. The quantum kinetic equation for the Wigner distribution function is derived and then solved to evaluate the second-order electric-field contributions to the current density. Both the Berry curvature dipole and the field-induced terms in the current are analyzed across a broad range of model parameters. We propose that our theory can be tested experimentally by measuring the dependence of the anomalous Hall resistivity on the in-plane magnetic field in the surface states of the topological insulator SnTe, in WTe$_2$ and WSe$_2$ monolayers, as well as in the Kondo lattice material Ce$_3$Bi$_4$Pd$_3$ at very low temperatures.

cond-mat.mes-hall↗

arXiv2026

Light induced magnetization in d-wave superconductors

We develop a microscopic theory of the inverse Faraday effect in $s$- and $d$-wave superconductors. An extended version of the Keldysh--Nambu quasiclassical formalism, which retains the particle-hole asymmetric terms responsible for the branch population imbalance, is used to compute the dc component of the nonlinear current density induced by an external monochromatic radiation. We demonstrate how the branch population imbalance produces a nonvanishing nonlinear and nonlocal dc response, evaluate the magnitude of the induced current, and obtain estimates for the induced static magnetization. For $d$-wave pairing we identify a qualitatively new contribution: the radiation induces a linear-in-field oscillation of the order-parameter amplitude -- the Schmid-Higgs mode -- in the $B_{1g}$ channel, which feeds the rectified response with a weight proportional to the pair susceptibility and is therefore resonantly enhanced at the pair-breaking threshold. This contribution is symmetry-forbidden for an isotropic $s$-wave gap, so the light-induced magnetization serves both as a dc-channel probe of the Higgs mode and as a discriminator of the pairing symmetry. Experimental implications of our theory and future extensions of our work are briefly discussed.

cond-mat.supr-con↗

arXiv2026

Strong photogalvanic effect in Weyl materials due to magnetic resonances

We study the photogalvanic effect in Weyl semimetals under a magnetic field, focusing on the shift current. Using the Kubo formalism for an ideal, clean Weyl node at zero temperature, we derive a general analytic expression valid for arbitrary light frequency, Fermi energy, and magnetic field strength. We identify a series of resonances that can be probed experimentally. To complement the microscopic analysis, we employ the semiclassical Boltzmann approach, which allows us to incorporate finite scattering phenomenologically. Unlike most previous studies using this method, we do not treat the magnetic field perturbatively; instead, we solve the Boltzmann equation for a Weyl node exactly within the limits of validity of the semiclassical theory. Our solution reproduces the low-frequency resonances and elucidates the role of finite scattering.

cond-mat.mes-hall↗

arXiv2025

Nonunitary spin-triplet superconductors in Zeeman magnetic field

We study spin-triplet superconductivity with both unitary and nonunitary pairing in the presence of an external Zeeman magnetic field. Within a mean-field framework, we exactly diagonalize the Bogoliubov-de Gennes Hamiltonian and derive general expressions for the quasiparticle spectrum, superconducting gap, critical temperature, and spin magnetization, valid for arbitrary magnetic-field strengths and temperatures. We analyze in detail the nonlinear spin susceptibility and the field evolution of the superconducting gap and transition temperature, highlighting qualitative differences between unitary and nonunitary pairing states. Our results are broadly applicable to a wide range of materials, including systems with both weak and strong spin-orbit coupling. We show that systematic measurements of the critical temperature and spin susceptibility as functions of the magnitude and orientation of the magnetic field provide a powerful means to identify the structure of the spin-triplet order parameter, and we discuss implications of our findings for candidate materials such as 4Hb-TaS$_2$ and PrOs$_4$Sb$_{12}$.

cond-mat.supr-con↗

arXiv2025

Unveiling a Hidden Percolation Transition in Monitored Clifford Circuits: Inroads from ZX-Calculus

We revisit the measurement-induced phase transition (MPT) in Clifford circuits, which are both classically simulable and exhibit critical behavior widely believed to be distinct from classical percolation theory, using ZX-calculus. We analyze the MPT in a dynamical model composed of CNOT, SWAP, identity gates, and Bell-pair measurements, respectively, arranged randomly in a brickwork pattern. Our circuits exhibit a transition that is seemingly distinct from classical percolation based on standard arguments, that is in line with the prevailing understanding in the field. In contrast, by employing ZX-calculus based simplification techniques, we unveil a hidden percolation transition within the circuit structure. Over a range of parameters tied to the probabilities for applying different gates, we demonstrate that the classical percolation transition in the ZX-simplified network coincides with the MPT observed through mutual information. Our findings suggest that the MPT in Clifford circuits is, in fact, controlled by a classical percolation transition in disguise.

quant-ph↗

arXiv2025

Instability of the critical Ngai's coupling and two-boson mechanism in metals

We study the properties of a Fermi liquid coupled to a quantum critical boson via the two-boson interaction known as Ngai's coupling. We find that the original quantum critical point is generally unstable, resulting in a finite-momentum spatially modulated state unless two conditions are satisfied: (i) the critical boson is polar and transverse, and (ii) the ratio of the Fermi velocity to the transverse-boson velocity is sufficiently large. If these conditions hold and the uniform state remains stable, we demonstrate that the system enters a strong-coupling regime below a certain energy scale. In this regime, we discuss a self-consistent solution at criticality in two-dimensional systems and show that the critical boson field develops a nontrivial anomalous dimension, $η=1/2$. Our findings highlight the significant role of two-boson coupling in critical theories, challenging the conventional view that its effects are subdominant to linear coupling.

cond-mat.str-el↗

arXiv2024

Magnetic field induces giant nonlinear optical response in Weyl semimetals

We study the second-order optical response of Weyl semimetals in the presence of a magnetic field. We consider an idealized model of a perfectly linear Weyl node and use the Kubo formula at zero temperature to calculate the intrinsic contribution to photocurrent and second harmonic generation conductivity components. We obtain exact analytical expressions applicable at arbitrary values of frequency, chemical potential, and magnetic field. Our results show that finite magnetic field significantly enhances the nonlinear optical response in semimetals, while magnetic resonances lead to divergences in nonlinear conductivity. In realistic systems, these singularities are regularized by a finite scattering rate, but result in pronounced peaks which can be detected experimentally, provided the system is clean and interactions are weak. We also perform a semiclassical calculation that complements and confirms our microscopic results at small magnetic fields and frequencies.

cond-mat.mes-hall↗

arXiv2023

The field theory of a superconductor with repulsion

A superconductor emerges as a condensate of electron pairs, which bind despite their strong Coulomb repulsion. Eliashberg's theory elucidates the mechanisms enabling them to overcome this repulsion and predicts the transition temperature and pairing correlations. However, a comprehensive understanding of how repulsion impacts the phenomenology of the resulting superconductor remains elusive. We present a formalism that addresses this challenge by applying the Hubbard-Stratonovich transformation to an interaction including instantaneous repulsion and retarded attraction. We first decompose the interaction into frequency scattering channels and then integrate out the fermions. The resulting bosonic action is complex and the saddle point corresponding to Eliashberg's equations generally extends into the complex plane and away from the physical axis. We numerically determine this saddle point using the gradient descent method, which is particularly well-suited for the case of strong repulsion. We then turn to consider fluctuations around this complex saddle point. The matrix controlling fluctuations about the saddle point is found to be a non-Hermitian symmetric matrix, which generally suffers from exceptional points that are tuned by different parameters. These exceptional points may influence the thermodynamics of the superconductor. For example, within the quadratic approximation the upper critical field sharply peaks at a critical value of the repulsion strength related to an exceptional point appearing at $T_c$. Our work facilitates the mapping between microscopic and phenomenological theories of superconductivity, particularly in the presence of strong repulsion. It has the potential to enhance the accuracy of theoretical predictions for experiments in systems where the pairing mechanism is unknown.

cond-mat.supr-con↗

arXiv2023

Linear magneto-conductivity as a DC probe of time-reversal symmetry breaking

Several optical experiments have shown that in magnetic materials the principal axes of response tensors can rotate in a magnetic field. Here we offer a microscopic explanation of this effect, and propose a closely related DC transport phenomenon -- an off-diagonal \emph{symmetric} conductivity linear in a magnetic field, which we refer to as linear magneto-conductivity (LMC). Although LMC has the same functional dependence on magnetic field as the Hall effect, its origin is fundamentally different: LMC requires time-reversal symmetry to be broken even before a magnetic field is applied, and is therefore a sensitive probe of magnetism. We demonstrate LMC in three different ways: via a tight-binding toy model, density functional theory calculations on MnPSe$_3$, and a semiclassical calculation. The third approach additionally identifies two distinct mechanisms yielding LMC: momentum-dependent band magnetization and Berry curvature. Finally, we propose an experimental geometry suitable for detecting LMC, and demonstrate its applicability using Landauer-Büttiker simulations. Our results emphasize the importance of measuring the full conductivity tensor in magnetic materials, and introduce LMC as a new transport probe of symmetry.

cond-mat.mes-hall↗

arXiv2023

A theory of criticality for quantum ferroelectric metals

A variety of compounds, for example doped paraelectrics and polar metals, exhibit both ferroelectricity and correlated electronic phenomena such as low-density superconductivity and anomalous transport. Characterizing such properties is tied to understanding the quantum dynamics of inversion symmetry breaking in the presence of itinerant electrons. Here, we present a comprehensive analysis of the normal state properties of a metal near a quantum critical transition to a ferroelectric state, in both two and three dimensions. Starting from a minimal model of electrons coupled to a \emph{transverse} polar phonon via a Rashba-type spin-orbit interaction, we compute the dynamical response of both electrons and phonons. We find that the system can evince both Fermi and non-Fermi liquid phases, as well as enhanced pairing in both singlet and triplet channels. Furthermore, we systematically compute corrections to one-loop theory and find a tendency to quantum order-by-disorder, leading to a phase diagram that can include second order, first order, and finite-momentum phase transitions. Finally, we show that the entire phase diagram can be controlled via application of external strain, either compressive or volume-preserving. Our results provide a map of the dynamical and thermodynamical phase space of quantum ferroelectic metals, which can serve in characterizing existing materials and in seeking applications for quantum technologies.

cond-mat.str-el↗

arXiv2023

Spin-triplet superconductivity from inter-valley Goldstone modes in magic-angle graphene

We consider magic-angle graphene in the doping regime around charge neutrality and study the connection between a recently proposed inter-valley coherent insulator at zero doping and the neighboring superconducting domes. The magic-angle graphene continuum model has an emergent U(1) valley-charge conservation symmetry, and an emergent SU(2) symmetry corresponding to opposite spin rotations in the two valleys. The inter-valley coherent insulator spontaneously breaks both these emergent symmetries, and as a result has four Goldstone modes which couple to doped charge carriers. We derive the effective interaction mediated by the Goldstone modes, and study its role in electron pair formation. The SU(2) Goldstone modes generate a ferromagnetic interaction, which is attractive in spin-triplet pairing channels and repulsive in spin-singlet channels. From a weak-coupling BCS calculation, we find the leading superconducting instability in the p-wave channel.

cond-mat.str-el↗

arXiv2022

Synergetic ferroelectricity and superconductivity in zero-density Dirac semimetals near quantum criticality

We study superconductivity in a three-dimensional zero-density Dirac semimetal in proximity to a ferroelectric quantum critical point. We find that the interplay of criticality, inversion-symmetry breaking, and Dirac dispersion gives rise to a robust superconducting state at the charge-neutrality point, where no Fermi surface is present. Using Eliashberg theory, we show that the ferroelectric quantum critical point is unstable against the formation of a ferroelectric density wave (FDW), whose fluctuations, in turn, lead to a first-order superconducting transition. Surprisingly, long-range superconducting and FDW orders are found to cooperate with each other, in contrast to the more usual scenario of phase competition. Therefore, we suggest that driving charge neutral Dirac materials, e.g., Pb$_x$Sn$_{1-x}$Te, through a ferroelectric quantum critical point may lead to superconductivity intertwined with FDW order.

cond-mat.supr-con↗

arXiv2022

Direct Geometric Probe of Singularities in Band Structure

The band structure of a crystal may have points where two or more bands are degenerate in energy and where the geometry of the Bloch state manifold is singular, with consequences for material and transport properties. Ultracold atoms in optical lattices have been used to characterize such points only indirectly, e.g., by detection of an Abelian Berry phase, and only at singularities with linear dispersion (Dirac points). Here, we probe band-structure singularities through the non-Abelian transformation produced by transport directly through the singular points. We prepare atoms in one Bloch band, accelerate them along a quasi-momentum trajectory that enters, turns, and then exits the singularities at linear and quadratic touching points of a honeycomb lattice. Measurements of the band populations after transport identify the winding numbers of these singularities to be 1 and 2, respectively. Our work opens the study of quadratic band touching points in ultracold-atom quantum simulators, and also provides a novel method for probing other band geometry singularities.

cond-mat.quant-gas↗

arXiv2021

Intrinsic anomalous Hall conductivity in a nonuniform electric field

We study how the intrinsic anomalous Hall conductivity is modified in two-dimensional crystals with broken time-reversal symmetry due to weak inhomogeneity of the applied electric field. Focusing on a clean noninteracting two-band system without band crossings, we derive the general expression for the Hall conductivity at small finite wave vector $q$ to order $q^2$, which governs the Hall response to the second gradient of the electric field. Using the Kubo formula, we show that the answer can be expressed through the Berry curvature, Fubini-Study quantum metric, and the rank-3 symmetric tensor which is related to the quantum geometric connection and physically corresponds to the gauge-invariant part of the third cumulant of the position operator. We further compare our results with the predictions made within the semiclassical approach. By deriving the semiclassical equations of motion, we reproduce the result obtained from the Kubo formula in some limits. We also find, however, that the conventional semiclassical description in terms of the definite position and momentum of the electron is not fully consistent because of singular terms originating from the Heisenberg uncertainty principle. We thus present a clear example of a case when the semiclassical approach inherently suffers from the uncertainty principle, implying that it should be applied to systems in nonuniform fields with extra care.

cond-mat.mes-hall↗

arXiv2020

Quantized Thermoelectric Hall Effect Induces Giant Power Factor in a Topological Semimetal

Thermoelectrics are promising by directly generating electricity from waste heat. However, (sub-)room-temperature thermoelectrics have been a long-standing challenge due to vanishing electronic entropy at low temperatures. Topological materials offer a new avenue for energy harvesting applications. Recent theories predicted that topological semimetals at the quantum limit can lead to a large, non-saturating thermopower and a quantized thermoelectric Hall conductivity approaching a universal value. Here, we experimentally demonstrate the non-saturating thermopower and quantized thermoelectric Hall effect in the topological Weyl semimetal (WSM) tantalum phosphide (TaP). An ultrahigh longitudinal thermopower Sxx= 1.1x10^3 muV/K and giant power factor ~525 muW/cm/K^2 are observed at ~40K, which is largely attributed to the quantized thermoelectric Hall effect. Our work highlights the unique quantized thermoelectric Hall effect realized in a WSM toward low-temperature energy harvesting applications.

cond-mat.mes-hall↗

arXiv2020

The effect of interorbital scattering on superconductivity in doped Dirac materials

Unconventional superconductivity has been discovered in a variety of doped materials, including topological insulators, semimetals and twisted bilayers. A unifying property of these systems is strong orbital hybridization, which involves pairing of states with non-trivial Bloch wave functions. In contrast to naive expectation, many of these superconductors are relatively resilient to disorder. Here we study the effects of a generic disorder on superconductivity in doped 3D Dirac systems, which serve as a paradigmatic example for the dispersion near a band crossing point. We argue that due to strong orbital hybridization, interorbital scattering processes are naturally present and must be taken into account. We calculate the reduction of the critical temperature for a variety of pairing states and scattering channels using Abrikosov-Gor'kov theory. In that way, the role of disorder is captured by a single parameter $Γ$, the pair scattering rate. This procedure is very general and can be readily applied to different band structures and disorder configurations. Our results show that interorbital scattering has a significant effect on superconductivity, where the robustness of different pairing states highly depends on the relative strength of the different interorbital scattering channels. Our analysis also reveals a protection, analogous to the Anderson's theorem, of the odd-parity pairing state with total angular momentum zero (the B-phase of superfluid $^3$He). This odd-pairty state is a singlet of partners under $\mathcal{CT}$ symmetry (rather than $\mathcal{T}$ symmetry in the standard Anderson's theory), where $\mathcal{C}$ and $\mathcal{T}$ are chiral and time-reversal symmetries, respectively. As a result, it is protected against any disorder potential that respects $\mathcal{CT}$ symmetry, which includes a family of time-reversal odd (magnetic) impurities.

cond-mat.supr-con↗

arXiv2020

Interactions remove the quantization of the chiral photocurrent at Weyl points

The chiral photocurrent or circular photogalvanic effect (CPGE) is a photocurrent that depends on the sense of circular polarization. In a disorder-free, noninteracting chiral Weyl semimetal, the magnitude of the effect is approximately quantized with a material-independent quantum $e^3/h^2$ for reasons of band topology. We study the first-order corrections due to the Coulomb and Hubbard interactions in a continuum model of a Weyl semimetal in which known corrections from other bands are absent. We find that the inclusion of interactions generically breaks the quantization. The corrections are similar but larger in magnitude than previously studied interaction corrections to the (nontopological) linear optical conductivity of graphene, and have a potentially observable frequency dependence. We conclude that, unlike the quantum Hall effect in gapped phases or the chiral anomaly in field theories, the quantization of the CPGE in Weyl semimetals is not protected but has perturbative corrections in interaction strength.

cond-mat.str-el↗

arXiv2020

DMFT reveals the non-Hermitian topology in heavy-fermion systems

We find that heavy fermion systems can have bulk "Fermi arcs", with the use of the non-Hermitian topological theory. In an interacting electron system, the microscopic many-body Hamiltonian is Hermitian, but the one-body quasiparticle Hamiltonian is non-Hermitian due to the finite quasiparticle lifetime. We focus on heavy electron systems as a stage of finite lifetime quasiparticles with two lifetimes, since quasiparticle lifetimes for f-electrons and c-electrons should be different. Two lifetimes induce exceptional points (EPs) of the non-Hermitian quasiparticle Hamiltonian matrix in momentum space. The line connecting between two EPs characterizes the bulk Fermi arcs. With the use of the dynamical mean field theory (DMFT) calculation, we confirm our statement in Kondo insulators with a momentum-dependent hybridization in two-dimensions. We show that the concept of the EPs in the non-Hermitian quasiparticle Hamiltonian is one of powerful tools to predict new phenomena in strongly correlated electron systems.

cond-mat.str-el↗