arXiv subjects

M. V. Feigel'man

Publications and source records attributed to M. V. Feigel'man.

At least 19 recordsLinked to original sources

arXiv2024

Andreev conductance in disordered SF junctions with spin-orbit scattering

We calculate the conductance of a junction between a disordered superconductor and a very strong half-metallic ferromagnet admitting electrons with only one spin projection. A usual mechanism of Andreev reflection is strongly suppressed in this case since Cooper pairs are composed of electrons with opposite spins. However, this obstacle can be overcome if we take into account spin-orbit scattering inside the superconductor. Spin-orbit scattering induces a fluctuational (zero on average) spin-triplet component of the superconducting condensate, which is enough to establish Andreev transport into a strong ferromagnet. This remarkably simple mechanism is quite versatile and can explain long-range triplet proximity effect in a number of experimental setups. One particular application of the suggested effect is to measure the spin-orbit scattering time $τ_{\text{SO}}$ in disordered superconducting materials. The value of Andreev conductance strongly depends on the parameter $Δτ_\text{SO}$ and can be noticeable even in very disordered but relatively light metals like granular aluminum.

cond-mat.supr-con↗

arXiv2022

Microwave response of type-II superconductors at weak pinning

Theory of linear microwave response of thin films of type-II superconductors in the mixed state is developed taking into account random spatial fluctuations of the parameters of the system, such as the order parameter, diffusion coefficient, or film thickness. In the regime of collective pinning the microwave response of the system exhibits strong frequency dispersion, arising from nonequilibrium vortex core quasiparticles. The corresponding contribution to the ac conductivity is controlled by the inelastic relaxation time, and may exceed the usual Bardeen-Stephen conductivity. It is caused by the Debye-type inelastic relaxation. Debye mechanism of microwave losses may be responsible for strong effects of electromagnetic noise upon dc conductivity in the mixed state at low temperatures.

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arXiv2022

High-frequency transport and zero-sound in an array of SYK quantum dots

We study an array of strongly correlated quantum dots of complex SYK type and account for the effects of quadratic terms added to the SYK Hamiltonian; both local terms and inter-dot tunneling are considered in the non-Fermi-liquid temperature range $T \gg T_{FL}$. We take into account soft-mode fluctuations and demonstrate their relevance for physical observables. Electric $σ(ω,p)$ and thermal $κ(ω,p)$ conductivities are calculated as functions of frequency and momentum for arbitrary values of the particle-hole asymmetry parameter $\mathcal{E}$. At low-frequencies $ω\ll T$ we find the Lorenz ratio $L = κ(0,0)/Tσ(0,0)$ to be non-universal and temperature-dependent. At $ω\gg T$ the conductivity $σ(ω,p)$ contains a pole with nearly linear dispersion $ω\approx sp\sqrt{\ln\fracω{T}}$ reminiscent of the "zero-sound", known for Fermi-liquids. We demonstrate also that the developed approach makes it possible to understand the origin of heavy Fermi liquids with anomalously large Kadowaki-Woods ratio.

cond-mat.str-el↗

arXiv2021

Non-equilibrium Sachdev-Ye-Kitaev model with quadratic perturbation

We consider a non-equilibrium generalization of the mixed SYK$_4$+SYK$_2$ model and calculate the energy dissipation rate $W(ω)$ that results due to periodic modulation of random quadratic matrix elements with a frequency $ω$. We find that $W(ω)$ possesses a peak at $ω$ close to the polaron energy splitting $ω_R$ found recently (PRL 125, 196602), demonstrating the physical significance of this energy scale. Next, we study the effect of energy pumping with a finite amplitude at the resonance frequency $ω_R$ and calculate, in presence of this pumping, non-equilibrium dissipation rate due to low-frequency parametric modulation. We found an unusual phenomenon similar to "dry friction" in presence of pumping.

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arXiv2020

Perturbed Sachdev-Ye-Kitaev model: a polaron in the hyperbolic plane

We study the SYK$_4$ model with a weak SYK$_2$ term of magnitude $Γ$ beyond the simplest perturbative limit considered previously. For intermediate values of the perturbation strength, $J/N \ll Γ\ll J/\sqrt{N}$, fluctuations of the Schwarzian mode are suppressed, and the SYK$_4$ mean-field solution remains valid beyond the timescale $t_0 \sim N/J$ up to $t_* \sim J/Γ^2$. Out-of-time-order correlation function displays at short time intervals exponential growth with maximal Lyapunov exponent $2πT$, but its prefactor scales as $T$ at low temperatures $T \leq Γ$.

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arXiv2020

Conductivity of superconductors in the flux flow regime

We develop a theory of conductivity of type-II superconductors in the flux flow regime taking into account random spatial fluctuations of the system parameters, such as the gap magnitude $Δ$(r) and the diffusion coefficient D(r). We find a contribution to the conductivity that is proportional to the inelastic relaxation time $τ_{in}$, which is much longer than the elastic relaxation time. This new contribution is due to Debye-type relaxation, and it can be much larger than the conventional flux flow conductivity due to Bardeen and Stephen. The new contribution is expected to dominate in clean superconductors at low temperatures and in magnetic fields much smaller than $H_{c2}$.

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arXiv2020

Spectroscopic evidence for strong correlations between local superconducting gap and local Altshuler-Aronov density-of-states suppression in ultrathin NbN films

Disorder has different profound effects on superconducting thin films. For a large variety of materials, increasing disorder reduces electronic screening which enhances electron-electron repulsion. These fermionic effects lead to a mechanism described by Finkelstein: when disorder combined to electron-electron interactions increases, there is a global decrease of the superconducting energy gap $Δ$ and of the critical temperature $T_c$, the ratio $Δ$/$k_BT_c$ remaining roughly constant. In addition, in most films an emergent granularity develops with increasing disorder and results in the formation of inhomogeneous superconducting puddles. These gap inhomogeneities are usually accompanied by the development of bosonic features: a pseudogap develops above the critical temperature $T_c$ and the energy gap $Δ$ starts decoupling from $T_c$. Thus the mechanism(s) driving the appearance of these gap inhomogeneities could result from a complicated interplay between fermionic and bosonic effects. By studying the local electronic properties of a NbN film with scanning tunneling spectroscopy (STS) we show that the inhomogeneous spatial distribution of $Δ$ is locally strongly correlated to a large depletion in the local density of states (LDOS) around the Fermi level, associated to the Altshuler-Aronov effect induced by strong electronic interactions. By modelling quantitatively the measured LDOS suppression, we show that the latter can be interpreted as local variations of the film resistivity. This local change in resistivity leads to a local variation of $Δ$ through a local Finkelstein mechanism. Our analysis furnishes a purely fermionic scenario explaining quantitatively the emergent superconducting inhomogeneities, while the precise origin of the latter remained unclear up to now.

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arXiv2018

SYK model with quadratic perturbations: the route to a non-Fermi-liquid

We study the stability of the SYK$_4$ model with a large but finite number of fermions $N$ with respect to a perturbation, quadratic in fermionic operators. We develop analytic perturbation theory in the amplitude of the SYK$_2$ perturbation and demonstrate the stability of the SYK$_4$ infra-red asymptotic behavior characterized by a Green function $G(τ) \propto 1/τ^{3/2} $, with respect to weak perturbation. This result is supported by exact numerical diagonalization. Our results open the way to build a theory of non-Fermi-liquid states of strongly interacting fermions.

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arXiv2018

Quantum meets classical phase transition: Low-temperature anomaly in disordered superconductors near $B_{c2}$

Strongly disordered superconductors in a magnetic field display many characteristic properties of type-II superconductivity--- except at low temperatures where an anomalous linear $T$-dependence of the resistive critical field $B_{c2}$ is routinely observed. This behavior violates the conventional theory of superconductivity, and its origin remains a long-standing puzzle. Here we report on systematic measurements of the critical magnetic field and current on amorphous indium oxide films of various levels of disorder. Surprisingly, our measurements show that the $B_{c2}$ anomaly near zero-temperature is accompanied by a clear mean-field like scaling behavior of the critical current. We demonstrate theoretically that these are consequences of the vortex-glass ground state and its thermal fluctuations. This theory further predicts the linear-$T$ anomaly to occur in films as well as bulk superconductors with a slope that depends on the normal-state sheet resistance---in agreement with experimental data. Thus, our combined experimental and theoretical study reveals universal low-temperature behavior of $B_{c2}$ in a large class of disordered superconductors.

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arXiv2018

Dielectric response of Anderson and pseudogapped insulators

Using a combination of analytic and numerical methods, we study the polarizability of a (non-interacting) Anderson insulator in one, two, and three dimensions and demonstrate that, in a wide range of parameters, it scales proportionally to the square of the localization length, contrary to earlier claims based on the effective-medium approximation. We further analyze the effect of electron-electron interactions on the dielectric constant in quasi-1D, quasi-2D and 3D materials with large localization length, including both Coulomb repulsion and phonon-mediated attraction. The phonon-mediated attraction (in the pseudogapped state on the insulating side of the Superconductor-Insulator Transition) produces a correction to the dielectric constant, which may be detected from a linear response of a dielectric constant to an external magnetic field.

cond-mat.mes-hall↗

arXiv2017

Perturbed Kitaev model: excitation spectrum and long-ranged spin correlations

We developed general approach to the calculation of power-law infrared asymptotics of spin-spin correlation functions in the Kitaev honeycomb model with different types of perturbations. We have shown that in order to find these correlation functions, one can perform averaging of some bilinear forms composed out of free Majorana fermions, and we presented the method for explicit calculation of these fermionic densities. We demonstrated how to derive an effective Hamiltonian for the Majorana fermions, including the effects of perturbations. For specific application of the general theory, we have studied the effect of the Dzyaloshinskii-Moriya anisotropic spin-spin interaction; we demonstrated that it leads, already in the second order over its relative magnitude $D/K$, to a power-law spin correlation functions, and calculated dynamical spin structure factor of the system. We have shown that an external magnetic field $h$ in presence of the DM interaction, opens a gap in the excitation spectrum of magnitude $Δ\propto D h$.

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arXiv2017

Microwave properties of superconductors close to SIT

Strongly disordered pseudogapped superconductors are expected to display arbitrary high values of kinetic inductance close to superconductor-insulator transition (SIT) that makes them attractive for the implementation of dissipationless superinductance. We develop the theory of the collective modes in these superconductors and discuss associated dissipation at microwave frequencies. We obtain the collective mode spectra dependence on the disorder level and conclude that collective modes become relevant source of dissipation and noise in the far vicinity of SIT.

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arXiv2017

Low-energy dynamical response of an Anderson insulator with local attraction

The low-frequency dynamical response of an Anderson insulator is dominated by so-called Mott resonances: hybridization of pairs of states close in energy, but separated spatially. We study the effect of interaction on Mott resonances in the model of spinful fermions (electrons) with local attraction. This model is known to exhibit a so-called pseudogap: a suppression of the low-energy single-particle excitations. Correspondingly, the low-energy dynamical response is also reduced. However this reduction has mostly quantitative character. In particular, the Mott formula for frequency-dependent conductivity preserves its functional asymptotic behavior at low frequencies, but with a small numerical prefactor. This result can be explained in terms of Mott resonances for electron pairs instead of single electrons.

cond-mat.mes-hall↗

arXiv2016

Ultrasonic attenuation in a pseudogapped superconductor

We develop a theory of ultrasound decay rate $α$ in a model of a superconductor with a large pseudogap $Δ_P$, and show that at low temperatures ($T \ll T_c$) the magnitude of the decay rate $α$ is controlled by the ratio of $T/Δ$, where $Δ\ll Δ_P$ is a superconducting collective gap. Thus we propose new method to measure the collective gap $Δ$ in a situation when strong pseudogap is present.

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arXiv2016

Long-range spin correlations in a honeycomb spin model with magnetic field

We consider spin-$\frac{1}{2}$ model on the honeycomb lattice in the presence of weak magnetic field $h\ll J$. Such a perturbation treated in the second order over $h$ leads to the power-law decay of irreducible spin correlation function $S(\mathbf{r},t)=\left\langle \left\langle s^{z}_r(t)s^{z}_0(0)\right\rangle \right\rangle \propto h_{z}^{2}f(t,\mathbf{r})$, where $f(t,\mathbf{r})\propto \lbrack \max (t,Jr)]^{-4}$ is an oscillating function of $\mathbf{r}$, with a wavelength equal to 3 lattice constants. In the present Letter we sum main terms in all orders of the perturbation theory for the correlation function $S(\mathbf{r},t)$ in the limit of large $r,t$. Our results can be understood in terms of the effective low-energy Hamiltonian written in terms of Majorana fermions, which in the presence of magnetic field acquire vector potential $A_x \propto h_z^2$. Correspondingly, the wave vector of the oscillations in $S(\mathbf{r},t)$ changes according to $δk \propto h_z^2$. We also compute the dynamic structure factor $S(\mathbf{p},ω)$; in the vicinity of $\mathbf{p}_K$ corresponding to the inter-conical points excitations it reads as $S(\mathbf{p},ω)-S(\mathbf{p}_K,ω)\propto\sqrt{ω^2-3J^2(\mathbf{p}-\mathbf{p}_K)^2}$.

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arXiv2015

Superfluid density of a pseudogapped superconductor near SIT

We analyze critical behavior of superfluid density $ρ_s$ in strongly disordered superconductors near superconductor-insulator transition and compare it with the behavior of the spectral gap $Δ$ for collective excitations. We show that in contrast to conventional superconductors, the superconductors with preformed pairs display unusual scaling relation $ρ_s \propto Δ^2$ close to superconductor-insulator transition. This relation have been reported in very recent experiments.

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arXiv2015

Magnetic field-induced giant enhancement of electron-phonon energy transfer in strongly disordered conductors

Relaxation of soft modes (e.g. charge density in gated semiconductor heterostructures, spin density in the presence of magnetic field) slowed down by disorder may lead to giant enhancement of energy transfer (cooling power) between overheated electrons and phonons at low bath temperature. We show that in strongly disordered systems with time-reversal symmetry broken by external or intrinsic exchange magnetic field the cooling power can be greatly enhanced. The enhancement factor as large as $10^{2}$ at magnetic field $B \sim 10$ Tesla in 2D {\rm InSb} films is predicted. A similar enhancement is found for the ultrasound attenuation.

cond-mat.mes-hall↗

arXiv2014

Admittance of a long diffusive SNS junction

The dynamical properties of hybrid normal metal/superconductor structures have recently come into research focus both experimentally and theoretically. Recent experimental studies of the coherent admittance $% Y(ω)$ of SNS rings as function of the phase difference $ϕ_{0}$ are still not fully understood. Here we concentrate on the linear response regime, calculating $Y(ω)$ by solving Usadel equations, linearised in electric field. Although partially reproducing previously known results, we find qualitatively different behaviour in the collisionless regime of $τ_{in}^{-1}\ll ω\lesssim E_{Th}$ and high temperature $T\gg E_{Th}$ and low temperature $T\lesssim E_{Th}$ near the minigap closing $ϕ_{0}\sim π$. We find that the dissipative part $\mbox{Re}Y(ω)$ peaks when the minigap closes (at a phase difference of $π$) even at high temperatures, when the equilibrium supercurrent is fully suppressed.

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