arXiv subjects

Alex Levchenko

Publications and source records attributed to Alex Levchenko.

At least 91 records · Page 5Linked to original sources

arXiv2021

Hybridization mechanism of the dual proximity effect in superconductor-topological insulator interfaces

In this communication we consider generalities of the proximity effect in a contact between a conventional $s$-wave superconductor (S) nano-island and a thin film of a topological insulator (TI). A local hybridization coupling mechanism is considered and a corresponding model is corroborated that captures not only the induced unconventional superconductivity in a TI, but also predicts the spreading of topologically protected surface states into the superconducting over-layer. This dual nature of the proximity effect leads specifically to a modified description of topological superconductivity in these systems. Experimentally accessible signatures of this phenomenon are discussed in the context of scanning tunneling microscopy measurements. For this purpose an effective density of states is computed in both the superconductor and topological insulator. As a guiding example, practical applications are made for Nb islands deposited on a surface of Bi$_2$Se$_3$. The obtained results are general and can be applied beyond the particular material system used. Possible implications of these results to proximity circuits and hybrid hardware devices for quantum computation processing are discussed.

cond-mat.supr-con↗

arXiv2020

Anomalous thermodynamic properties of quantum critical superconductors

Recent high-precision measurements employing different experimental techniques have unveiled an anomalous peak in the doping dependence of the London penetration depth which is accompanied by anomalies in the heat capacity in iron-pnictide superconductors at the optimal composition associated with the hidden antiferromagnetic quantum critical point. We argue that finite temperature effects can be a cause of observed features. Specifically we show that quantum critical magnetic fluctuations under superconducting dome can give rise to a nodal-like temperature dependence of both specific heat and magnetic penetration depth in a fully gapped superconductor. In the presence of line nodes in the superconducting gap fluctuations can lead to the significant renormalization of the relative slope of $T$-linear penetration depth which is steepest at the quantum critical point. The results we obtain are general and can be applied beyond the model we use.

cond-mat.supr-con↗

arXiv2020

Hydrodynamic electron transport near charge neutrality

We develop the theory of hydrodynamic electron transport in a long-range disorder potential for conductors in which the underlying electron liquid lacks Galilean invariance. For weak disorder, we express the transport coefficients of the system in terms of the intrinsic kinetic coefficients of the electron liquid and the correlation function of the disorder potential. We apply these results to analyze the doping and temperature dependence of transport coefficients of graphene devices. We show that at charge neutrality, long-range disorder increases the conductivity of the system above the intrinsic value. The enhancement arises from the predominantly vortical hydrodynamic flow caused by local deviations from charge neutrality. Its magnitude is inversely proportional to the shear viscosity of the electron liquid and scales as the square of the disorder correlation radius. This is qualitatively different from the situation away from charge neutrality. In that case, the flow is predominantly potential, and produces negative viscous contributions to the conductivity, which are proportional to the sum of shear and bulk viscosities, and inversely proportional to the square of disorder correlation radius.

cond-mat.mes-hall↗

arXiv2020

Transport properties of strongly coupled electron-phonon liquids

In this work we consider the hydrodynamic behavior of a coupled electron-phonon fluid, focusing on electronic transport under the conditions of strong phonon drag. This regime occurs when the rate of phonon equilibration due to e.g. umklapp scattering is much slower than the rate of normal electron-phonon collisions. Then phonons and electrons form a coupled out-of-equilibrium state where the total quasi-momentum of the electron-phonon fluid is conserved. A joint flow-velocity emerges as a collective hydrodynamic variable. We derive the equation of motion for this fluid from the underlying microscopic kinetic theory and elucidate its effective viscosity and thermal conductivity. In particular, we derive decay times of arbitrary harmonics of the distribution function and reveal its corresponding super-diffusive relaxation on the Fermi surface. We further consider several applications of this theory to magneto-transport properties in the Hall-bar and Corbino-disk geometries, relevant to experiments. In our analysis we allow for general boundary conditions that cover the crossover from no-slip to no-stress flows. Our approach also covers a crossover from the Stokes to the Ohmic regime under the conditions of the Gurzhi effect. In addition, we consider the frequency dependence of the surface impedance and non-equilibrium noise. For the latter, we notice that in the diffusive regime, a Fokker-Planck approximation, applied to the electron-phonon collision integral in the Eliashberg form, reduces it to a differential operator with Burgers nonlinearity. As a result, the non-equilibrium distribution function has a shock-wave structure in the energy domain. The consequence of this behavior for the Fano factor of the noise is investigated. In conclusion we discuss connections and limitations of our results in the context of recent electron-phonon drag measurements in Dirac and Weyl semimetals.

cond-mat.mes-hall↗

arXiv2020

Dirac surface states in superconductors: a dual topological proximity effect

In this paper we present scanning tunneling microscopy of Bi$_2$Se$_3$ with superconducting Nb deposited on the surface. We find that the topologically protected surface states of the Bi$_2$Se$_3$ leak into the superconducting over-layer, suggesting a dual topological proximity effect. Coupling between theses states and the Nb states leads to an effective pairing mechanism for the surface states, leading to a modified model for a topological superconductor in these systems. This model is consistent with fits between the experimental data and the theory.

cond-mat.supr-con↗

arXiv2020

Fluctuational Anomalous Hall and Nernst Effects in Superconductors

In this work we develop microscopic kinetic theory of the anomalous Hall and Nernst effects in superconductors induced by fluctuations in the vicinity of the critical transition temperature. The technical analysis is carried out within the Matsubara diagrammatic technique and linear response Kubo formula with disorder averaging and an analytical continuation. It is shown that asymmetric skew-scattering due to spin-orbit interaction gives rise to a new anomalous Hall conductance promoted by Maki-Thompson interference and density of states fluctuational effects. The side-jump mechanism of the anomalous Hall effect is present only in the density of states contributions. The anomalous Nernst effect is found due to a special nonlinear Aslamazov-Larkin term from the quantum-crossing of interacting fluctuations in a Hikami box with the skew-scattering process. In addition to calculations in the weak-coupling BCS model we also sketch an approach for the account of fluctuational transport effects in the strong-coupling limit. In particular we explore an example of how Keldysh version of the Eliashberg theory can in principle be constructed and estimate fluctuation-induced conductivity within conventional electron-phonon coupling scenario. These ideas may pave the way for further developments of strong coupling theories in applications to fluctuations in unconventional superconductors and anomalous transport responses in particular.

cond-mat.supr-con↗

arXiv2020

Temperature Dependence of the Anomalous Hall Effect from Electron Interactions

We consider the impact of electron-electron interactions on the temperature dependence of the anomalous Hall effect in disordered conductors. The microscopic analysis is carried out within the diagrammatic approach of the linear response Kubo-Streda formula with an account of both extrinsic skew-scattering and side-jump mechanisms of the anomalous Hall effect arising in the presence of spin-orbit coupling. We demonstrate the importance of electron interactions in the Cooper channel even for nominally non-superconducting materials and find that the corresponding low-temperature dependence of the anomalous Hall conductivity is asymptotically of the form $\sqrt{T}/\ln(T_0/T)$ in three dimensions and $\ln[\ln(T_0/T)]$ in two dimensions, where the scale of $T_0$ is parametrically of the order of Fermi energy. These results, in particular, may provide an explanation for the recently observed unconventional temperature dependence of the anomalous Hall effect in HgCr$_2$Se$_4$.

cond-mat.mes-hall↗

arXiv2019

Quasiclassical circuit-theory of contiguous disordered multiband superconductors

We consider a general problem of a Josephson contact between two multiband superconductors with coexisting superconducting and magnetic phases. As a particular example, we use the quasiclassical theory of superconductivity to study the properties of a Josephson contact between two disordered $s^{\pm}$-wave superconductors allowing for the coexistence between superconductivity and spin-density-wave orders. The intra- and inter-band scattering effects of disorder are treated within the self-consistent Born approximation. We calculate the spatial profile of the corresponding order parameters on both sides of the interface assuming that the interface has finite reflection coefficient and use our results to evaluate the local density of states at the interface as well as critical supercurrent through the junction as a function of phase or applied voltage. Our methods are particularly well suited for describing spatially inhomogeneous states of iron-based superconductors where controlled structural disorder can be created by an electron irradiation. We reveal the connection between our theory and the circuit-theory of Andreev reflection and extend it to superconducting junctions of arbitrary nature. Lastly, we outline directions for further developments in the context of proximity circuits of correlated electron systems.

cond-mat.str-el↗

arXiv2019

Anomalous mesoscopic kinetics in disordered superconductors

We study anomalous mesoscopic transport effects at the onset of the superconducting transition focusing on the observed large Nernst-Ettinghausen signal in disordered thin films. In the vicinity of the transition, as the Ginzburg-Landau coherence length of preformed Cooper pairs diverges, short-range mesoscopic fluctuations are equivalent to local fluctuations of the critical temperature. As a result, the dynamical susceptibility function of pair propagation acquires a singular mesoscopic component, and consequently, superconducting correlations give rise to enhanced mesoscopic fluctuations of thermodynamic and transport characteristics. In contrast to disordered normal metals, the root-mean-square value of mesoscopic conductivity fluctuations ceases to be universal and displays strong dependence on dimensionality, temperature, and under certain conditions can exceed its quantum normal state value by a large factor. Interestingly, we find different universality as magnetic susceptibility, conductivity, and transverse magnetic thermopower coefficients all display the same temperature dependence. Finally, we discuss an enhancement of mesoscopic effects in the Seebeck thermoelectricity and Hall conductivity fluctuations as mediated by emergent superconductivity.

cond-mat.supr-con↗

arXiv2019

Topological supercurrents interaction and fluctuations in the multiterminal Josephson effect

We study the Josephson effect in the multiterminal junction of topological superconductors. We use the symmetry-constrained scattering matrix approach to derive band dispersions of emergent sub-gap Andreev bound states in a multidimensional parameter space of superconducting phase differences. We find distinct topologically protected band crossings that serve as monopoles of finite Berry curvature. Particularly, in a four-terminal junction the admixture of $2π$ and $4π$ periodic levels leads to the appearance of finite energy Majorana-Weyl nodes. This topological regime in the junction can be characterized by a quantized nonlocal conductance that measures the Chern number of the corresponding bands. In addition, we calculate current-phase relations, variance, and cross-correlations of topological supercurrents in multiterminal contacts and discuss the universality of these transport characteristics. At the technical level these results are obtained by integrating over the group of a circular ensemble that describes the scattering matrix of the junction. We briefly discuss our results in the context of observed fluctuations of the gate dependence of the critical current in topological planar Josephson junctions and comment on the possibility of parity measurements from the switching current distributions in multiterminal Majorana junctions.

cond-mat.supr-con↗

arXiv2019

Negative Viscosity and Eddy Flow of Imbalanced Electron-Hole Liquid in Graphene

We present a hydrodynamic theory for electron-hole magnetotransport in graphene incorporating carrier-population imbalance, energy, and momentum relaxation processes. We focus on the electric response and find that the carrier and energy imbalance relaxation processes strongly modify the shear viscosity, so that an effective viscosity can be negative in the vicinity of charge neutrality. We predict an emergent eddy flow pattern of swirling currents and explore its manifestation in nonlocal resistivity oscillations in a strip of graphene driven by a source current.

cond-mat.mes-hall↗

arXiv2018

Josephsonic diagnostic of competing orders in quantum critical multiband superconductors

Motivated by the recent experimentally observed manifestations of the quantum critical point fluctuations in the thermodynamic properties of multiband superconductors, we derive a general expression for the Josephson current of various junctions between two superconductors in the phase of superconductivity coexistence with the spin-density-wave. We demonstrate that the critical current peaks at the quantum critical point that separates pure and mixed superconducting phases. We argue that our results are generic and, in particular, can be adopted to explain the recent observations of a nonmonotonic dependence of the supercurrent on the external pressure in the heavy fermion superconductor CeRhIn$_5$, and on the chemical doping in iron-based superconductors such as Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$.

cond-mat.supr-con↗

arXiv2018

Josephson currents in chaotic quantum dots

We study theoretically the Josephson current-phase relationship in a chaotic quantum dot coupled to superconductors by ballistic contacts. In this regime, strong proximity effect induces superconductivity in the quantum dot that leads to a significant modification in the electron density of states and formation of multiple sub-gaps. The magnitude of the resulting supercurrent depends on the phase difference of the superconducting order parameter in the leads and shows strongly anharmonic skewed behavior. We find that when the Thouless energy on the dot exceeds the superconducting energy gap, the second harmonic of the supercurrent becomes comparable in magnitude to the first harmonic. To address these effects on the technical level, we use the nonlinear $σ$-model Keldysh formalism in the framework of the circuit theory to compute dependence of the density of states, Josephson energy, and current on the superconducting phases in the leads. We analyze how these quantities change as a function of the Thouless energy and the superconducting gap. Finally, we briefly discuss sub-gap tail states, mesoscopic supercurrent fluctuations, weak localization correction, and also touch on anharmonicity of gatemon qubits with quantum dot Josephson junctions.

cond-mat.supr-con↗

arXiv2018

Superconductivity-localization interplay and fluctuation magnetoresistance in epitaxial BaPb$_{1-x}$Bi$_x$O$_3$ thin films

BaPb$_{1-x}$Bi$_x$O$_3$ is a superconductor, with transition temperature $T_c=11$ K, whose parent compound BaBiO$_3$ possess a charge ordering phase and perovskite crystal structure reminiscent of the cuprates. The lack of magnetism simplifies the BaPb$_{1-x}$Bi$_{x}$O$_3$ phase diagram, making this system an ideal platform for contrasting high-$T_c$ systems with isotropic superconductors. Here we use high-quality epitaxial thin films and magnetotransport to demonstrate superconducting fluctuations that extend well beyond $T_c$. For the thickest films (thickness above $\sim100$ nm) this region extends to $\sim27$ K, well above the bulk $T_c$ and remarkably close to the higher $T_c$ of Ba$_{1-x}$K$_x$BiO$_3$ ($T_c=31$ K). We drive the system through a superconductor-insulator transition by decreasing thickness and find the observed $T_c$ correlates strongly with disorder. This material manifests strong fluctuations across a wide range of thicknesses, temperatures, and disorder presenting new opportunities for understanding the precursor of superconductivity near the 2D-3D dimensionality crossover.

cond-mat.supr-con↗

arXiv2018

Thermalization of dipole oscillations in confined systems by rare collisions

We study the relaxation of the center-of-mass, or dipole oscillations in the system of interacting fermions confined spatially. With the confinement frequency $ω_{\perp}$ fixed the particles were considered to freely move along one (quasi-1D) or two (quasi-2D) spatial dimensions. We have focused on the regime of rare collisions, such that the inelastic collision rate, $1/τ_{in} \ll ω_{\perp}$. The dipole oscillations relaxation rate, $1/τ_{\perp}$ is obtained at three different levels: by direct perturbation theory, solving the integral Bethe-Salpeter equation and applying the memory function formalism. As long as anharmonicity is weak, $1/τ_{\perp} \ll 1/ τ_{in}$ the three methods are shown to give identical results. In quasi-2D case $1/τ_{\perp} \neq 0$ at zero temperature. In quasi-1D system $1/τ_{\perp} \propto T^3$ if the Fermi energy, $E_F$ lies below the critical value, $E_F < 3 ω_{\perp}/4$. Otherwise, unless the system is close to integrability, the rate $1/τ_{\perp}$ has the temperature dependence similar to that in quasi-2D. In all cases the relaxation results from the excitation of particle-hole pairs propagating along unconfined directions resulting in the relationship $1/τ_{\perp} \propto 1/τ_{in}$, with the inelastic rate $1/τ_{in} \neq 0$ as the phase-space opens up at finite energy of excitation, $\hbar ω_{\perp}$. While $1/τ_{\perp} \propto τ_{in}$ in the hydrodynamic regime, $ω_{\perp} \ll 1/τ_{in}$, in the regime of rare collisions, $ω_{\perp} \gg 1/τ_{in}$, we obtain the opposite trend $1/τ_{\perp} \propto 1/τ_{in}$.

cond-mat.mes-hall↗

arXiv2018

Controlled-Z gate for transmon qubits coupled by semiconductor junctions

We analyze the coupling of two qubits via an epitaxial semiconducting junction. In particular, we consider three configurations that include pairs of transmons or gatemons as well as gatemon-like two qubits formed by an epitaxial four-terminal junction. These three configurations provide an electrical control of the interaction between the qubits by applying voltage to a metallic gate near the semiconductor junction and can be utilized to naturally realize a controlled-Z gate (CZ). We calculate the fidelity and timing for such CZ gate. We demonstrate that in the absence of decoherence, the CZ gate can be performed under $50\ {\rm ns}$ with gate error below $10^{-4}$.

quant-ph↗

arXiv2017

Hydrodynamic flows of non-Fermi liquids: magnetotransport and bilayer drag

We consider a hydrodynamic description of transport for generic two dimensional electron systems that lack Galilean invariance and do not fall into the category of Fermi liquids. We study magnetoresistance and show that it is governed only by the electronic viscosity provided that the wavelength of the underlying disorder potential is large compared to the microscopic equilibration length. We also derive the Coulomb drag transresistance for double-layer non-Fermi liquid systems in the hydrodynamic regime. As an example, we consider frictional drag between two quantum Hall states with half-filled lowest Landau levels, each described by a Fermi surface of composite fermions coupled to a $U(1)$ gauge field. We contrast our results to prior calculations of drag of Chern-Simons composite particles and place our findings in the context of available experimental data.

cond-mat.str-el↗

arXiv2017

Topological Andreev bands in three-terminal Josephson junctions

We study the emergent band topology of subgap Andreev bound states in the three-terminal Josephson junctions. We scrutinize the symmetry constraints of the scattering matrix in the normal region connecting superconducting leads that enable the topological nodal points in the spectrum of Andreev states. When the scattering matrix possesses time-reversal symmetry, the gap closing occurs at special stationary points that are topologically trivial as they carry vanishing Berry fluxes. In contrast, for the time-reversal broken case we find topological monopoles of the Berry curvature and corresponding phase transition between states with different Chern numbers. The latter is controlled by the structure of the scattering matrix that can be tuned by a magnetic flux piercing through the junction area in a three-terminal geometry. The topological regime of the system can be identified by nonlocal conductance quantization that we compute explicitly for a particular parametrization of the scattering matrix in the case where each reservoir is connected by a single channel.

cond-mat.mes-hall↗