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K. B. Efetov

Publications and source records attributed to K. B. Efetov.

At least 19 recordsLinked to original sources
arXiv2019

Spin polarization and orbital effects in superconductor-ferromagnet structures

We study theoretically spontaneous currents and magnetic field induced in a superconductor-ferromagnet (S-F) bilayer due to direct and inverse proximity effects. The induced currents {are Meissner currents that appear even in the absence of an external magnetic field due to the magnetic moment in the ferromagnet }and {to the magnetization } in the superconductor . The latter is induced by the inverse proximity effect over a distance of the order of the superconducting correlation length $ξ_{S}$. On the other hand the magnetic induction $B$, caused by Meissner currents, penetrates the S film over the London length $λ_{S}$. Even though $λ_{S}$ usually exceeds considerably the correlation length, the amplitude and sign of $B$ at distances much larger than $ξ_{S}$ depends crucially on the strength of the exchange energy in the ferromagnet and on the magnetic moment induced in the in the S layer.

cond-mat.supr-con↗
arXiv2015

Specular Interband Andreev Reflections in Graphene

Electrons incident from a normal metal onto a superconductor are reflected back as holes - a process called Andreev reflection. In a normal metal where the Fermi energy is much larger than a typical superconducting gap, the reflected hole retraces the path taken by the incident electron. In graphene with ultra low disorder, however, the Fermi energy can be tuned to be smaller than the superconducting gap. In this unusual limit, the holes are expected to be reflected specularly at the superconductor-graphene interface due to the onset of interband Andreev processes, where the effective mass of the reflected holes change sign. Here we present measurements of gate modulated Andreev reflections across the low disorder van der Waals interface formed between graphene and the superconducting NbSe2. We find that the conductance across the graphene-superconductor interface exhibits a characteristic suppression when the Fermi energy is tuned to values smaller than the superconducting gap, a hallmark for the transition between intraband retro- and interband specular- Andreev reflections.

cond-mat.mes-hall↗
arXiv2015

Quantum criticality in two dimensions and Marginal Fermi Liquid

Kinetic properties of a two dimensional model of fermions interacting with antiferromagnetic spin excitations near the quantum critical point (QCP) are considered. The temperature or doping are assumed to be sufficiently high, such that the pseudogap does not appear. In contrast to standard spin-fermion models, it is assumed that there are intrinsic inhomogeneities in the system suppressing space correlations of the antiferromagnetic excitations. It is argued that the inhomogeneities in the spin excitations in the "strange metal" phase can be a consequence of existence of $π$-shifted domain walls in the doped antiferromagnetic phase. Averaging over the inhomogeneities and calculating physical quantities like resistivity and some others one can explain unusual properties of cuprates unified under the name "Marginal Fermi Liquid" (MFL). The dependence of the slope of the linear temperature dependence of the resistivity on doping is compared with experimental data.

cond-mat.str-el↗
arXiv2014

Vortices and charge order in high-T_c superconductors

We theoretically investigate the vortex state of the cuprate high-temperature superconductors in the presence of magnetic fields. Assuming the recently derived nonlinear $σ$-model for fluctuations in the pseudogap phase, we find that the vortex cores consist of two crossed regions of elliptic shape, in which a static charge order emerges. Charge density wave order manifests itself as satellites to the ordinary Bragg peaks directed along the axes of the reciprocal copper lattice. Quadrupole density wave (bond order) satellites, if seen, are predicted to be along the diagonals. The intensity of the satellites should grow linearly with the magnetic field, in agreement with the result of recent experiments.

cond-mat.supr-con↗
arXiv2014

Radiation-induced quantum Fano-type resonances in the transport of $N$-$P$-$N$ graphene based junctions

We present a theoretical study of quantum resonances in the ballistic transport of graphene based $N$-$P$-$N$ junction subject to an externally applied electromagnetic field (EF). By making use of the Floquet analysis and the quasi-classical approach we analyze the dynamics of electrons in the presence of time and coordinate dependent potential $U(z,t)$. In the absence of EF the resonant tunneling results in a set of sharp resonances in the dependence of dc conductance $σ$ on the gate voltage $V_g$. In irradiated $N$-$P$-$N$ junctions we obtain the Fano-type resonances in the dependence of $σ(V_g)$. This \emph{coherent quantum-mechanical phenomenon} is due to the interplay of two effects: the resonant tunneling through quasi-bound states and the quantum-interference effect in the region between the "resonant points", where the resonant absorption (emission) of photons occurs, and junction interfaces.

cond-mat.mes-hall↗
arXiv2014

Cascade of phase transitions in the vicinity of a quantum critical point

We study the timely issue of charge order checkerboard patterns observed in a variety of cuprate superconductors. We suggest a minimal model in which strong quantum fluctuations in the vicinity of a single antiferromagnetic quantum critical point generate the complexity seen in the phase diagram of cuprates superconductors and, in particular, the evidenced charge order. The Fermi surface is found to fractionalize into hotspots and antinodal regions, where physically different gaps are formed. In the phase diagram, this is reflected by three transition temperatures for the formation of pseudogap, charge density wave, and superconductivity (or quadrupole density wave if a sufficiently strong magnetic field is applied). The charge density wave is characterized by modulations along the bonds of the CuO lattice with wave vectors connecting points of the Fermi surface in the antinodal regions. These features, previously observed experimentally, are so far unique to the quantum critical point in two spatial dimensions and shed a new light on the interplay between strongly fluctuating critical modes and conduction electrons in high-temperature superconductors.

cond-mat.str-el↗
arXiv2013

Effect of magnetic field on competition between superconductivity and charge order below the pseudogap state

We theoretically investigate the T-B phase diagram of cuprates building on the SU(2) pseudogap order recently obtained from the spin-fermion model. At low temperatures, our analysis reveals an almost temperature-independent critical magnetic field B_{QDW} at which d-wave superconductivity switches to the quadrupole density wave (QDW) phase. For temperatures beyond a certain value of the order of the zero-field superconducting transition temperature T_c, the critical field B_{QDW}(T) sharply grows. We compare our results derived within the effective SU(2) non-linear sigma-model with the phase diagram recently obtained by sound velocity measurements by LeBoeuf et al. [Nat. Phys. 9, 79 (2013)].

cond-mat.str-el↗
arXiv2013

Pseudogap state from quantum criticality

Upon application of an external tuning parameter, a magnetic state can be driven to a normal metal state at zero temperature. This phenomenon is known as quantum criticality and leads to fascinating responses in thermodynamics and transport of the compound. In the standard picture, a single quantum critical point occurs at zero temperature, which results in a nontrivial critical behaviour in its vicinity. Here we show that in two dimensions the scenario is considerably more complex due to the enormous amount of quantum fluctuations. Instead of the single point separating the antiferromagnet from the normal metal, we have discovered a broad region between these two phases where the magnetic order is destroyed but certain areas of the Fermi surface are closed by a large gap. This gap reflects the formation of a novel quantum state characterised by a superposition of d-wave superconductivity and a quadrupole-density wave, id est a state in which an electron quadrupole density spatially oscillates with a period drastically different from the one of the original spin-density wave. At moderate temperatures both orders co-exist at short distances but thermal fluctuations destroy the long-range order. Below a critical temperature the fluctuations are less essential and superconductivity becomes stable. This new phenomenon may shed some light on the origin of the mysterious pseudogap state and of the high-temperature transition into the superconducting state in cuprates. Our results demonstrate that quantum phase transitions between antiferromagnets and normal metals in layered materials may be the proper playground for search of new high temperature superconductors.

cond-mat.str-el↗
arXiv2012

Fermionic and bosonic ac conductivities at strong disorder

We study the ac conduction in a system of fermions or bosons strongly localised in a disordered array of sites with short-range interactions at frequencies larger than the intersite tunnelling but smaller than the characteristic fluctuation of the on-site energy. While the main contribution $σ_0(ω)$ to the conductivity comes from local dipole-type excitations on close pairs of sites, coherent processes on three or more sites lead to an interference correction $σ_1(ω)$, which depends on the statistics of the charge carriers and can be suppressed by magnetic field. For bosons the correction is always positive, while for fermions it can be positive or negative depending on whether the conduction is dominated by effective single-particle or single-hole processes. We calculate the conductivity explicitly assuming a constant density of states of single-site excitations. Independently of the statistics, $σ_0(ω)=const$. For bosons $σ_1(ω)\propto \log(C/ω)$. For fermions $σ_1(ω)\propto\log[\max(A,ω)/ω]-\log[\max(B,ω)/ω]$, where the first and the second term are respectively the particle and hole contributions, $A$ and $B$ being the particle and hole energy cutoffs. The ac magnetoresistance has the same sign as $σ_1(ω)$.

cond-mat.mes-hall↗
arXiv2012

Low energy excitations and singular contributions in the thermodynamics of clean Fermi liquids

Using a recently suggested method of bosonization in an arbitrary dimension, we study the anomalous contribution of the low energy spin and charge excitations to thermodynamic quantities of a two-dimensional (2D) Fermi liquid. The method is slightly modified for the present purpose such that the effective supersymmetric action no longer contains the high energy degrees of freedom but still accounts for effects of the finite curvature of the Fermi surface. Calculating the anomalous contribution $δc(T)$ to the specific heat, we show that the leading logarithmic in temperature corrections to $δc(T)/T^2$ can be obtained in a scheme combining a summation of ladder diagrams and renormalization group equations. The final result is represented as the sum of two separate terms that can be interpreted as coming from singlet and triplet superconducting excitations. The latter may diverge in certain regions of the coupling constants, which should correspond to the formation of triplet Cooper pairs.

cond-mat.str-el↗
arXiv2012

Strong quantum interference in strongly disordered bosonic insulators

We study the variable-range hopping (VRH) of bosons in an array of sites with short-range interactions and a large characteristic coordination number. The latter leads to strong quantum interference phenomena yet allows for their analytical study. We develop a functional renormalization group scheme that repeatedly eliminates high-energy sites properly renormalizing the tunnelling between the low-energy ones. Using this approach we determine the temperature and magnetic field dependence of the hopping conductivity and find a large positive magnetoresistance. With increasing magnetic field the behaviour of the conductivity crossovers from the Mott's law to an activational behaviour with the activation gap proportional to the magnetic field.

cond-mat.mes-hall↗
arXiv2012

Josephson-like spin current in junctions composed of antiferromagnets and ferromagnets

We study Josephson-like junctions formed by materials with antiferromagnetic (AF) order parameters. As an antiferromagnet, we consider a two-band material in which a spin density wave (SDW) arises. This could be Fe-based pnictides in the temperature interval ${T_{\text{c}}\leq T\leq T_{N}}$, where $T_{c}$ and $T_{N}$ are the critical temperatures for the superconducting and antiferromagnetic transitions, respectively. The spin current $j_{\text{Sp}}$ in AF/F/AF junctions with a ballistic ferromagnetic layer and in tunnel AF/I/AF junctions is calculated. It depends on the angle between the magnetization vectors in the AF leads in the same way as the Josephson current depends on the phase difference of the superconducting order parameters in S/I/S tunnel junctions. It turns out that in AF/F/AF junctions, two components of the SDW order parameter are induced in the F\nobreakdash-layer. One of them oscillates in space with a short period ${ξ_{\text{F,b}} \sim \hbar v/\mathcal{H}}$ while the other decays monotonously from the interfaces over a long distance of the order ${ξ_{\text{N,b}}=\hbar v/2πT}$ (where $v$, $\mathcal{H}$ and $T$ are the Fermi velocity, the exchange energy and the temperature, respectively; the subindex $\text{b}$ denotes the ballistic case). This is a clear analogy with the case of Josephson S/F/S junctions with a nonhomogeneous magnetization where short- and long\nobreakdash-range condensate components are induced in the F\nobreakdash-layer. However, in contrast to the charge Josephson current in S/F/S junctions, the spin current in AF/F/AF junctions is not constant in space, but oscillates in the ballistic F\nobreakdash-layer. We also calculate the dependence of $j_{\text{Sp}}$ on the deviation from the ideal nesting in the AF/I/AF junctions.

cond-mat.supr-con↗
arXiv2011

Possibility of superconductivity due to electron-phonon interaction in graphene

We discuss the possibility of superconductivity in graphene taking into account both electron-phonon and electron-electron Coulomb interactions. The analysis is carried out assuming that the Fermi energy is far away from the Dirac points, such that the density of the particles (electrons or holes) is high. We derive proper Eliashberg equations that allow us to estimate the critical superconducting temperature. The most favorable is pairing of electrons belonging to different valleys in the spectrum. Using values of electron-phonon coupling estimated in other publications we obtain the critical temperature T_c as a function of the electron (hole) density. This temperature can reach the order of 10 K at the Fermi energy of order 1-2 eV. We show that the dependence of the intervalley pairing on the impurity concentration should be weak.

cond-mat.supr-con↗
arXiv2011

Interaction of Josephson and magnetic oscillations in Josephson tunnel junctions with a ferromagnetic layer

We study the dynamics of Josephson junctions with a thin ferromagnetic layer F [superconductor-ferromagnet-insulator-ferromagnet-superconductor (SFIFS) junctions]. In such junctions, the phase difference $ϕ$ of the superconductors and magnetization $M$ in the F layer are two dynamic parameters coupled to each other. We derive equations describing the dynamics of these two parameters and formulate the conditions of validity. The coupled Josephson plasma waves and oscillations of the magnetization $M$ affect the form of the current-voltage ($I$-$V$) characteristics in the presence of a weak magnetic field (Fiske steps). We calculate the modified Fiske steps and show that the magnetic degree of freedom not only changes the form of the Fiske steps but also the overall view of the $I$-$V$ curve (new peaks related to the magnetic resonance appear). The $I$-$V$ characteristics are shown for different lengths of the junction including those which correspond to the current experimental situation. We also calculate the power $P$ absorbed in the system if a microwave radiation with an ac in-plane magnetic field is applied (magnetic resonance). The derived formula for the power $P$ essentially differs from the one which describes the power absorption in an isolated ferromagnetic film. In particular, this formula describes the peaks related to the excitation of standing plasma waves as well as the peak associated with the magnetic resonance.

cond-mat.supr-con↗
arXiv2011

Localization and critical diffusion of quantum dipoles in two dimensions

We discuss quantum propagation of dipole excitations in two dimensions. This problem differs from the conventional Anderson localization due to existence of long range hops. We found that the critical wavefunctions of the dipoles always exist which manifest themselves by a scale independent diffusion constant. If the system is T-invariant the states are critical for all values of the parameters. Otherwise, there can be a "metal-insulator" transition between this "ordinary" diffusion and the Levy-flights (the diffusion constant logarithmically increasing with the scale). These results follow from the two-loop analysis of the modified non-linear supermatrix $σ$-model.

cond-mat.dis-nn↗
arXiv2011

Quasiclassical description of a superconductor with a spin density wave

We derive equations for the quasiclassical Green's functions $\check{g}$ within a simple model of a two-band superconductor with a spin-density-wave (SDW). The elements of the matrix $\check{g}$ are the retarded, advanced, and Keldysh functions each of which is an $8\times 8$ matrix in the Gor'kov-Nambu, the spin and the band space. In equilibrium, these equations are a generalization of the Eilenberger equation. On the basis of the derived equations we analyze the Knight shift, the proximity and the dc Josephson effects in the superconductors under consideration. The Knight shift is shown to depend on the orientation of the external magnetic field with respect to the direction of the vector of the magnetization of the SDW. The proximity effect is analyzed for an interface between a superconductor with the SDW and a normal metal. The function describing both superconducting and magnetic correlations is shown to penetrate the normal metal or a metal with the SDW due to the proximity effect. The dc Josephson current in an $S_{SDW}/N/S_{SDW}$ junction is also calculated as a function of the phase difference $ϕ$. It is shown that in our model the Josephson current does not depend on the mutual orientation of the magnetic moments in the superconductors $S_{SDW}$ and is proportional to $ \sin ϕ$. The dissipationless spin current $j_{sp}$ depends on the angle $α$ between the magnetization vectors in the same way ($j_{sp} \sim \sin α$) and is not zero above the superconducting transition temperature.

cond-mat.supr-con↗
arXiv2011

Photocurrent in a visible-light graphene photodiode

We calculate the photocurrent in a clean graphene sample normally irradiated by a monochromatic electromagnetic field and subject to a step-like electrostatic potential. We consider the photon energies $\hbarΩ$ that significantly exceed the height of the potential barrier, as is the case in the recent experiments with graphene-based photodetectors. The photocurrent comes from the resonant absorption of photons by electrons and decreases with increasing ratio $\hbarΩ/U_0$. It is weakly affected by the background gate voltage and depends on the light polarization as $\propto\sin^2γ$, $γ$ being the angle between the potential and the polarization plane.

cond-mat.mes-hall↗