arXiv subjects

K. B. Efetov

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

At least 55 records · Page 3Linked to original sources

arXiv2007

Superbosonization formula and its application to random matrix theory

Starting from Gaussian random matrix models we derive a new supermatrix field theory model. In contrast to the conventional non-linear sigma models, the new model is applicable for any range of correlations of the elements of the random matrices. We clarify the domain of integration for the supermatrices, and give a demonstration of how the model works by calculating the density of states for an ensemble of almost diagonal matrices. It is also shown how one can reduce the supermatrix model to the conventional sigma model.

cond-mat.mes-hall↗

arXiv2007

Nonhomogeneous magnetization and superconductivity in superconductor-ferromagnet structures

We study two different superconductor-ferromagnet (S/F) structures. We consider first a Josephson junction which consists of two S/F bilayers separated by an insulating layer. We show that for an antiparallel alignment of the magnetization in the two F layers the Josephson critical current $I_c$ increases with increasing exchange field $h$. The second system we consider is a S/F structure with a local inhomogeneity of the magnetization in the ferromagnet near the S/F interface. Due to the proximity effect not only a singlet but also a triplet component of the superconducting condensate is induced in the ferromagnet. The latter penetrates over the length $\sqrt{D/ε}$ ($D$ is the diffusion coefficient and $ε$ the energy). In the case of temperatures of the order of the Thouless energy this length is comparable to the length of the ferromagnet. This long-range penetration leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature $T_c$. Contrary to the case of the singlet component, the contribution to the conductance due to the odd triplet component is not zero at $T = 0$ and $V = 0$ ($V$ is the voltage) and decays with increasing temperature T in a monotonic way

cond-mat.supr-con↗

arXiv2007

Scattering by magnetic and spin-orbit impurities and the Josephson current in superconductor-ferromagnet-superconductor junctions

We analyze the Josephson current in a junction consisting of two superconductors (S) and a ferromagnetic layer (F) for arbitrary impurity concentration. In addition to non-magnetic impurities, we consider also magnetic ones and spin-orbit scattering. In the limit of weak proximity effect we solve the linearized Eilenberger equation and derive an analytical expression for the Josephson critical current valid in a broad range of parameters. This expression enables us to obtain not only known results in the dirty and clean limits but also in a intermediate region of the impurity concentration, which may be very important for comparison with experimental data.

cond-mat.supr-con↗

arXiv2007

Josephson effect due to the long-range odd-frequency triplet superconductivity in SFS junctions with Neel domain walls

We consider a SFS Josephson junction made of two superconductors S and a multidomain ferromagnet F with an in-plane magnetization. We assume that the neighboring domains of the ferromagnet are separated by Neel domain walls. An odd-frequency triplet long-range component of superconducting correlations arises in the domain walls and spreads into the domains over a long distance of the order $ξ_T = \sqrt{D / 2 πT}$, where $D$ is the diffusion coefficient (dirty limit is implied). We calculate the contribution of this component to the Josephson current in the situation when conventional short-range components exponentially decay over the thickness of the F layer and can be neglected. In the limit when the thickness of the F layer is much smaller than the penetration length of the long-range component, we find that the junction is in the $π$ state. We also analyze a correction to the density of states due to the long-range triplet component.

cond-mat.supr-con↗

arXiv2007

Electromagnetic field induced suppression of transport through $n$-$p$ junctions in graphene

We study quasi-particle transmission through an $n $-$p$ junction in a graphene irradiated by an electromagnetic field (EF). In the absence of EF the electronic spectrum of undoped graphene is gapless, and one may expect the perfect transmission of quasi-particles flowing perpendicular to the junction. We demonstrate that the resonant interaction of propagating quasi-particles with the component of EF parallel to the junction induces a \textit{non-equilibrium dynamic gap} $(2Δ_R)$ between electron and hole bands in the quasi-particle spectrum of graphene. In this case the strongly suppressed quasi-particle transmission is only possible due to interband tunnelling. The effect may be used for controlling transport properties of diverse structures in graphene, like, e.g., $n$-$p$-$n$ transistors, single electron transistors, quantum dots, etc., by variation of the intensity $S$ and frequency $ω$ of the external radiation.

cond-mat.mes-hall↗

arXiv2007

Quantum dots in graphene

We suggest a way of confining quasiparticles by an external potential in a small region of a graphene strip. Transversal electron motion plays a crucial role in this confinement. Properties of thus obtained graphene quantum dots are investigated theoretically for different types of the boundary conditions at the edges of the strip. The (quasi)bound states exist in all systems considered. At the same time, the dependence of the conductance on the gate voltage carries an information about the shape of the edges.

cond-mat.mes-hall↗

arXiv2006

Proximity Effects in Ferromagnet/Superconductor Heterostructures

We review the present status of the experimental and theoretical research on the proximity effect in heterostructures composed of superconducting (S) and ferromagnetic (F) thin films. First, we discuss traditional effects originating from the oscillatory behavior of the superconducting pair wave function in the F-layer. Then, we concentrate on recent theoretical predictions for S/F layer systems. These are a) generation of odd triplet superconductivity in the F-layer and b) ferromagnetism induced in the S-layer below the superconducting transition temperature $T_{c}$ (inverse proximity effect). The second part of the review is devoted to discussion of experiments relevant to the theoretical predictions of the first part. In particular, we present results of measurements of the critical temperature $T_{c}$ as a function of the thickness of F-layers and we review experiments indicating existence of odd triplet superconductivity, cryptoferromagnetism and inverse proximity effect.

cond-mat.supr-con↗

arXiv2006

Collective mode description for a clean Fermi gas with repulsion in arbitrary dimensions: interaction of spin excitations and its consequences

We discuss a recent bosonization method developed to study clean Fermi gases with a repulsion in any dimensions. The method enables one to consider both density and spin excitations. It is demonstrated that due to a non-abelian structure of the effective theory, the spin excitations interact with each other, which leads to new logarithmic in temperature corrections to physical quantities. Using a renormalization group scheme constructed for the effective low energy field theory these logarithms are summed up in all orders. Temperature dependent corrections to the specific heat and spin susceptibility are obtained for all dimensions d=1,2,3.

cond-mat.str-el↗

arXiv2006

Hall resistivity of granular metals

We calculate the Hall conductivity $\sig_{xy}$ and resistivity $ρ_{xy}$ of a granular system at large tunneling conductance $g_{T}\gg 1$. We show that in the absence of Coulomb interaction the Hall resistivity depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula $ρ_{xy}=H/(n^* e c)$, where $n^*$ differs from the carrier density $n$ inside the grains by a numerical coefficient determined by the shape of the grains. The Coulomb interaction gives rise to logarithmic in temperature $T$ correction to $ρ_{xy}$ in the range $\Ga \lesssim T \lesssim \min(g_T E_c,\ETh)$, where $\Ga$ is the tunneling escape rate, $E_c$ is the charging energy and $\ETh$ is the Thouless energy of the grain.

cond-mat.mes-hall↗

arXiv2006

Effect of Disorder on Transport in Graphene

Quenched disorder in graphene is characterized by 5 constants and experiences the logarithmic renormalization even from the spatial scales smaller than the Fermi wavelength. We derive and solve renormalization group equations (RGEs) describing the system at such scales. At larger scales, we derive a non-linear supermatrix $σ$-model completely describing localization and crossovers between different ensembles. The parameters of this $σ$% -model are determined by the solutions of the RGEs.

cond-mat.dis-nn↗

arXiv2006

Comment on ''Superconducting decay length in a ferromagnetic metal''

In the paper " Superconducting decay length in a ferromagnetic metal" by Gusakova, Kupriyanov and Golubov [Pis'ma v ZhETF 83, 487 (2006); cond-mat/0605137], the authors claim that they solved the linearized Eilenberger equation in the ferromagnetic region of an S/F heterostructure at arbitrary mean free path. In this comment we show that the solution suggested by the authors is not correct and explain details of the exact solution found by us in an earlier work several years ago (Phys. Rev. B 64, 134506, (2001).

cond-mat.supr-con↗

arXiv2006

Temperature dependence of the spin susceptibility of a clean Fermi gas with repulsion

Spin susceptibility of a clean Fermi gas with repulsion in any dimension is considered using a supersymmetric low energy theory of interacting spin excitations and renormalization scheme recently proposed by Aleiner and Efetov [cond-mat/0602309]. We generalize this method to include the coupling to the magnetic field. As a result, we obtain for the correction $δχ$ to the Pauli susceptibility a non-analytic temperature dependence of the form $ T^{d-1}γ_{b}^{2}(T)$ in dimensions $d=2,3,$ where $γ_{b}(T)$ is an effective $d$-dependent logarithmically renormalized backscattering amplitude. In one dimension, $δχ$ is proportional to $γ_{b}(T)$, and we reproduce a well known result obtained long ago by a direct calculation.

cond-mat.str-el↗

arXiv2006

Granular Electronic Systems

A granular metal is an array of metallic nano-particles imbedded into an insulating matrix. Tuning the intergranular coupling strength a granular system can be transformed into either a good metal or an insulator and, in case of superconducting particles, experience superconductor-insulator transition. The ease of adjusting electronic properties of granular metals makes them most suitable for fundamental studies of disordered solids and assures them a fundamental role for nanotechnological applications. This Review discusses recent important theoretical advances in the study of granular metals, emphasizing on the interplay of disorder, quantum effects, fluctuations and effects of confinement in formation of electronic transport and thermodynamic properties of granular materials.

cond-mat.mes-hall↗

arXiv2006

Supersymmetric low-energy theory and renormalization group for a clean Fermi gas with a repulsion in arbitrary dimensions

We suggest a new method of calculations for a clean Fermi gas with a repulsion in any dimension. This method is based on writing equations for quasiclassical Green functions and reducing them to equations for collective spin and charge excitations. The spin excitations interact with each other and this leads to non-trivial physics. Writing the solution of the equations and the partition function in terms of a functional integral over supervectors and averaging over fluctuating fields we come to an effective field theory describing the spin excitations. In some respects, the theory is similar to bosonization but also includes the ``ghost'' excitations which prevents overcounting of the degrees of freedom. Expansion in the interaction reveals logarithmic in temperature corrections. This enables us to suggest a renormalization group scheme and derive renormalization group equations. Solving these equations and using their solutions for calculating thermodynamic quantities we obtain explicit expression for the specific heat containing only an effective amplitude of the backward scattering. This amplitude has a complicated dependence on the logarithm of temperature, which leads to a non-trivial temperature dependence of the specific heat.

cond-mat.str-el↗

arXiv2006

Oscillations of Induced Magnetization in Superconductor-Ferromagnet Heterostructures

We study a change in the spin magnetization of a superconductor-ferromagnet (SF) heterostructure, when temperature is lowered below the superconducting transition temperature. It is assumed that the SF interface is smooth on the atomic scale and the mean free path is not too short. Solving the Eilenberger equation we show that the spin magnetic moment induced in the superconductor is an oscillating sign-changing function of the product $hd$ of the exchange field $h$ and the thickness $d$ of the ferromagnet. Therefore the total spin magnetic moment of the system in the superconducting state can be not only smaller (screening) but also greater (anti-screening) than that in the normal state, in contrast with the case of highly disordered (diffusive) systems, where only screening is possible. This surprising effect is due to peculiar periodic properties of localized Andreev states in the system. It is most pronounced in systems with ideal ballistic transport (no bulk disorder in the samples, smooth ideally transparent interface), however these ideal conditions are not crucial for the very existence of the effect. We show that oscillations exist (although suppressed) even for arbitrary low interface transparency and in the presence of bulk disorder, provided that $h τ\gg 1$ ($τ$ -- mean free path). At low interface transparency we solve the problem for arbitrary strength of disorder and obtain oscillating magnetization in ballistic regime ($h τ\gg 1$) and nonoscillating magnetization in diffusive one ($h τ\ll 1$) as limiting cases of one formula.

cond-mat.mes-hall↗

arXiv2005

Observation of Periodic pi-Phase Shifts in Ferromagnet-Superconductor Multilayers

We report complementary studies of the critical temperature and the critical current in ferromagnet (Ni) - superconductor (Nb) multilayers. The observed oscillatory behavior of both quantities upon variation of the thickness of the ferromagnetic layer is found to be in good agreement with theory. The length scale of oscillations is identical for both quantities and is set by the magnetic length corresponding to an exchange field of 200 meV in Ni. The consistency between the behavior of the two quantities provides strong evidence for periodic pi- phase shifts in these devices.

cond-mat.supr-con↗

arXiv2005

Long-range odd triplet superconductivity in SF structures with Neel walls

We consider a multidomain superconductor/ferromagnet (SF) structure with an in-plane magnetization, assuming that the neighboring domains are separated by the Neel domain walls. We show that an odd triplet long-range component arises in the domain walls and spreads into domains over a long distance of the order ξ_T = \sqrt{D/2πT} (in the dirty limit). The density of states variation in the domains due to this component changes over distances of the order ξ_T and turns to zero in the middle of domains if the magnetization rotates in the same direction in all domain walls.

cond-mat.supr-con↗

arXiv2005

Odd Triplet Superconductivity and Related Phenomena in Superconductor-Ferromagnet Structures

We consider novel unusual effects in superconductor-ferromagnet (S/F) structures. In particular we analyze the triplet component (TC) of the condensate generated in those systems.This component is odd in frequency and even in the momentum, which makes it insensitive to non-magnetic impurities. If the exchange field is not homogeneous in the system the triplet component is not destroyed even by a strong exchange field and can penetrate the ferromagnet over long distances. Some other effects considered here and caused by the proximity effect are: enhancement of the Josephson current due to the presence of the ferromagnet, induction of a magnetic moment in superconductors resulting in a screening of the magnetic moment, formation of periodic magnetic structures due to the influence of the superconductor, etc. We compare the theoretical predictions with existing experiments.

cond-mat.supr-con↗