arXiv subjects

K. B. Efetov

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

At least 91 records · Page 5Linked to original sources

arXiv2002

Effect of spin on electron motion in a random magnetic field

We consider properties of a two-dimensional electron system in a random magnetic field. It is assumed that the magnetic field not only influences orbital electron motion but also acts on the electron spin. For calculations, we suggest a new trick replacing the initial Hamiltonian by a Dirac Hamiltonian. This allows us to do easily a perturbation theory and derive a supermatrix sigma model, which takes a form of the conventional sigma model with the unitary symmetry. Using this sigma model we calculate several correlation functions including a spin-spin correlation function. As compared to the model without spin, we get different expressions for the single-particle lifetime and the transport time. The diffusion constant turns out to be 2 times smaller than the one for spinless particles.

cond-mat.mes-hall↗

arXiv2002

Ballistic versus diffusive magnetoresistance of a magnetic point contact

The quasiclassical theory of a nanosize point contacts (PC) between two ferromagnets is developed. The maximum available magnetoresistance values in PC are calculated for ballistic versus diffusive transport through the area of a contact. In the ballistic regime the magnetoresistance in excess of few hundreds percents is obtained for the iron-group ferromagnets. The necessary conditions for realization of so large magnetoresistance in PC, and the experimental results by Garcia et al are discussed

cond-mat.mtrl-sci↗

arXiv2002

Multivalued dependence of the magnetoresistance on the quantized conductance in nanosize magnetic contacts

We calculate the quantized conductance of nanosize point contacts between two ferromagnets for different mutual orientations of the magnetic moments. It is found that the magnetoresistance MR is a multivalued function of the quantized conductance at the parallel alignment of the magnetizations sigma^F. This leads us to the conclusion that experimentally observed large fluctuations of MR versus sigma^F are rather due to the conductance quantization than to measurement errors or a poor reproducibility of the results. Using the results of the calculations we are able to understand experimental data obtained by Garcia et al for MR of the magnetic nanocontacts.

cond-mat.mtrl-sci↗

arXiv2002

Transition from insulating to a non-insulating temperature dependence of the conductivity in granular metals

We consider interaction effects in a granular normal metal at not very low temperatures. Assuming that all weak localization effects are suppressed by the temperature we replace the initial Hamiltonian by a proper functional of phases and study the possibility for a phase transition depending on the tunneling conductance $g$. It is demonstrated for any dimension that, while at small $g$ the conductivity decays with temperature exponentially, its temperature dependence is logarithmic at large $g.$ The formulae obtained are compared with an existing experiment and a good agreement is found.

cond-mat.dis-nn↗

arXiv2002

Transport and triplet superconducting condensate in mesoscopic ferromagnet-superconductor structures

We calculate the conductance of a superconductor/ferromagnet (S/F) mesoscopic structure in the dirty limit. First we assume that the ferromagnet exhibits a homogeneous magnetization and consider the case that the penetration of the condensate into the F wire is negligible and the case in which the proximity effect is taken into account. It is shown that if the exchange field is large enough, the conductance below the critical temperature $T_C$, is always smaller than the conductance in the normal state. At last, we calculate the conductance for a F/S structure with a local inhomogeneity of the magnetization in the ferromagnet. We demonstrate that a triplet component of the condensate is induced in the F wire.This leads to a increase of the conductance below $T_C$.

cond-mat.supr-con↗

arXiv2002

Superconducting Fluctuations in Granular Metals with a Large Coupling Between the Grains

We study the fluctuation conductivity of superconducting granular metals at low temperatures and strong magnetic field destroying the Cooper pairs. Explicit calculations are performed for larger values of the coupling between the grains than those considered in previous works. We show that in a broad region of the coupling constants the superconducting fluctuations still significantly reduce the conductivity leading to a negative magnetoresistance.

cond-mat.mes-hall↗

arXiv2001

Local density of states in superconductor-strong ferromagnet structures

We study the dependence of the local density of states (LDOS) on coordinates for a superconductor-ferromagnet (S/F) bilayer and a S/F/S structure assuming that the exchange energy h in the ferromagnet is sufficiently large: $% hτ>>1,$ where $τ$ is the elastic relaxation time. This limit cannot be described by the Usadel equation and we solve the more general Eilenberger equation. We demonstrate that, in the main approximation in the parameter $% (hτ)^{-1}$, the proximity effect does not lead to a modification of the LDOS in the S/F system and a non-trivial dependence on coordinates shows up in next orders in $(hτ) ^{-1}.$ In the S/F/S sandwich the correction to the LDOS is nonzero in the main approximation and depends on the phase difference between the superconductors. We also calculate the superconducting critical temperature $T_{c}$ for the bilayered system and show that it does not depend on the exchange energy of the ferromagnet in the limit of large h and a thick F layer.

cond-mat.supr-con↗

arXiv2001

Josephson current in superconductor-ferromagnet structures with a nonhomogeneous magnetization

We calculate the dc Josephson current $I_J$ for two types of superconductor-ferromagnet (S/F) Josephson junctions. The junction of the first type is a S/F/S junction. On the basis of the Eilenberger equation, the Josephson current is calculated for an arbitrary impurity concentration. If $% hτ\ll1$ the expression for the Josephson critical current $I_c$ is reduced to that which can be obtained from the Usadel equation ($h$ is the exchange energy, $τ$ is the momentum relaxation time). In the opposite limit $hτ\gg1$ the superconducting condensate oscillates with period $% v_F/h$ and penetrates into the F region over distances of the order of the mean free path $l$. For this kind of junctions we also calculate $I_J$ in the case when the F layer presents a nonhomogeneous (spiral) magnetic structure with the period $2π/Q$. It is shown that for not too low temperatures, the $π$-state which occurs in the case of a homogeneous magnetization (Q=0) may disappear even at small values of $Q$. In this nonhomogeneous case, the superconducting condensate has a nonzero triplet component and can penetrate into the F layer over a long distance of the order of $ξ_{T}=% \sqrt{D/2πT}$. The junction of the second type consists of two S/F bilayers separated by a thin insulating film. It is shown that the critical Josephson current $I_{c}$ depends on the relative orientation of the effective exchange field $h$ of the bilayers. In the case of an antiparallel orientation, $I_{c}$ increases with increasing $h$. We establish also that in the F film deposited on a superconductor, the Meissner current created by the internal magnetic field may be both diamagnetic or paramagnetic.

cond-mat.supr-con↗

arXiv2001

Long-range effects in superconductor-ferromagnet structures

We analyze the proximity effect in a superconductor/ferromagnet (S/F) structure with a local inhomogeneity of the magnetization in the ferromagnet near the S/F interface. We demonstrate that not only the singlet but also the triplet component of the superconducting condensate is induced in the ferromagnet due to the proximity effect. Although the singlet component of the condensate penetrates into the ferromagnet over a short length $ξ_{h}=\sqrt{D/h}$ ($h$ is the exchange field in the ferromagnet and $D$ the diffusion coefficient), the triplet component, being of the order of the singlet one at the S/F interface, penetrates over a long length $\sqrt{D/ε}$ ($ε$ is the energy). This long-range penetration into the ferromagnet leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature $T_{c}$.

cond-mat.supr-con↗

arXiv2001

Enhancement of the Josephson current by an exchange field in superconductor-ferromagnet structures

We calculate the dc Josephson current for two superconductor-ferromagnet (S/F) bilayers separated by a thin insulating film. It is demonstrated that the critical Josephson current $I_{c}$ in the junction strongly depends on the relative orientation of the effective exchange field $h$ of the bilayers. We found that in the case of an antiparallel orientation, $I_{c}$ increases at low temperatures with increasing $h$ and at zero temperature has a singularity when $h$ equals the superconducting gap $Δ$. This striking behavior contrasts suppression of the critical current by the magnetic moments aligned in parallel and is an interesting new effect of the interplay between superconductors and ferromagnets.

cond-mat.supr-con↗

arXiv2001

Effect of Long Range Disorder on Electron Motion in Two Dimensions

We consider a two-dimensional electron gas with long range disorder. Assuming that time reversal symmetry is broken either by an external magnetic field or, as in the case of a delta-correlated random magnetic field, by the disorder itself, we derive a supermatrix $σ$-model. As an intermediate step, we provide a microscopic derivation of the ballistic $σ$-model, and find that certain corrections to its usual form may become important. We then integrate out degrees of freedom corresponding to short length scales to derive a low-energy supermatrix $σ$-model. We find an extra term in the free energy that couples to the correlator of local currents. Use of a proper ultraviolet regularisation procedure that preserves gauge invariance indicates that the contribution of the extra term seems finally to become irrelevant. Within the scope of our analyis, we therefore do not find any deviation of the scaling behaviour of the delta-correlated random magnetic field model from that of the conventional unitary ensemble. We then generalize the discussion to include models of even longer-ranged disorder, plus short-range disorder. When the disorder is sufficiently long-ranged that the local currents become delta-correlated, a new term appears in the free energy that does give rise to logarithmic corrections to the conductivity. A renormalisation group analysis of the free energy yields a scaling form for the diffusion coefficient which contains both a positive correction, that represents classical superdiffusion, and a negative correction, which is the usual weak localization correction. The fact that both corrections are of the same order and opposite sign leads to the interesting possibility of a quantum phase transition at weak disorder in two dimensions, tuned by the relative disorder strengths.

cond-mat.mes-hall↗

arXiv2000

Two-scale localization of wave functions in disordered wires in a weak magnetic field

Using the supersymmetry technique combined with the transfer matrix method we calculate different physical quantities characterizing localization in disordered wires. In particular, we analyze the density-density correlation function and study effects of an external magnetic field $H$ on tails of wave functions. At zero and very strong magnetic fields, we obtain explicit expressions, valid at arbitrary distances, for all moments and for the entire distribution function of the density-density correlations. A two-scale decay is shown to be a typical feature of infinitely long wires at weak magnetic fields: The far tail of the wave functions decays twice as slow as their main body. Extending Mott's physical picture for the localized states we present a qualitative description of the crossover in the magnetic field. Our arguments can be used for any dimensionality indicating that the concept of the two-scale localization in a weak magnetic field is a general feature of localization. The effect is very sensitive to a level smearing and cannot be seen in the transmittance of a system with metallic leads. This means that the Borland conjecture may not be used for a numerical check of the two-scale localization.

cond-mat.mes-hall↗

arXiv2000

Quantum interference and Coulomb interaction in arrays of tunnel junctions

We study the electronic properties of an array of small metallic grains connected by tunnel junctions. Such an array serves as a model for a granular metal. Previous theoretical studies of junction arrays were based on models of quantum dissipation which did not take into account the diffusive motion of electrons within the grains. We demonstrate that these models break down at sufficiently low temperatures: for a correct description of the screening properties of a granular metal at low energies the diffusive nature of the electronic motion within the grains is crucial. We present both a diagrammatic and a functional integral approach to analyse the properties of junction arrays. In particular, a new effective action is obtained which enables us to describe the array at arbitrary temperature. In the low temperature limit, our theory yields the correct, dynamically screened Coulomb interaction of a normal metal, whereas at high temperatures the standard description in terms of quantum dissipation is recovered.

cond-mat.mes-hall↗

arXiv2000

Comment on ``Search for two-scale localization in disordered wires in a magnetic field''

In a recent work Schomerus and Beenakker [Phys. Rev. Lett. 84, 3927 (2000)] tried to check numerically our prediction about a two-scale localization in disordered wires in a weak magnetic field. According to our theory an exponential decay of wave functions with the localization length of the orthogonal ensemble is followed at larger distances by another exponential decay with the length of the unitary ensemble. Studying the transmittance, Schomerus and Beenakker did not confirm our theory. The most probable explanation they suggested is that only rare states could have the two scales while ``typical'' states have the only localization length. We suggest another explanation of the discrepancy, namely, that the Borland conjecture relating the transmittance and wave functions to each other may not be used for studying the effect of the magnetic field on the tails of wave functions.

cond-mat.mes-hall↗

arXiv2000

Cryptoferromagnetic state in superconductor-ferromagnet multilayers

We study a possibility of a non-homogeneous magnetic order (cryptoferromagnetic state) in heterostructures consisting of a bulk superconductor and a ferromagnetic thin layer that can be due to the influence of the superconductor. The exchange field in the ferromagnet may be strong and exceed the inverse mean free time. A new approach based on solving the Eilenberger equations in the ferromagnet and the Usadel equations in the superconductor is developed. We derive a phase diagram between the cryptoferromagnetic and ferromagnetic states and discuss the possibility of an experimental observation of the CF state in different materials.

cond-mat.supr-con↗

arXiv2000

Antilocalization in a 2D Electron Gas in a Random Magnetic Field

We construct a supersymmetric field theory for the problem of a two-dimensional electron gas in a random, static magnetic field. We find a new term in the free energy, additional to those present in the conventional unitary sigma-model, whose presence relies on the long-range nature of the disorder correlations. Under a perturbative renormalization group analysis of the free energy, the new term contributes to the scaling function at one-loop order and leads to antilocalization.

cond-mat.mes-hall↗

arXiv2000

Localization-delocalization transition in disordered systems with a direction

Using the supersymmetry technique, we study the localization-delocalization transition in quasi-one-dimensional non-Hermitian systems with a direction. In contrast to chains, our model captures the diffusive character of carriers' motion at short distances. We calculate the joint probability of complex eigenvalues and some other correlation functions. We find that the transition is abrupt and occurs as a result of an interplay between two saddle-points in the free energy functional.

cond-mat.dis-nn↗

arXiv1999

Statistics of Wave Functions in Coupled Chaotic Systems

Using the supersymmetry technique, we calculate the joint distribution of local densities of electron wavefunctions in two coupled disordered or chaotic quantum billiards. We find novel spatial correlations that are absent in a single chaotic system. Our exact result can be interpreted for small coupling in terms of the hybridization of eigenstates of the isolated billiards. We show that the presented picture is universal, independent of microscopic details of the coupling.

cond-mat.mes-hall↗