arXiv subjects

Konstantin B. Efetov

Publications and source records attributed to Konstantin B. Efetov.

31 records · Page 2Linked to original sources

arXiv2015

Spin Current in Junctions composed of Multi-band Superconductors with a Spin-density Wave

We calculate a nondissipative spin current and show that it can flow with or without a charge current. We consider a two-band model which can be applied to the description of Fe-based pnictides in coexistence regime of superconductivity and spin-density wave. Using quasiclassical Green's functions approach and tunneling Hamiltonian method we show that there exists a possibility to switch off the Josephson current while leaving the spin current finite. Moreover, it is possible to have the critical Josephson current and the critical spin current being proportional to each other, thus giving a possibility to measure the spin current via the Josephson current. The underlying mechanism is the interfering hopping of electron--hole pairs between different bands of the superconductors composing the junction. This is an intrinsic property of the system and provides a unique and natural way to utilize junctions made solely of pnictides in promising applications in spintronics devices.

cond-mat.supr-con↗

arXiv2014

Topological Defects in Systems with Two Competing Order Parameters: Application to Superconductors with Charge- and Spin-Density Waves

On the basis of coupled Ginzburg--Landau equations we study nonhomogeneous states in systems with two order parameters~(OP). Superconductors with superconducting OP~$Δ$, and charge- or spin-density wave (CDW or SDW) with amplitude~$W$ are examples of such systems. When one of OP, say~$Δ$, has a form of a topological defect, like, e.g., vortex or domain wall between the domains with the phases~$0$ and~$π$, the other OP~$W$ is determined by the Gross--Pitaevskii equation and is localized at the center of the defect. We consider in detail the domain wall defect for~$Δ$ and show that the shape of the associated solution for~$W$ depends on temperature and doping (or on the curvature of the Fermi surface)~$μ$. It turns out that, provided temperature or doping level are close to some discrete values~$T_{n}$ and~$μ_{n}$, the spacial dependence of the function~$W(x)$ is determined by the form of the eigenfunctions of the linearized Gross--Pitaevskii equation. The spacial dependence of~$W_{0}$ corresponding to the ground state has the form of a soliton, while other possible solutions~$W_{n}(x)$ have nodes. Inverse situation~when~$W(x)$ has the form of a topological defect and~$Δ(x)$ is localized at the center of this defect is also possible. In particular, we predict a surface or interfacial superconductivity in a system where a superconductor is in contact with a material that suppresses~$W$. This superconductivity should have rather unusual temperature dependence existing only in certain intervals of temperature. Possible experimental realizations of such non-homogeneous states of OPs are discussed.

cond-mat.supr-con↗

arXiv2014

Dynamics of Order Parameters near Stationary States in Superconductors with a Charge-Density Wave

We consider a simple model of a quasi-one-dimensional conductor in which two order parameters (OP) may coexist, i.e., the superconducting OP $Δ$ and the OP $W$ that characterizes the amplitude of a charge-density wave (CDW). In the mean field approximation we present equations for the matrix Green's functions $G_{ik}$, where $i$ relates to the one of the two Fermi sheets and $k$, operates in the Gor'kov-Nambu space. Using the solutions of these equations, we find stationary states for different values of the parameter describing the curvature of the Fermi surface, $μ$, which can be varied, e.g., by doping. It is established that in the interval $μ_1<μ<μ_2$ the self-consistency equations have a solution for coexisting OPs $Δ$ and $W$. However, this solution corresponds to a saddle point in the energy functional $Φ(Δ, W)$, i.e., it is unstable. Stable states are: 1)the state with the CDW at $μ< μ_{2}$; and 2) the purely superconducting state at $μ_1<μ$. At $μ<μ_0$, the state 1) corresponds to a global minimum, and at $μ_0<μ$, the state 2) has a lower energy, i.e., only the superconducting state survives at large $μ$. We study the dynamics of the variations $δΔ$ and $δW$ from these states in the collisionless limit. It is characterized by two modes of oscillations, the fast and the slow one. The fast mode is analogous to damped oscillations in conventional superconductors. The frequency of slow modes depends on the curvature $μ$ and is much smaller than $2Δ$ if the coupling constants for superconductivity and CDW are close to each other. The considered model can be applied to high-$T_c$ superconductors where the parts of the Fermi surface near the `hot' spots may be regarded as the considered two Fermi sheets. We also discuss relation of the considered model to the simplest model for Fe-based pnictides.

cond-mat.supr-con↗

arXiv2014

Time-dependent equation for the magnetic order parameter near the quantum critical point in multiband superconductors with a spin density wave

Using a simple two-band model for Fe-based pnictides and the generalized Eilenberger equation, we present a microscopic derivation of a time-dependent equation for the amplitude of the spin density wave near the quantum critical point where it turns to zero. This equation describes the dynamics of the magnetic---$m$, as well as the superconducting order parameter---$Δ$. It is valid at low temperatures $T$ and small $m$ (${T, m \ll Δ}$) in a region of coexistence of both order parameters, $m$ and $Δ$. The boundary of this region is found in the space of the nesting parameter $\{μ_{0},μ_ϕ\}$ where $μ_{0}$ describes the relative position of the electron and the hole pockets on the energy scale, and $μ_ϕ$ accounts for the ellipticity of the electron pocket. At low $T$ the number of quasiparticles is small due to the presence of the energy gap $Δ$, and therefore the quasiparticles do not play a role in the relaxation of $m$. This circumstance allows one to derive the time-dependent equation for $m$ in contrast to the case of conventional superconductors for which the time-dependent Ginzburg--Landau equation can be derived near $T_{\text{c}}$ only in some special cases (high concentration of paramagnetic impurities. In the stationary case the derived equation is valid at arbitrary temperatures. We find a solution of the stationary equation which describes a domain wall in the magnetic structure. In the center of the domain wall the superconducting order parameter has a maximum, which means a local enhancement of superconductivity. Using the derived time-dependent equation for $m$, we investgate also the stability of a uniform commensurate SDW and obtain the values of $\{μ_{0}, μ_ϕ\}$ at which the first order transition into the state with ${m = 0}$ takes place or the transition to the state with an inhomogeneous SDW occurs.

cond-mat.supr-con↗

arXiv2013

Realization of the $π$-state in junctions formed by multi-band superconductors with a spin-density-wave

Using a simple model of multi-band superconductors, which can be applied in particular to Fe\nobreakdash-based pnictides, we calculate the Josephson current $I_{\text{J}}$ in a tunnel junction composed by such superconductors. We employ the tunneling Hamiltonian method and quasiclassical Green's functions. We study both the case of coexistence of the superconducting ($Δ$) and magnetic (SDW---spin density wave) order parameters and the case when only the superconducting order parameter exists. We show that the current $I_{\text{J}}$ depends on the mutual orientation of magnetization of the SDW in the case of non-ideal nesting when the coexistence of superconducting and magnetic order parameters is possible as it takes place in Fe\nobreakdash-based pnictides. It is found that the realization of the $π$\nobreakdash-junction is possible in both cases. We compare our results for multi-band superconductors without the SDW with those obtained earlier and find that they coincide if the tunneling matrix elements are real. If these elements are complex, a new term appears in the formula for the Josephson critical current.

cond-mat.supr-con↗

arXiv2011

Conversion of self-assembled monolayers into nanocrystalline graphene: Structure and electric transport

Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tuneable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfil this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this process, the SAM is converted into a nanocrystalline graphene sheet with well defined thickness and arbitrary dimensions. Electric transport data demonstrate that this transformation is accompanied by an insulator to metal transition that can be utilized to control electrical properties such as conductivity, electron mobility and ambipolar electric field effect of the fabricated graphene sheets. The suggested route opens broad prospects towards the engineering of free-standing 2D carbon materials with tuneable properties on various solid substrates and on holey substrates as suspended membranes.

cond-mat.mes-hall↗

arXiv2009

Anomalous Hall effect in granular ferromagnetic metals and effects of weak localization

We theoretically investigate the anomalous Hall effect in a system of dense-packed ferromagnetic grains in the metallic regime. Using the formalism recently developed for the conventional Hall effect in granular metals, we calculate the residual anomalous Hall conductivity $σ_{xy}$ and resistivity $ρ_{xy}$ and weak localization corrections to them for both skew-scattering and side-jump mechanisms. We find that, unlike for homogeneously disordered metals, the scaling relation between $ρ_{xy}$ and the longitudinal resistivity $ρ_{xx}$ does not hold. The weak localization corrections, however, are found to be in agreement with those for homogeneous metals. We discuss recent experimental data on the anomalous Hall effect in polycrystalline iron films in view of the obtained results.

cond-mat.mes-hall↗

arXiv2009

Excitonic condensation in a double-layer graphene system

The possibility of excitonic condensation in a recently proposed electrically biased double-layer graphene system is studied theoretically. The main emphasis is put on obtaining a reliable analytical estimate for the transition temperature into the excitonic state. As in a double-layer graphene system the total number of fermionic "flavors" is equal to N=8 due to two projections of spin, two valleys, and two layers, the large-$N$ approximation appears to be especially suitable for theoretical investigation of the system. On the other hand, the large number of flavors makes screening of the bare Coulomb interactions very efficient, which, together with the suppression of backscattering in graphene, leads to an extremely low energy of the excitonic condensation. It is shown that the effect of screening on the excitonic pairing is just as strong in the excitonic state as it is in the normal state. As a result, the value of the excitonic gap $\De$ is found to be in full agreement with the previously obtained estimate for the mean-field transition temperature $T_c$, the maximum possible value $Δ^{\rm max},T_c^{\rm max}\sim 10^{-7} ε_F$ ($ε_F$ is the Fermi energy) of both being in $ 1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} ε_F$ really sets the upper bound for the transition temperature and invalidates the recently expressed conjecture about the high-temperature first-order transition into the excitonic state. These findings suggest that, unfortunately, the excitonic condensation in graphene double-layers can hardly be realized experimentally.

cond-mat.str-el↗

arXiv2008

Electron screening and excitonic condensation in double-layer graphene systems

We theoretically investigate the possibility of excitonic condensation in a system of two graphene monolayers separated by an insulator, in which electrons and holes in the layers are induced by external gates. In contrast to the recent studies of this system, we take into account the screening of the interlayer Coulomb interaction by the carriers in the layers, and this drastically changes the result. Due to a large number of electron species in the system (two projections of spin, two valleys, and two layers) and to the suppression of backscattering in graphene, the maximum possible strength of the screened Coulomb interaction appears to be quite small making the weak-coupling treatment applicable. We calculate the mean-field transition temperature for a clean system and demonstrate that its highest possible value $T_c^\text{max}\sim 10^{-7}ε_F\lesssim 1 \text{mK}$ is extremely small ($ε_F$ is the Fermi energy). In addition, any sufficiently short-range disorder with the scattering time $τ\lesssim \hbar /T_c^\text{max}$ would suppress the condensate completely. Our findings renders experimental observation of excitonic condensation in the above setup improbable even at very low temperatures.

cond-mat.str-el↗

arXiv2008

Mesoscopic conductance fluctuations in graphene samples

Mesoscopic conductance fluctuations in graphene samples at energies not very close to the Dirac point are studied analytically. We demonstrate that the conductance variance $<[δG]^2>$ is very sensitive to the elastic scattering breaking the valley symmetry. In the absence of such scattering (disorder potential smooth at atomic scales, trigonal warping negligible), the variance $<[δG]^2 > = 4 < [δG]^2 >_\text{metal}$ is four times greater than that in conventional metals, which is due to the two-fold valley degeneracy. In the absence of intervalley scattering, but for strong intravalley scattering and/or strong warping $<[δG]^2 > =2 < [δG]^2 >_\text{metal}$. Only in the limit of strong intervalley scattering $<[δG]^2 > = < [δG]^2 >_\text{metal}$. Our theory explains recent numerical results and can be used for comparison with existing experiments.

cond-mat.mes-hall↗

arXiv2007

Hall Effect in Granular Metals: Weak Localization Corrections

We study the effects of localization on the Hall transport in a granular system at large tunneling conductance $g_{T}\gg 1$ corresponding to the metallic regime. We show that the first-order in 1/g_T weak localization correction to Hall resistivity of a two- or three-dimensional granular array vanishes identically, $\de ρ_{xy}^{WL}=0$. This result is in agreement with the one for ordinary disordered metals. Being due to an exact cancellation, our result holds for arbitrary relevant values of temperature T and magnetic field H, both in the ``homogeneous'' regime of very low T and H corresponding to ordinary disordered metals and in the ``structure-dependent'' regime of higher values of T or H.

cond-mat.mes-hall↗

arXiv2007

Hall Transport in Granular Metals and Effects of Coulomb Interactions

We present a theory of Hall effect in granular systems at large tunneling conductance $g_{T}\gg 1$. Hall transport is essentially determined by the intragrain electron dynamics, which, as we find using the Kubo formula and diagrammatic technique, can be described by nonzero diffusion modes inside the grains. We show that in the absence of Coulomb interaction the Hall resistivity $ρ_{xy}$ 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 and type of granular lattice. Further, we study the effects of Coulomb interactions by calculating first-order in $1/g_T$ corrections and find that (i) in a wide range of temperatures $T \gtrsim \Ga$ exceeding the tunneling escape rate $\Ga$, the Hall resistivity $ρ_{xy}$ and conductivity $\sig_{xy}$ acquire logarithmic in $T$ corrections, which are of local origin and absent in homogeneously disordered metals; (ii) large-scale ``Altshuler-Aronov'' correction to $\sig_{xy}$, relevant at $T\ll\Ga$, vanishes in agreement with the theory of homogeneously disordered metals.

cond-mat.mes-hall↗

arXiv1994

Localization and dynamic persistent currents in long cylinders

A dynamic response to a magnetic field in a long disordered cylinder is considered. We show that, although at high frequencies conduction is classical in all directions, the low frequency behavior corresponds to localization in the longitudinal direction and to a diamagnetic dynamic persistent current in the transversal one. The current density does not vanish even in the limit of the infinitely long cylinder.

cond-mat↗