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A. F. Volkov

Publications and source records attributed to A. F. Volkov.

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↗
arXiv2013

Electronic transport through ferromagnetic and superconducting junctions with spin-filter tunneling barriers

We present a theoretical study of the quasiparticle and subgap conductance of generic $X/I_{sf}/S_{M}$ junction with a spin-filter barrier $I_{sf}$, where $X$ is either a normal $N$ or a ferromagnetic metal $F$ and $S_{M}$ is a superconductor with a built-in exchange field. Our study is based on the tunneling Hamiltonian and the Green's function technique. First, we focus on the quasiparticle transport, both above and below the superconducting critical temperature. We obtain a general expression for the tunneling conductance which are valid for arbitrary values of the exchange field and arbitrary magnetization directions in the electrodes and in the spin-filter barrier. In the second part we consider the subgap conductance of a normal metal-superconductor junction with a spin-filter barrier. We provide a heuristic derivation of new boundary conditions for the quasiclassical Green's functions which take into account the spin-filter effect at the interface. With the help of these boundary conditions, we show how the proximity effect and the subgap conductance is suppressed by spin-filtering in a $N/I_{sf}/S$ junction, where $N$ is a normal metal. Our work provides useful tools for the study of spin-polarized transport in hybrid structures both in the normal and the superconducting state.

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arXiv2012

Spin-polarized Josephson and quasiparticle currents in superconducting spin-filter tunnel junctions

We present a theoretical study of the effect of spin-filtering on the Josephson and dissipative quasiparticle currents in a superconducting tunnel junction. By combining the quasiclassical Green's functions and the tunneling Hamiltonian method we describe the transport properties of a generic junction consisting of two superconducting leads with an effective exchange field h separated by a spin-filter insulating barrier. We show that besides the tunneling of Cooper pairs with total spin-projection Sz = 0 there is another contribution to the Josephson current due to equal-spin Cooper pairs. The latter is finite and not affected by the spin-filter effect provided that the fields h and the magnetization of the barrier are non-collinear . We also determine the quasiparticle current for a symmetric junction and show that the differential conductance may exhibit peaks at different values of the voltage depending on the polarization of the spin-filter, and the relative angle between the exchange fields and the magnetization of the barrier. Our findings provide a plausible explanation for existing experiments on Josephson junctions with magnetic barriers, predict new effects and show how spin-polarized supercurrents in hybrid structures can be created.

cond-mat.supr-con↗
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.

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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.

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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↗
arXiv2010

Comment on "Composite excitation of Josephson phase and spin waves in ferromagnetic Josephson junctions" (S.Hikino, M.Mori, S.Takahashi, and S.Maekawa, arXiv:1009.3551)

We clarify the applicability of the quasistatic approximation used in Ref. [\onlinecite{VE}], where coupled spin and Josephson plasma waves have been predicted to exist in SIFS Josephson junctions. We show, contrary to the claim of the authors of Ref. [\onlinecite{Maekawa}], that this approximation is very accurate in realistic systems studied experimentally.

cond-mat.supr-con↗
arXiv2010

Odd spin-triplet superconductivity in a multilayered superconductor-ferromagnet Josephson junction

We study the dc Josephson effect in a diffusive multilayered SF'FF'S structure, where S is a superconductor and F,F' are different ferromagnets. We assume that the exchange energies in the F' and F layers are different ($% h $ and $H$, respectively) and the middle F layer consists of two layers with parallel or antiparallel magnetization vectors $M$. The $M$ vectors in the left and right F' layers are generally not collinear to those in the F layer. In the limit of a weak proximity effect we use a linearized Usadel equation. Solving this equation, we calculate the Josephson critical current for arbitrary temperatures, arbitrary thicknesses of the F' and F layers ($% L_{h}$ and $L_{H}$) in the case of parallel and antiparallel $M$ orientations in the F layer. The part of the critical current $I_{cSR}$ formed by the short-range (SRC) singlet and S=0 triplet condensate components decays on a short length $ξ_{H}=\sqrt{D/H}$, whereas the part $% I_{cLR}$ due to the long-range triplet $|S|=1$ component (LRTC) decreases with increasing $L_{H}$ on the length $ξ_{N}=\sqrt{D/πT}$. Our results are in agreement with the experiment \cite{Birge}.

cond-mat.supr-con↗
arXiv2010

Nonlinear Resonance of Superconductor/Normal Metal Structures to Microwaves

We study the variation of the differential conductance $G=dj/dV$ of a normal metal wire in a Superconductor/Normal metal heterostructure with a cross geometry under external microwave radiation applied to the superconducting parts. Our theoretical treatment is based on the quasiclassical Green's functions technique in the diffusive limit. Two limiting cases are considered: first, the limit of a weak proximity effect and low microwave frequency, second, the limit of a short dimension (short normal wire) and small irradiation amplitude.

cond-mat.supr-con↗
arXiv2009

Inhomogeneous state in non-equilibrium superconductor/normal metal tunnel structures: a LOFF-like phase for non-magnetic systems

We analyze non-equilibrium states in a tunnel superconductor-normal metal (NSN) structure in the presence of a tunnel current $I$. We use an approximation of an effective temperature $T$ and calculate the current-voltage I-V characteristics. It is shown that the I-V dependence may have an S-shaped form. We determine nonuniform current $I(x)$ and temperature $T(x)$ distributions that arise as a result of instability of the uniform state with negative differential conductance ($dI/dV<0$). We discuss an analogy with equilibrium superconductors with an exchange field in which nonuniform states predicted by Larkin-Ovchinnikov and Fulde-Ferrel are possible.

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arXiv2009

Hybridization of Spin and Plasma Waves in Josephson Tunnel Junctions Containing a Ferromagnetic Layer

We study dynamics of tunnel Josephson junctions with a thin ferromagnetic layer F [superconductor-insulator-ferromagnet-superconductor (SIFS) junctions]. On the basis of derived equations relating the superconducting phase and magnetic moment to each other we analyze collective excitations in the system and find a new mode which is a hybrid of plasmalike and spin waves. The latter are coupled together in a broad range of parameters characterizing the system. Using the solution describing the collective modes we demonstrate that besides the Fiske steps new peaks appear on the I-V characteristics due to oscillations of the magnetic moment M in the ferromagnetic layer. Thus, by measuring the I-V curve of the SIFS junctions, one can extract information about the spectrum of spin excitations in the ferromagnet F.

cond-mat.supr-con↗
arXiv2009

Proximity effect and its enhancement by ferromagnetism in high-temperature superconductor-ferromagnet structures

We consider a bi-layer consisting of a $d-$wave layerd superconductor and diffusive ferromagnet with a domain wall (DW). The $c-$axis in the superconductor and DW in the ferromagnet are assumed to be perpendicular to the interface. We demonstrate that in such a heterostructure the inhomogeneous exchange field enhances the proximity effect. It is shown that, whereas in the absence of the exchange field the $d-$wave condensate decays in the normal metal on the mean free path $l$, the superconductivity penetrates the ferromagnet along the DW over much larger distances. This happens because the presence of DW results in a generation of an odd frequency triplet s-wave component of the condensate. The phenomenon discovered here may help to explain a recent experiment on high temperature superconductor-ferromagnet bi-lyers.

cond-mat.supr-con↗
arXiv2008

Odd triplet superconductivity in superconductor-ferromagnet structures with a narrow domain wall

We study the proximity effect in superconductor-ferromagnet (SF) structure with a narrow domain wall (DW) at the SF interface. The width of the domain wall is assumed to be larger than the Fermi wave length, but smaller than other characteristic lengths (for example, the ''magnetic'' length). The transmission coefficient is supposed to be small so that we deal with a weak proximity effect. Solving the linearized Eilenberger equation, we find analytical expressions for quasiclassical Green's functions. These functions describe the short-range (SR) condensate components, singlet and triplet with zero projection of the total spin on the quantization z-axis, induced in F due to the proximity effect as well as long-range odd triplet component (LRTC) with a nonzero projection of the total spin of Cooper pairs on the $z$% -axis. The amplitude of the LRTC essentially depends on the product $hτ$ and increases with increasing the exchange energy $h$ ($τ$ is the elastic scattering time). We calculate the Josephson current in SFS junction with a thickness of the F layer much greater than the penetration length of the SR components. The Josephson critical current caused by the LRTC may be both positive and negative depending on chirality of the magnetic structure in F. The density of states (DOS) in a diffusive SF bilayer is also analyzed. It is shown that the contributions of the SR and LR components to the DOS in F have a different dependence on the thickness $d$ of the F layer (nonmonotonous and monotonous).

cond-mat.supr-con↗
arXiv2007

Charge imbalance and Josephson effects in superconductor-normal metal mesoscopic structures

We consider a $SBS$ Josephson junction the superconducting electrodes $S$ of which are in contact with normal metal reservoirs ($B$ means a barrier). For temperatures near $T_{c}$ we calculate an effective critical current $% I_{c}^{\ast}$ and the resistance of the system at the currents $I<$ $% I_{c}^{\ast}$ and $I>>I_{c}^{\ast}$. It is found that the charge imbalance, which arises due to injection of quasiparticles from the $N$ reservoirs into the $S$ wire, affects essentially the characteristics of the structure. The effective critical current $I_{c}^{\ast}$ is always larger than the critical current $I_{c}$ in the absence of the normal reservoirs and increases with decreasing the ratio of the length of the $S$ wire $2L$ to the charge imbalance relaxation length $l_{Q}$. It is shown that a series of peaks arises on the $I-V$ characteristics due to excitation of the Carlson-Goldman collective modes. We find the position of Shapiro steps which deviates from that given by the Josephson relation.

cond-mat.supr-con↗
arXiv2007

Collective Modes in Two-band Superconductors

We analyze collective modes in two-band superconductors in the dirty limit. It is shown that these modes exist at all temperatures $T$ below $T_{c}$ provided the frequency of the modes is higher than the inelastic scattering rate and lower than the energy gaps $Δ_{a,b}$. At low temperatures these modes are related to counterphase oscillations of the condensate currents in each band. The spectrum of the collective oscillations is similar to the spectrum of the Josephson ''plasma'' modes in a tunnel Josephson junction but the velocity of the mode propagation in the case under consideration is much lower. At higher temperatures ($Δ_{b}<T<T_{c}$) the spectrum consists of two branches. One of them is gapless (sound-like) and the second one has a threshhold that depends on coupling between the bands. We formulate the conditions under which both types of collective modes can exist. The spectrum of the collective modes can be determined by measuring the I-V characteristics of a Josephson junction in a way as it was done by Carlson and Goldman.

cond-mat.supr-con↗
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.

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