arXiv subjects

A. F. Volkov

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

At least 55 records · Page 3Linked to original sources

arXiv2003

Odd triplet superconductivity in superconductor/ferromagnet multilayered structures

We demonstrate that in multilayered superconductor-ferromagnet structures a non-collinear alignment of the magnetizations of different ferromagnetic layers generates a triplet superconducting condensate which is odd in frequency. This triplet condensate coexists in the superconductors with the conventional singlet one but decays very slowly in the ferromagnet, which should lead to a large Josephson effect between the superconductors separated by the ferromagnet. Depending on the mutual direction of the ferromagnetic moments the Josephson coupling can be both of 0 and $π$ type.

cond-mat.supr-con↗

arXiv2003

Comment on: ''Radiation-Induced ''Zero-Resistance State'' and the Photon Assisted Transport''

Shi and Xie [8] predicted the N-shape current-voltage characteristic $(CVC)$ for a 2DEG in a magnetic field where the zero-resistance state has been observed in recent experiments. However it is known that in the absence of a magnetic field the zero resistance state (zero differential resistance state) is achieved in a system with the S-shape $CVC$. The difference in the behaviour of systems with N- and S-shape $CVC$ was studied more than three decades ago (see for example the review [9] and references therein) and is briefly explained in this Comment. At present it is not clear whether the N-shape $CVC$ may lead to the zero resistance state in a 2DEG in a magnetic field.

cond-mat.mes-hall↗

arXiv2002

Resistance of a domain wall in the quasiclassical approach

Starting from a simple microscopic model, we have derived a kinetic equation for the matrix distribution function. We employed this equation to calculate the conductance $G$ in a mesoscopic F'/F/F' structure with a domain wall (DW). In the limit of a small exchange energy $J$ and an abrupt DW, the conductance of the structure is equal to $G_{2d}=4σ_{\uparrow}σ_{\downarrow }/(σ_{\uparrow}+σ_{\downarrow})L$. Assuming that the scattering times for electrons with up and down spins are close to each other we show that the account for a finite width of the DW leads to an increase in this conductance. We have also calculated the spatial distribution of the electric field in the F wire. In the opposite limit of large $J$ (adiabatic variation of the magnetization in the DW) the conductance coincides in the main approximation with the conductance of a single domain structure $% G_{1d}=(σ_{\uparrow}+σ_{\downarrow})/L$. The account for rotation of the magnetization in the DW leads to a negative correction to this conductance. Our results differ from the results in papers published earlier.

cond-mat.mtrl-sci↗

arXiv2002

Electron-hole imbalance in superconductor-normal metal mesoscopic structures

We analysed the electron-hole or, in another words, branch imbalance (BI) and the related electric potential $V_{imb}$ which may arise in a mesoscopic superconductor/normal metal (S/N) structure under non-equilibrium conditions in the presence of a supercurrent. Non-equilibrium conditions can be created in different ways: a) a quasiparticle current flowing between the N reservoirs; b) a temperature gradient between the N reservoirs and no quasiparticle current. It is shown that the voltage $V_{imb}$ oscillates with the phase difference $ϕ$. In a cross-geometry structure the voltage $V_{imb}$ arises in the vertical branch and affects the conditions for a transition into the $π$-state.

cond-mat.supr-con↗

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↗

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↗

arXiv2000

Josephson effects in a superconductor-normal metal mesoscopic structure with a dangling superconducting arm

We studied a mesoscopic cross-like normal metal structure connected to two superconducting (S) and two normal (N) reservoirs. We observed the Josephson effect under unusual conditions when there is no current through one of the two S/N interfaces. The potential difference between the S reservoirs was zero unless the voltage applied between S and N reservoirs exceeded a critical value although the electric potential in the N wire connecting the superconductors varied in a nonmonotonic way. The observed effects are discussed theoretically.

cond-mat.supr-con↗

arXiv2000

Giant thermoemf in multiterminal superconductor/normal metal mesoscopic structures

We considered a mesoscopic superconductor/normal metal (S/N) structure in which the N reservoirs are maintained at different temperatures. It is shown that in the absence of current between the N reservoirs a voltage difference $V_{T}$ arises between the superconducting and normal conductors. The voltage $V_{T}$ oscillates with increasing phase difference $ϕ$ between the superconductors, and its magnitude does not depend on the small parameter $(T/ε_{F}).$

cond-mat.supr-con↗

arXiv2000

Weak localization correction to the FS interface resistance

The classical resistance of a contact between a ferromagnet (F) and a superconductor (S) acquires an additional contact term as compared to the contact between a ferromagnet and a normal metal. The necessity to match spin-polarized current in a ferromagnet to spin-less current in the superconductor results in the accumulation of non-equilibrium polarization near the F/S interface. In the present work, we show that the weak localization correction to the classical diffusion coefficient, $δD$, is dependent on the degree of polarization, with majority spins more likely to be reflected from the interface than minority spins. Taking into account the change in the spin polarized particle distribution in the F wire arising from $δD$, we calculate the weak localization correction to the F/S contact resistance.

cond-mat.mes-hall↗

arXiv2000

Phase-coherent effects in multiterminal superconductor/normal metal mesoscopic structures

In this report we analyse three effects which may arise in a mesoscopic multiterminal S/N structure (two normal and two superconducting reservoirs). We show that the Josephson critical current is a non-monotonic function of a voltage between the normal reservoirs. The influence of the spin-polarized electrons on the Josephson critical current was also studied. We show that if there is a temperature difference between the normal reservoirs, a voltage between the superconductors and normal conductors arises which oscillates as the phase difference varies. Its magnitude is much greater than the thermoemf in the case of the ordinary thermoelectric effect.

cond-mat.supr-con↗

arXiv2000

Suppression and enhancement of the critical current in multiterminal S/N/S mesoscopic structures

We analyse the measured critical current $I_{m\text{}}$ in a mesoscopic 4-terminal S/N/S structure. The current through the S/N interface is shown to consist not only of the Josephson component $I_{c}\sin ϕ,$ but also a phase-coherent part $I_{sg}\cos ϕ$ of the subgap current. The current $I_{m}$ is determined by the both components $I_{c}$ and $I_{sg},$ and depends in a nonmonotonic way on the voltage $V$ between superconductors and normal reservoirs reaching a maximum at $V\cong Δ/e$. The obtained theoretical resultas are in qualitative agreement with recent experimental data.

cond-mat.supr-con↗

arXiv1999

Anomalous behaviour of S/N mesoscopic structures near T_c

We observe a maximum in the conductance of Aluminum / n-GaAs junctions at temperatures 20 mK lower than the superconducting transition temperature. This is the first observation of a non- monotonic behavior of the conductance near the superconducting transition in superconducting normal junctions. To accommodate this effect we calculate the full temperature dependence of the conductance of these structures, invoking quasiclassical Green's functions in the diffusive limit. In addition to the well-known low temperature peak at temperatures on the order of the Thouless energy, we find a maximum near the critical temperature. This peak has the same origin as the subgap conductance observed in superconducting-normal junctions at low temperatures. Its calculated magnitude and position are in good agreement with our experimental observation.

cond-mat.supr-con↗

arXiv1999

Phase-coherent tunneling through mesoscopic superconductor coupled with superconducting and normal metal electrodes

Phase-coherent diffusive transport through mesoscopic hybrid superconductor/normal metal tunneling structures is investigated. For a N-s-S two-barrier tunneling system with bulk S and N electrodes coupled by a mesoscopic superconducting constriction s, zero-bias conductance and non-linear I-V curves are calculated under the assumption that the dwell times of quasiparticles in the s region is shorter than inelastic relaxation time. It is shown that the low voltage conductance of this system determined by the Andreev reflection processes may exceed the conductance in the normal state and its value is very sensitive to the weak pairing interaction of electrons in the s region. We show that even weak pairing electron interaction may result in the significant qualitative and quantitative change of the conductance temperature dependence with respect to the case of structures with the normal mesoscopic region. We calculate the I-V curves and show that they depend on the applied voltage in a non-monotonic way, therefore differential conductance becomes negative with increasing voltage. Such behavior is due to the voltage dependence of the order parameter in the constriction and the phase difference, phi, between the S and s superconductors. It is shown that if the tunneling processes determine the form of the quasiparticle distribution function in the s superconductor, the phase, phi, is stationary at arbitrary voltages. For quasiparticle tunneling interferometers in which the mesoscopic superconductor,s, couples the superconductor,S, and the normal metal,N, the zero bias conductance, as a function of the phase difference between the S electrodes is investigated. It is shown that the amplitude of the conductance oscillations may exceed the conductance of this structure in the normal state.

cond-mat.supr-con↗

arXiv1999

Interplay between spin-relaxation and Andreev reflection in ferromagnetic wires with superconducting contacts

We analyze the change in the resistance of a junction between a diffusive ferromagnetic (F) wire and normal metal electrode, due to the onset of superconductivity (S) in the latter and a double Andreev scattering process leading to a complete internal reflection of a large fraction of the spin-polarized electrons back into the ferromagnet. The superconducting transition results in an additional contact resistance arising from the necessity to match spin-polarized current in F-wire to spin-less current in S-reservoir, which is comparable to the resistance of a piece of a F-wire with the length equal to the spin-relaxation length.