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 $I_{c}\sin \phi ,$ but also a phase-coherent part $I_{sg}\cos \phi$ 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 $V\cong \Delta /e$. The obtained theoretical resultas are in qualitative agreement with recent experimental data.Comment: 4 page, 3 figures. To be puplished in PRB Rapid co

Nonequilibrium Josephson effect in short-arm diffusive SNS interferometers

We study non-equilibrium Josephson effect and phase-dependent conductance in three-terminal diffusive interferometers with short arms. We consider strong proximity effect and investigate an interplay of dissipative and Josephson currents co-existing within the same proximity region. In junctions with transparent interfaces, the suppression of the Josephson current appears at rather large voltage, $eV\sim \Delta$, and the current vanishes at $eV\geq\Delta$. Josephson current inversion becomes possible in junctions with resistive interfaces, where the inversion occurs within a finite interval of the applied voltage. Due to the presence of considerably large and phase-dependent injection current, the critical current measured in a current biased junction does not coincide with the maximum Josephson current, and remains finite when the true Josephson current is suppressed. The voltage dependence of the conductance shows two pronounced peaks, at the bulk gap energy, and at the proximity gap energy; the phase oscillation of the conductance exhibits qualitatively different form at small voltage $eV<\Delta$, and at large voltage $eV>\Delta$.Comment: 11 pages, 9 figures, revised version, to be published in Phys. Rev.

Magnetic field influence on the proximity effect in semiconductor - superconductor hybrid structures and their thermal conductance

We show that a magnetic field can influnce the proximity effect in NS junctions via diamagnetic screening current flowing in the superconductor. Using ballistic quasi-one-dimensional (Q1D) electron channels as an example, we show that the supercurrent flow shifts the proximity-induced minigap in the excitation spectrum of a Q1D system from the Fermi level to higher quasiparticle energies. Thermal conductance of a Q1D channel (normalized by that of a normal Q1D ballistic system) is predicted to manifest such a spectral feature as a nonmonotonic behavior at temperatures corresponding to the energy of excitation into the gapful part of the spectrum.Comment: 5 pages, 3 figures, revised version with a new titl

Coherent Charge Transport in Metallic Proximity Structures

We develop a detailed microscopic analysis of electron transport in normal diffusive conductors in the presence of proximity induced superconducting correlation. We calculated the linear conductance of the system, the profile of the electric field and the densities of states. In the case of transparent metallic boundaries the temperature dependent conductance has a non-monotoneous ``reentrant'' structure. We argue that this behavior is due to nonequilibrium effects occuring in the normal metal in the presence of both superconducting correlations and the electric field there. Low transparent tunnel barriers suppress the nonequilibrium effects and destroy the reentrant behavior of the conductance. If the wire contains a loop, the conductance shows Aharonov-Bohm oscillations with the period $\Phi_0=h/2e$ as a function of the magnetic flux $\Phi$ inside the loop. The amplitude of these oscillations also demonstrates the reentrant behavior vanishing at $T=0$ and decaying as $1/T$ at relatively large temperatures. The latter behavior is due to low energy correlated electrons which penetrate deep into the normal metal and ``feel'' the effect of the magnetic flux $\Phi$. We point out that the density of states and thus the ``strengh'' of the proximity effect can be tuned by the value of the flux inside the loop. Our results are fully consistent with recent experimental findings.Comment: 16 pages RevTeX, 23 Postscript figures, submitted to Phys. Rev.

Proximity-induced transport in hybrid mesoscopic normal-superconducting metal structures

Using an approach based on quasiclassical Green's functions we present a theoretical study of transport in mesoscopic S/N structures in the diffusive limit. The subgap conductance in S/N structures with barriers (zero bias and finite bias anomalies) are discused. We also analyse the temperature dependence of the conductance variation $\delta S(T)$ for a Andreev interferometer. We show that besides the well know low temperature maximum a second maximum near $T_c$ may appear. We present the results of studies on the Josephson effect in 4 terminal S/N/S contacts and on the possible sign reversal of the Josephson critical current.Comment: 8 pages, 5 figures, with added refrence

Direct demonstration of circulating currents in a controllable π\pi-SQUID generated by a 0 to π\pi transition of the weak links

A controllable $\pi$-SQUID is a DC SQUID with two controllable $\pi$-junctions as weak links. A controllable $\pi$-junction consists of a superconducting - normal metal - superconducting Josephson junction with two additional contacts to the normal region of the junction. By applying a voltage $V_c$ over these contacts it is possible to control the sate of the junction, i.e. a conventional (0) state or a $\pi$-state, depending on the magnitude of $V_c$. We demonstrate experimentally that, by putting one junction into a $\pi$-state, a screening current is generated around the SQUID loop at integer external flux. To be able to do this, we have fabricated controllable $\pi$-junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at 1.4 K only the Nb-Ag device shows the transition to a $\pi$-state as a function of $V_c$ consistent with theoretical predictions. In a controllable $\pi$ SQUID based on Nb-Ag we observe, a part from a screening current at integer external flux, a phase shift of $\pi$ of the $V_{SQUID}-B$ oscillations under suitable current bias, depending on the magnitude of $V_c$.Comment: 11 pages, 12 figures, subm. to Phys. Rev.

Influence of Inorganic Nano-powders on the Structure and Conductive Properties of the Network Polymer Electrolytes for Lithium Batteries

The paper describes investigation on the network polymer electrolytes based on polyethylene glycol diacrylates and polyester diacrylates PEDA with introduction the nanopowders TiO2, Li2TiO3 and SiO2, with different size and shape. Much attention is paid to effects of nanoparticles additives on the ionic conductivity of network polymer electrolytes. The work is aimed to explanation of the mechanism of additives action on Li+ - ion transport and structural changes of the polymer chains and the solvent molecules. For these purposes the NMR method with rotation under a magic corner on nuclei 1H and NMR method with a pulsed magnetic field gradient at the nuclei 7Li were used. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3551

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 $\sqrt{D/\epsilon}$ ($D$ is the diffusion coefficient and $\epsilon$ 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 wa

Nonequilibrium Josephson effect in mesoscopic ballistic multiterminal SNS junctions

We present a detailed study of nonequilibrium Josephson currents and conductance in ballistic multiterminal SNS-devices. Nonequilibrium is created by means of quasiparticle injection from a normal reservoir connected to the normal part of the junction. By applying a voltage at the normal reservoir the Josephson current can be suppressed or the direction of the current can be reversed. For a junction longer than the thermal length, $L\gg\xi_T$, the nonequilibrium current increases linearly with applied voltage, saturating at a value equal to the equilibrium current of a short junction. The conductance exhibits a finite bias anomaly around $eV \sim \hbar v_F/L$. For symmetric injection, the conductance oscillates $2\pi$-periodically with the phase difference $\phi$ between the superconductors, with position of the minimum ($\phi=0$ or $\pi$) dependent on applied voltage and temperature. For asymmetric injection, both the nonequilibrium Josephson current and the conductance becomes $\pi$-periodic in phase difference. Inclusion of barriers at the NS-interfaces gives rise to a resonant behavior of the total Josephson current with respect to junction length with a period $\sim \lambda_F$. Both three and four terminal junctions are studied.Comment: 21 pages, 19 figures, submitted to Phys. Rev.

Free boundary problems describing two-dimensional pulse recycling and motion in semiconductors

An asymptotic analysis of the Gunn effect in two-dimensional samples of bulk n-GaAs with circular contacts is presented. A moving pulse far from contacts is approximated by a moving free boundary separating regions where the electric potential solves a Laplace equation with subsidiary boundary conditions. The dynamical condition for the motion of the free boundary is a Hamilton-Jacobi equation. We obtain the exact solution of the free boundary problem (FBP) in simple one-dimensional and axisymmetric geometries. The solution of the FBP is obtained numerically in the general case and compared with the numerical solution of the full system of equations. The agreement is excellent so that the FBP can be adopted as the basis for an asymptotic study of the multi-dimensional Gunn effect.Comment: 19 pages, 9 figures, Revtex. To appear in Phys. Rev.