Electron transport in a mesoscopic superconducting / ferromagnetic hybrid conductor

We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors. Received 9 July 2002 / Received in final form 22 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: herve.courtois@grenoble.cnrs.fr RID="b" ID="b"associated to Université Joseph Fourier     

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 SN structures in the diffusive limit. The subgap conductance in SN structures with barriers (zero bias and finite bias anomalies) are discussed. We also analyse the temperature dependence of the conductance variationδS (T) for a Andreev interferometer. We show that besides the well know low temperature maximum a second maximum near T_cmay appear. We present the results of studies on the Josephson effect in 4-terminal SNS contacts and on the possible sign reversal of the Josephson critical current.     

Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, current injection and electromagnetic radiation. In particular we will consider the ‘giant’ Josephson critical current in ‘long’ SIS tunnel junctions and the regular and anomalous nonequilibrium Josepson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).     

Study of transport properties in superconducting junctions of double insulating barrier

In this work we analyze the Tomasch effect in double barrier insulating superconducting N₁ISIN₂ (N: normal metal, I: insulator and S: superconductor) junctions. From the solution of the Bogoliubov–de Gennes equations we find that the differential conductance presents resonances when the applied voltage changes. These resonances are originated by the formation of quasibound states in the superconducting region and depend on the symmetry of the pair potential. We develop an analytical model in order to find the quasibound states energies and its lifetimes. This model allows us to calculate the voltage at which each resonance appears and the resonance widths. We calculate and analyze the dependence of the transmission coefficients with the thickness of the superconducting layer.     

Vortex quasi-crystals in mesoscopic superconducting samples

There seems to be a one to one correspondence between the phases of atomic and molecular matter(AMOM) and vortex matter(VM) in superfluids and superconductors. Crystals, liquids, and glasses have been experimentally observed in both AMOM and VM. Here, we propose a vortex quasi-crystal state which can be stabilized due to boundary and surface energy effects for samples of special shapes and sizes. For finite sized pentagonal samples, it is proposed that a phase transition between a vortex crystal and a vortex quasi-crystal occurs as a function of magnetic field and temperature as the sample size is reduced.     

Thermo-Coulomb effects in mesoscopic tunnel junctions

We present a study of the thermoelectric effect in tunnel junctions. In particular we calculate the thermopower coefficientS and the Peltier coefficient . For macroscopic junctions we demonstrate the sensitivity ofS and to the structure of the density of states. For mesoscopic junctions we show that Coulomb effects modify the ordinary Onsager picture and the relation =TS. The coefficientsS and are found to be very sensitive to the coupling of the junctions to the external world. We analyze measurements of the thermopower in granular films in terms of the thermo-Coulomb effects. We comment on the relevance of these effects to scanning tunneling microscope measurements.     

Superconductivity in mesoscopic high-Tc superconducting particles

Based on the Hubbard model in the framework of non-phonon kinematical mechanism and taking into account the discreetness of an electronic energy spectrum, the superconducting critical temperature of a mesoscopic high-T_c sphere is analyzed as a function of doping and as a function of particle's radius. The critical temperature T_c is found to be an oscillating function of the radius of a particle. The size-dependent doping regime is revealed in high-T_c nanoparticles. Our analysis shows that each oscillation in T_c corresponds to the increase in a number of the energy levels in the sphere by 1. The amplitude of oscillations of T_c increases with decreasing R and can reach a value of 6 K for nanoparticles with sizes about 25 nm, in good agreement with experimental studies of YBa₂Cu₃O_7−δ nanoparticles.     

Noise current induced electrical transport anomalies near the superconducting transition in mesoscopic aluminum lines

We investigate whether unintentionally applied currents may have induced the previously reported resistive and current-voltage characteristic anomalies near the superconducting transition temperature in quasi-one-dimensional aluminum lines. Our samples show no intrinsic anomalies, but a low frequency current deliberately added to the ordinary measuring current induces a resistive peak near the transition. Additional a.c. current applied to the voltage leads instead of the current leads induces an excess voltage peak near the critical current in V(I) curves. The resistive peak near the transition results from mixing, but we cannot explain the voltage peak by a simple mixing analysis. The similarity of the resistive and V(I) peaks to previous observations suggests possible extrinsic effects in earlier work.     

Electronic transport in mesoscopic superconductor/2D electron gas junctions in strong magnetic field

The hybrid superconductor/2D electron gas (S/2DEG) structures based on InGaAs-InP hetero-junctions with a high-mobility 2D electron gas and superconducting NbN electrodes have been investigated. The electronic transport and current-voltage characteristics of S/2DEG/normal metal (S/2DEG/N) structures in strong perpendicular magnetic fields have been studied. Oscillations in the magnetoresistance of S/2DEG/N structures have been found in strong magnetic fields. It is shown that at bias voltages lower than the superconducting gap the amplitude of oscillations in S/2DEG/N structures significantly exceeds the oscillation amplitude in the reference N/2DEG/N samples. The experimental results can be explained within the quasiclassical theory of magnetotransport in S/2DEG structures developed by N.M. Chtchelkatchev and I.S. Burmistrov (Phys. Rev. B, 2007, vol. 75, 214 510).     

三维介观超导环的涡旋结构

在Ginzburg-Landan理论的框架下, 运用有限差分法研究了在圆环电流产生磁场下的介观超导圆环内的涡旋结构, 讨论了超导圆环尺寸和不同空间分布的磁场对涡旋形成的影响, 得到在一般超导圆环体内的基态多是巨涡旋态、而多涡旋态多以激发态形式存在的结论, 说明磁场一般从超导圆环的环孔穿过, 而很难穿过超导圆环体.     

Critical fluctuations in a mesoscopic superconducting ring

The nonlocal magnetoconductivity fluctuations in a superconducting submicron ring, with radius comparable to the Ginzburg-Landau coherence length, are studied. The order parameter mode separation yields to the solution of the time-dependent Ginzburg-Landau equation and the paraconductivity Fourier components are calculated in the vicinity of the critical temperature, including the critical fluctuation region. The homogeneous component has a logarithmic singularity at T(c) while the other components are found to be not singular.     

Proximity effect, Andreev reflections, and charge transport in mesoscopic superconducting/semiconducting heterostructures

In the quasi-two-dimensional (Q2D) electron gas of an InAs channel between an AlSb substrate and superconducting niobium layers, the proximity effect induces a pair potential so that a Q2D mesoscopic superconducting/normal/superconducting (SNS) junction forms in the channel. The pair potential is calculated with quasiclassical Green’s functions in the clean limit. For such a junction, alternating Josephson currents and current–voltage characteristics (CVCs) are computed, using the nonequilibrium quasiparticle wavefunctions which solve the time-dependent Bogoliubov–de Gennes equations. The CVCs exhibit features found experimentally by the Kroemer group: a steep rise of the current at small voltages (‘foot’) changes at a ‘corner current’ to a much slower increase of current with higher voltages, and the zero-bias differential resistance increases with temperature. Phase-coherent multiple Andreev reflections and the associated Cooper pair transfers are the physical mechanisms responsible for the oscillating Josephson currents and the CVCs. Additional experimental findings not reproduced by the theory require model improvements, especially a consideration of the external current leads which should give rise to hybrid quasiparticle/collective-mode excitations.     

Modeling rectification effects in mesoscopic superconducting devices

We study thermal fluctuations and capacitive effects on small Josephson Junction Rings (JJR) that mimics the rectification phenomena recently observed in triangular shaped mesoscopic superconductors, due to the superposition of the field induced persistent current with the bias current. At finite temperature we predicted that the amplitude of the rectified signal depends strongly on the current contacts configuration on the JJR, in coincidence with experiments. In addition, we analyze the range of parameters where a closed loop of capacitive junctions is an appropriate model to explain the experimental observations. We conclude that the closed loop of weak links, a JJR, is a simple, robust and good model to explain the observed voltage rectification effects on mesoscopic superconducting samples for a wide range of temperature.     

Metastability and paramagnetism in superconducting mesoscopic disks

A projected order parameter is used to calculate not only local minima of the Ginzburg-Landau energy functional but also saddle points or energy barriers responsible for the metastabilities observed in superconducting mesoscopic disks [A. K. Geim et al., Nature (London) 396, 144 (1998)]. We calculate the local minima magnetization and find the energetic instability points between vortex configurations with different vorticity. We also find that, for any vorticity, the supercurrent can reverse its flow direction on decreasing the magnetic field before one vortex can escape.     

Stable vortex-antivortex molecules in mesoscopic superconducting triangles

A thermodynamically stable vortex-antivortex pattern has been revealed in equilateral mesoscopic type I superconducting triangles, contrary to type II superconductors where similar patterns are unstable. The stable vortex-antivortex "molecule" appears due to the interplay between two factors: a repulsive vortex-antivortex interaction in type I superconductors and the vortex confinement in the mesoscopic triangle.     

三维介观超导环涡旋态的研究

通过数值求解非线性金兹堡-朗道(G-L)方程组,研究了三维介观超导环中的涡旋态。发现了在细环中只能存在巨涡旋态,以及存在顺磁、抗磁迈斯纳效应和间隙性超导现象。在粗环中,发现了多涡旋态和巨涡旋态共存的混合态。相应讨论有助于理解介观超导环中涡旋态相变。