arXiv subjects

C. J. Lambert

Publications and source records attributed to C. J. Lambert.

81 records · Page 5Linked to original sources

arXiv1997

Large-scale superconductivity-induced conductance suppression in mesoscopic normal-superconducting structures

Experiments on hybrid superconducting normal-metal structures have revealed that even in the absence of tunnel junctions the onset of superconductivity can lead to a decrease in the electrical conductance by an amount many orders of magnitude greater than $e^2/h$. In this Letter we provide a theory of this phenomenon which shows that it originates from an instability in 4 - probe conductance measurements which is absent from 2-probe measurements. We compare the zero-bias,zero-temperature 4-probe conductances $G_{N}$ and $G_{S}$ of a normal diffusive metal in contact with a superconductor in both the normal (N) and superconducting (S) states respectively. In the absence of tunnel barriers, the ensemble average of the difference $δG=G_{S}-G_N$ vanishes, in agreement with quasi-classical theory. However we also predict that there exists macroscopic sample specific fluctuations in $δG$, which lie beyond quasi-classical theory and allow large negative values of $δG$ to occur.

cond-mat.supr-con↗

arXiv1997

Phase coherent transport in hybrid superconducting nanostructures

This article is an overview of recent experimental and theoretical work on transport in phase-coherent hybrid nanostructures, with particular emphasis on dc electrical conduction. A summary of multiple scattering theory and the quasi-classical methods is presented and comparisons between the two are made. Several paradigms of phase-coherent transport are discussed, including zero-bias anomalies, reentrant and long range proximity effects, Andreev interferometers and superconductivity-induced conductance suppression.

cond-mat.supr-con↗

arXiv1997

Superconductivity-Induced Anderson Localisation

We have studied the effect of a random superconducting order parameter on the localization of quasi-particles, by numerical finite size scaling of the Bogoliubov-de Gennes tight-binding Hamiltonian. Anderson localization is obtained in d=2 and a mobility edge where the states localize is observed in d=3. The critical behavior and localization exponent are universal within error bars both for real and complex random order parameter. Experimentally these results imply a suppression of the electronic contribution to thermal transport from states above the bulk energy gap.

cond-mat.supr-con↗

arXiv1996

Boundary conditions for quasiclassical equations in the theory of superconductivity

In this paper we derive effective boundary conditions connecting the quasiclassical Green's function through tunnel barriers in superconducting - normal hybrid (S-N or S-S') structures in the dirty limit. Our work extends previous treatments confined to the small transparency limit. This is achieved by an expansion in the small parameter $r^{-1}=T/2(1-T)$ where T is the transparency of the barrier. We calculate the next term in the $r^{-1}$ expansion for both the normal and the superconducting case. In both cases this involves the solution of an integral equation, which we obtain numerically. While in the normal case our treatment only leads to a quantitative change in the barrier resistance $R_b$, in the superconductor case, qualitative different boundary conditions are derived. To illustrate the physical consequences of the modified boundary conditions, we calculate the Josephson current and show that the next term in the $r^{-1}$ expansion gives rise to the second harmonic in the current-phase relation.

cond-mat.supr-con↗

arXiv1996

Phase coherent transport in hybrid superconducting structures: the case of d-wave superconductors

We examine the effect of d-wave symmetry on zero bias anomalies in normal-superconducting tunnel junctions and phase-periodic conductances in Andreev interferometers. In the presence of d-wave pairing, zero-bias anomalies are suppressed compared with the s-wave case. For Andreev interferometers with aligned islands, the phase-periodic conductance is insensistive to the nature of the pairing, whereas for non-aligned islands, the nature of the zero-phase extremum is reversed.

cond-mat↗

arXiv1995

Resonant Andreev Scattering In Phase-Coherent, Superconducting Nanostructures

Analytic predictions for resonant transport in three generic structures are presented. For a structure comprising a normal (N) contact - normal dot (NDOT) - superconducting (S) contact, we predict that finite voltage, differential conductance resonances are destroyed by the switching on of superconductivity in the S-contact. In the weak coupling limit, the surviving resonances have a double-peaked line-shape. Secondly, we demonstrate that resonant Andreev interferometers can provide galvonometric magnetic flux detectors, with a sensitivity in excess of the flux quantum. Finally, for a superconducting dot (SDOT) connected to normal contacts (N), we show that the onset of superconductivity can increase the sub-gap conductance, in contrast with the usual behaviour of a tunnel junction.

cond-mat↗

arXiv1995

Diffusive and ballistic motion in superconducting hybrid structures

We examine transport properties of superconducting hybrid mesoscopic structures, in both the diffusive and ballistic regimes. For diffusive structures, analytic results from quasi-classical theory are compared with predictions from numerical, multiple-scattering calculations performed on small structures. For all structures, the two methods yield comparable results and in some cases, quantitative agreement is obtained. These results not only demonstrate that quasi-classical theory can yield the ensemble averaged conductance $ $ of small structures of dimensions of order 10- 20 Fermi wavelengths, but also establish that numerical scattering calculations on such small structures can yield results for $ $ which are characteristic of much larger systems. Having compared the two approaches, we extend the multiple-scattering analysis to the ballistic limit, where the sample dimensions become smaller than the elastic mean free path and demonstrate that the properties of certain Andreev interferometers are unchanged in the clean limit.

cond-mat↗

arXiv1995

Giant Conductance Oscillations In Mesoscopic Andreev Interferometers

We analyze the electrical conductance $G(ϕ)$ of a two-dimensional, phase coherent structure in contact with two superconductors, which is known to be an oscillatory function of the phase difference $ϕ$ between the superconductors. It is predicted that for a metallic sample, the amplitude of oscillation is enhanced by placing a normal barrier at the interface and that, by tuning the strength of the barrier, can be orders of magnitude greater than values observed in recent experiments. Giant oscillations can also be obtained without a barrier, provided a crucial sum rule is broken. This can be achieved by disorder induced normal scattering. In the absence of zero phase inter-channel scattering, the conductance possesses a zero phase minimum.

cond-mat↗

arXiv1994

Coherent Transport Through a Quantum Trombone

We highlight a new "transverse" interference effect, arising from the coupling between electrons and holes, induced by a superconducting island in contact with a normal metal. As an example we compute the electrical conductance G of a T-shaped mesoscopic sample, formed by joining a horizontal bar of normal metal to a vertical leg of the same material. With a superconducting island located on the vertical leg and the current flowing horizontally, we examine the dependence of G on the distance L of the island from the horizontal bar. Of particular interest is the differential conductance G(E) at an applied voltage E=eV, which due to quantum interference between electrons and Andreev reflected holes, is predicted to oscillate with both L and E. For a system with a spherical Fermi surface, the period of oscillation with E is 3.142 (E_F/k_FL) and with L is 3.142 (E_F/k_F E), where E_F is the Fermi energy and k_F the Fermi wavevector.

cond-mat↗