Localization Effect in a 2D Superconducting Network without Disorder

The superconducting properties of a two-dimensional superconducting wire network with a new geometry have been measured as a function of the external magnetic field. The extreme localization effect recently predicted for this periodic lattice is revealed as a suppression of the critical current when the applied magnetic field corresponds to half a flux quantum per unit cell. For this particular magnetic field, the observed vortex state configuration is highly disordered.Comment: 6 pages, 2 eps figures, submitted to Physica C. Title change

Voltage-induced Shapiro steps in a superconducting multi-terminal structure

When a superconducting tunnel junction at a finite voltage is irradiated with microwaves, the interplay between the alternating Josephson current and the ac field gives rise to steps in the dc current known as Shapiro steps. In this work we predict that in a mesoscopic structure connected to several superconducting terminals one can induce Shapiro-like steps in the absence of any external radiation simply by tuning the voltages of the leads. To illustrate this effect we make quantitative predictions for a three-terminal structure which comprises a diffusive superconductor-normal metal-superconductor junction and a tunneling probe, a set-up which can be realized experimentally.Comment: revtex4, 5 pages, 5 figures, to appear in Phys. Rev.

Nanoengineered magnetic-field-induced superconductivity

The perpendicular critical fields of a superconducting film have been strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles) on top of the film. Magnetic-field-induced superconductivity is observed in these hybrid superconductor / ferromagnet systems due to the compensation of the applied field between the dots by the stray field of the dipole array. By switching between different magnetic states of the nanoengineered field compensator, the critical parameters of the superconductor can be effectively controlled.Comment: 4 pages, 4 figure

Landau levels in the case of two degenerate coupled bands: kagome lattice tight-binding spectrum

The spectrum of charged particles hopping on a kagome lattice in a uniform transverse magnetic field shows an unusual set of Landau levels at low field. They are unusual in two respects: the lowest Landau levels are paramagnetic so their energies decrease linearly with increasing field magnitude, and the spacings between the levels are not equal. These features are shown to follow from the degeneracy of the energy bands in zero magnetic field. We give a general discussion of Landau levels in the case of two degenerate bands, and show how the kagome lattice tight-binding model includes one special case of this more general problem. We also discuss the consequences of this for the behavior of the critical temperature of a kagome grid superconducting wire network, which is the experimental system that originally motivated this work.Comment: 18 pages, 8 figure

Re-entrance of the metallic conductance in a mesoscopic proximity superconductor

We present an experimental study of the diffusive transport in a normal metal near a superconducting interface, showing the re-entrance of the metallic conductance at very low temperature. This new mesoscopic regime comes in when the thermal coherence length of the electron pairs exceeds the sample size. This re-entrance is suppressed by a bias voltage given by the Thouless energy and can be strongly enhanced by an Aharonov Bohm flux. Experimental results are well described by the linearized quasiclassical theory.Comment: improved version submitted to Phys. Rev. lett., 4 pages, 5 included epsf figure

Magnetic Flux Periodic Response of Nano-perforated Ultrathin Superconducting Films

We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K $<T_{co} <$5 K. These regular perforations give the films a phase-sensitive periodic response to an applied magnetic field. By measuring this response in their resistive transitions, $R(T)$, we are able to distinguish regimes in which fluctuations of the amplitude, both the amplitude and phase, and the phase of the superconducting order parameter dominate the transport. The portion of $R(T)$ dominated by amplitude fluctuations is larger in lower $T_{co}$ films and thus, grows with proximity to the superconductor to insulator transition.Comment: Revised title, abstract, text, figure

Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements indicate that the Rashba coupling constant increases linearly with electrostatic doping. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates

Coherent low-energy charge transport in a diffusive S-N-S junction

We have studied the current voltage characteristics of diffusive mesoscopic Nb-Cu-Nb Josephson junctions with highly-transparent Nb-Cu interfaces. We consider the low-voltage and high-temperature regime eV<\epsilon_{c}<k_{B}T where epsilon_{c} is the Thouless energy. The observed excess current as well as the observed sub-harmonic Shapiro steps under microwave irradiation suggest the occurrence of low-energy coherent Multiple Andreev Reflection (MAR).Comment: 4 pages, 4 figures, final versio

Two interacting Hofstadter butterflies

The problem of two interacting particles in a quasiperiodic potential is addressed. Using analytical and numerical methods, we explore the spectral properties and eigenstates structure from the weak to the strong interaction case. More precisely, a semiclassical approach based on non commutative geometry techniques permits to understand the intricate structure of such a spectrum. An interaction induced localization effect is furthermore emphasized. We discuss the application of our results on a two-dimensional model of two particles in a uniform magnetic field with on-site interaction.Comment: revtex, 12 pages, 11 figure