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

Energy spectroscopy of Andreev levels between two superconductors

We perform energy spectroscopy of Andreev reflection processes occurring at two superconducting electrodes connected in series via a ballistic two-dimensional channel, by measuring the voltage dependence of the part of the conductance which is modulated by the macroscopic phase difference. The modulation amplitude oscillates as a function of energy and the phase exhibits an abrupt shift close to π at the energy for which the amplitude is minimum. We discuss how our findings are related to the properties of bound states formed between the two superconductors.

Electron-phonon and electron-electron interactions in organic field effect transistors

Recent experiments have demonstrated that the performances of organic FETs strongly depend on the dielectric properties of the gate insulator. In particular, it has been shown that the temperature dependence of the mobility evolves from a metallic-like to an insulating behavior upon increasing the dielectric constant of the gate material. This phenomenon can be explained in terms of the formation of small polarons, due to the polar interaction of the charge carriers with the phonons at the organic/dielectric interface. Building on this model, the possible consequences of the Coulomb repulsion between the carriers at high concentrations are analyzed.Comment: proceedings of the SMEC'07 conference, submitted to the Journal of Physics and Chemistry of Solid

Statistical significance of fine structure in the frequency spectrum of Aharonov-Bohm conductance oscillations

We discuss a statistical analysis of Aharonov-Bohm conductance oscillations measured in a two-dimensional ring, in the presence of Rashba spin-orbit interaction. Measurements performed at different values of gate voltage are used to calculate the ensemble-averaged modulus of the Fourier spectrum and, at each frequency, the standard deviation associated to the average. This allows us to prove the statistical significance of a splitting that we observe in the h/e peak of the averaged spectrum. Our work illustrates in detail the role of sample specific effects on the frequency spectrum of Aharonov-Bohm conductance oscillations and it demonstrates how fine structures of a different physical origin can be discriminated from sample specific features.Comment: accepted for publication in PR

Magnetoconductance Oscillations in Ballistic Semiconductor-Superconductor Junctions

The mechanism of the magnetoconductance oscillations in junctions of a ballistic semiconductor and a superconductor is discussed. The oscillations appear when both the normal and the Andreev reflection occur at the interface. The interplay between the classical cyclotron motion of a quasiparticle and the phase shift caused by the magnetic field is the origin of the conductance oscillations.Comment: 4 pages, 4 figure

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.

Contact resistance in graphene-based devices

We report a systematic study of the contact resistance present at the interface between a metal (Ti) and graphene layers of different, known thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate that the contact resistance is quantitatively the same for single-, bi-, and tri-layer graphene ($\sim800 \pm 200 \Omega \mu m$), and is in all cases independent of gate voltage and temperature. We argue that the observed behavior is due to charge transfer from the metal, causing the Fermi level in the graphene region under the contacts to shift far away from the charge neutrality point

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.

Conditions for Adiabatic Spin Transport in Disordered Systems

We address the controversy concerning the necessary conditions for the observation of Berry phases in disordered mesoscopic conductors. For this purpose we calculate the spin-dependent conductance of disordered two-dimensional structures in the presence of inhomogeneous magnetic fields. Our numerical results show that for both, the overall conductance and quantum corrections, the relevant parameter defining adiabatic spin transport scales with the square root of the number of scattering events, in generalization of Stern's original proposal [Phys. Rev. Lett. 68, 1022 (1992)]. This could hinder a clear-cut experimental observation of Berry phase effects in diffusive metallic rings.Comment: 5 pages, 4 figures. To appear in Phys. Rev. B (Rapid Communications