Critical currents in ballistic two-dimensional InAs-based superconducting weak links

The critical supercurrent Ic carried by a short (0.3 to 0.8 µm) ballistic two-dimensional InAs-based electron gas between superconducting niobium electrodes is studied. In relating the maximum value to the resistance of the weak link in the normal state Rn a much lower value is found than theoretically expected for a ballistic system. The energy scale which characterizes the observed temperature dependence is comparable to the energy associated with the product of Ic and Rn. We point out that although the transport between the electrodes is ballistic, in the InAs underneath the superconducting electrodes the transport is diffusive, an experimental case which has not yet been studied theoretically

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.

Enhanced conductance near zero voltage bias in mesoscopic superconductor-semiconductor junctions

We have studied the conductance enhancement near zero voltage bias of double-barrier Nb-p++Si-E junctions, where we chose for the counterelectrode E either Nb, Al, or W. The experiments show a large correction, ΔG ≈ 0.1GN, on the classical superconductor–insulator–normal-metal (SIN) conductance. We present measurements of the temperature, magnetic-field, and voltage dependence, and we interpret the observed results within the available theoretical models for coherent Andreev reflection, as provided by several authors.

Development of the Methods of Control of Radiation Structure of

Specific features of radiation structure of terahertz quantum  cascade  lasers  are determined  by wire geometry of their waveguides with small and sub-wavelength transverse dimensions  and  the  length much larger than the wavelength. Here we present an  overview  of  the  results  of  beam  profile investigations and of the methods proposed for the control of radiation structure of such lasers

Shubnikov-de Haas oscillations near the metal-insulator transition in a two-dimensional electron system in silicon

We have studied Shubnikov-de Haas oscillations in a two-dimensional electron system in silicon at low electron densities. Near the metal-insulator transition, only "spin" minima of the resistance at Landau-level filling factors 2, 6, 10, and 14 are seen, while the "cyclotron" minima at filling factors 4, 8, and 12 disappear. A simple explanation of the observed behavior requires a giant enhancement of the spin splitting near the metal-insulator transition.Comment: 4 pages, postscript figures include