Effects of Magnetic Field on Josephson Current in SNS System

The effect of a magnetic field on Josephson current has been studied for a superconductor/normal-metal/superconductor (SNS) system, where N is a two-dimensional electron gas in a confining potential. It is found that the dependence of Josephson currents on the magnetic field are sensitive to the width of the normal metal. If the normal metal is wide and contains many channels (subbands), the current on a weak magnetic field shows a dependence similar to a Fraunhofer-pattern in SIS system and, as the field gets strong, it shows another type of oscillatory dependence on the field resulting from the Aharonov-Bohm interference between the edge states. As the number of channels decreases (i.e. normal metal gets narrower), however, the dependence in the region of the weak field deviates from a clear Fraunhofer pattern and the amplitude of the oscillatory dependence in the region of the strong field is reduced.Comment: 14 pages, 9 figure

Direct measurement of the phase coherence length in a GaAs/GaAlAs square network

The low temperature magnetoconductance of a large array of quantum coherentloops exhibits Altshuler-Aronov-Spivak oscillations which periodicitycorresponds to 1/2 flux quantum per loop.We show that the measurement of the harmonics content in a square networkprovides an accurate way to determine the electron phase coherence length$L\_{\phi}$ in units of the lattice length without any adjustableparameters.We use this method to determine $L\_{\phi}$ in a network realised from a 2Delectron gas (2DEG) in a GaAS/GaAlAs heterojunction. The temperaturedependence follows a power law $T^{-1/3}$ from 1.3 K to 25 mK with nosaturation, as expected for 1D diffusive electronic motion andelectron-electron scattering as the main decoherence mechanism.Comment: Additional experimental data in version

Interference of two electrons entering a superconductor

The subgap conductivity of a normal-superconductor (NS) tunnel junction is thought to be due to tunneling of two electrons. There is a strong interference between these two electrons, originating from the spatial phase coherence in the normal metal at a mesoscopic length scale and the intrinsic coherence of the superconductor. We evaluated the interference effect on the transport through an NS junction. We propose the layouts to observe drastic Aharonov-Bohm and Josephson effects.Comment: 8 pages REVTex, [PostScript] figures upon reques

Exact Thermodynamics of Disordered Impurities in Quantum Spin Chains

Exact results for the thermodynamic properties of ensembles of magnetic impurities with randomly distributed host-impurity couplings in the quantum antiferromagnetic Heisenberg model are presented. Exact calculations are done for arbitrary values of temperature and external magnetic field. We have shown that for strong disorder the quenching of the impurity moments is absent. For weak disorder the screening persists, but with the critical non-Fermi-liquid behaviors of the magnetic susceptibility and specific heat. A comparison with the disordered Kondo effect experiments in dirty metallic alloys is performed.Comment: 4 pages Late

Quantum oscillations in mesoscopic rings and anomalous diffusion

We consider the weak localization correction to the conductance of a ring connected to a network. We analyze the harmonics content of the Al'tshuler-Aronov-Spivak (AAS) oscillations and we show that the presence of wires connected to the ring is responsible for a behaviour different from the one predicted by AAS. The physical origin of this behaviour is the anomalous diffusion of Brownian trajectories around the ring, due to the diffusion in the wires. We show that this problem is related to the anomalous diffusion along the skeleton of a comb. We study in detail the winding properties of Brownian curves around a ring connected to an arbitrary network. Our analysis is based on the spectral determinant and on the introduction of an effective perimeter probing the different time scales. A general expression of this length is derived for arbitrary networks. More specifically we consider the case of a ring connected to wires, to a square network, and to a Bethe lattice.Comment: 17 pages, 7 eps figure

Equivalence of Fokker-Planck approach and non-linear σ\sigma-model for disordered wires in the unitary symmetry class

The exact solution of the Dorokhov-Mello-Pereyra-Kumar-equation for quasi one-dimensional disordered conductors in the unitary symmetry class is employed to calculate all $m$-point correlation functions by a generalization of the method of orthogonal polynomials. We obtain closed expressions for the first two conductance moments which are valid for the whole range of length scales from the metallic regime ($L\ll Nl$) to the insulating regime ($L\gg Nl$) and for arbitrary channel number. In the limit $N\to\infty$ (with $L/(Nl)=const.$) our expressions agree exactly with those of the non-linear $\sigma$-model derived from microscopic Hamiltonians.Comment: 9 pages, Revtex, one postscript figur

Studies of h/e Aharonov-Bohm Photovoltaic Oscillations in Mesoscopic Au Rings

We have investigated a mesoscopic photovoltaic (PV) effect in micron-size Au rings in which a dc voltage Vdc is generated in response to microwave radiation. The effect is due to the lack of inversion symmetry in a disordered system. Aharonov-Bohm PV oscillations with flux period h/e have been observed at low microwave intensities for temperatures ranging from 1.4 to 13 K. For moderate microwave intensities the h/e PV oscillations are completely quenched providing evidence that the microwaves act to randomize the phase of the electrons. Studies of the temperature dependence of Vdc also provide evidence of the dephasing nature of the microwave field. A complete theoretical explanation of the observed behavior seems to require a theory for the PV effect in a ring geometry.Comment: 10 pages (RevTex twocolumn style), 8 figures-2 pages (one postscript file) To be published in Phys. Rev.

Conductance fluctuations in mesoscopic normal-metal/superconductor samples

We study the magnetoconductance fluctuations of mesoscopic normal-metal/superconductor (NS) samples consisting of a gold-wire in contact with a niobium film. The magnetic field strength is varied over a wide range, including values that are larger than the upper critical field B_c2 of niobium. In agreement with recent theoretical predictions we find that in the NS sample the rms of the conductance fluctuations (CF) is by a factor of 2.8 +/- 0.4 larger than in the high field regime where the entire system is driven normal conducting. Further characteristics of the CF are discussed.Comment: 4 pages, REVTEX, 3 eps-figures included. To be published in Phys. Rev. Lett.. Changes: one misplaced figure correcte

Boundary scattering and weak localization of electrons in a magnetic field

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Critical Conductance and Its Fluctuations at Integer Hall Plateau Transitions

Under periodic boundary condition in the transverse direction, we calculate the averaged zero-temperature two-terminal conductance ($$) and its statistical fluctuations ($$ for $n\le 4$) at the critical point of integer quantum Hall plateau transitions. We find {\it universal} values for $=(0.58\pm0.03){e^2\over h}$, and $=({e^2\over h})^{2n}A_{2n}$, where $A_{2,4,6,8}=0.081\pm0.005$; $0.013\pm0.003$; $0.0026\pm0.005$; and $(8\pm2)\times10^{-4}$ respectively. We also determine the leading finite size scaling corrections to these observables. Comparisons with experiments will be made.Comment: 13 pages, 3 Postscript figures included, Final version with minor corrections, To appear in Physical Review Letter