Session IV: Novel Phenomena

IV-1: Andreev states and resonance charge pumping through quantum S-N-S contacts

Valerii Vinokur
Argonne National Laboratory, USA

Electronic transport through a superconducting point contact is mediated by multiple Andreev reflections within the contact and can be visualized as spectral flow of the Andreev levels. We propose a mechanism of quantum pumping mediated by this spectral flow in a voltage-biased SINIS quantum junction. Pumping is realized via the sequential closing of the minigaps in the energy spectrum in resonance with the Josephson frequency. We show that the pumped dc current exhibits giant peaks at rational voltages. We further discuss the low-temperature current noise in the SINIS junctions and show that it provides information about the non-equilibrium population of the Andreev sub-gap states.

IV-2: Mixed-State Properties Associating with Two Gap Effect in MgB₂ Thin Films

Hai-Hu Wen
National Laboratory for Superconductivity, Institute of Physics and National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, P. R. China

Mixed state properties have been exclusively investigated in epitaxial MgB₂ thin films by measurements of resistive transition, current-voltage characteristics, Hall effect, point-contact tunnelling spectrum and Nernst. We found that unlike single gap superconductors with negligible vortex quantum fluctuations in which vortices are frozen at T=0 K, finite zero-temperature dissipation due to vortex motion exists in MgB₂ over a wide magnetic field range. This dissipation was found to be associated with proliferation of quasiparticles from the π-band of MgB₂. The result shows that the vortex fluctuations are enhanced by two-band superconductivity in MgB₂ and we suggest that the vortex quantum fluctuation is a possible cause of the non-vanishing zero-temperature dissipation.

Work collaborated with Y. Jia, Y. Huang, H. Yang, L. Shan, C. Ren at IOP, CAS and C. G. Zhuang, Y. Cui, Qi Li, Z. K. Liu, X. X. Xi at Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

IV-3: The Classical and Quantum Gauge Glass in 2D

Lei-Han Tang, Qinghu Chen and Peiqing Tong
Department of Physics, Hong Kong Baptist University, China

The two-dimensional (2D) gauge-glass model describes possible vortex glass behavior in disordered superconducting thin films and more recently has been suggested to be a bose metal at zero temperature under weak quantum fluctuations. We report recent numerical analysis of the ground state and low-lying excited states of the corresponding Coulomb gas problem which exhibits zero-temperature criticality in the classical limit. The power-law decay of the spin-wave stiffness with distance is consistently explained in a renormalized dielectric screening theory of gapless vortex-antivortex pair excitations. For the quantum rotor model with strong random frustration, we show that a phase glass phase, characterized by a finite Edwards-Anderson order parameter and zero compressibility, exists at sufficiently low quantum fluctuations. Transition to the Mott insulator state is studied using QMC methods.

IV-4: Free energy distribution of the (1+1)-dimensional random directed polymer problem

Vadim Geshkenbein
ETH Zürich, Switzerland

We consider two configurations of a random directed polymer of length L confined to a plane and ending in two points separated by 2u. Defining the mean free energy difference F and the energy difference of the two configurations, we determine the joint distribution function P(F, F') using replica approach. We find, that for large L and large negative free energies F, the joint distribution functions factorizes into longitudinal P(F) and transverse P(F') components, which furthermore coincide with results obtained previously via different independent routes.