An experimental study of nonequilibrium superconductivity

Abstract

Experimental investigations were made on the transport properties of several superconducting systems in which the superconducting state was out of thermodynamic equilibrium. In such systems, the dynamics of the pairing processes governs the restoration of equilibrium. Systematic investigations into the nonequilibrium dynamics were made possible by creating a steady state depairing region at a supernormal interface with a transport current. Information Was obtained by direct probing of the potential profiles in the depairing region. Measurements were made with superconducting microcircuits with high spatial resolution extending to the scale of the wavelength of light which limits our fabrication system. It was found that at the depairing region, the experimental results can be described by the existence of two distinct potentials, one for the single electron excitations and one for the electron pairs. A theory for the dynamics of the nonequilibrium process was proposed and succeeded in accounting for most of the experimental observations. The relaxation time associated with this dynamics is found to be longer than the usual equilibrium relaxation time for the condensed electron pairs. Similar measurements were extended to nonequilibrium regions close to current carrying interfaces between two different superconductors. In this case it was found that the two-potential measurements performed within the nonequilibrium region can be described in terms of a time dependent boundary condition at the interface. To study the nonequilibrium superconductivity associated with this time dependent boundary condition in a controlled way, two such superconducting boundaries were brought very close to each other, creating a well defined boundary region less than one micron in length. It was found that the boundary region showed Josephson-like effects as well as a unique phenomenon in which electromagnetic radiation increases the apparent superconducting transition temperature. A model for this situation was proposed in which two complex wavefunctions with independent phase angles interfere quantum mechanically inside the boundary region. This model successfully explains most of the observed phenomena associated with the boundary region in the voltage sustaining nonequilibrium state.