"Dynamical and non-equilibrium electron transport and fluctuation processes in superconducting bicrystal junctions and heterostructures"

Supported by: Russian Foundation for Basic Research, Project number 04-02-16818
Duration: 2004-2006
Supervisor: Karen Constantinian, IREE RAS, Moscow, Russia

Abstract (Results 2004)

Theoretical investigation was carried out of electron transport phenomena in superconducting heterostructures S-MB-N and S-MB-S with magnetic barriers (MB) determined by monolayer of ferromagnet (F) or spin-valve structures FNF or FIF (N is a normal metal, I is insulating layer). The I-V characteristics of S-MB-N structures were found for voltages including DC and AC (oscillating with definite frequency) components. The DC Josephson effect was studied for S-FIF-S and S-FcF-S structures (c is a constriction) for diffusive transport through F layers with arbitrary mutual orientation of the exchange fields (EF) in the F layers directed parallel to their planes. For these structures the theory of AC Josephson effect was constructed for the cases of parallel and antiparallel EF orientations in the F layers.

The I-V characteristics of cuprate bicrystal Josephson junctions have been experimentally investigated at different temperatures, barrier transparency factors and the phase difference of superconducting electrodes. Experimental data testify, that the most probable mechanism of current transport in such structures is a tunneling through a barrier with participation in the process of the mid-gap bound states, arising from multiple Andreev reflection. However no complete theory exists, which can explain experimental situation: deviations of waveforms of current-phase relation and magnetic-field dependences of a critical current, features caused by non-homogeneous barrier nature, arising from barrier faceting during the epitaxial film growth.

For bicrystal junctions with d-wave superconducting electrodes at high voltages V> 5 mV an excess noise larger than thermal fluctuations have been observed, conterminous with the dependence of shot noise as for the case of superconducting SIS tunnel junctions. In the region of small voltages V <1 mV such junctions show peak of noise (excess over 2e of effective transferred charge), a characteristic, inherent to superconducting junctions with multiple Andreev reflection.

In planar heterojunctions Nb/Au/YBa2Cu3Ox with an inclined c-axis of the YBa2Cu3Ox film a reduction of the maximal height of the first Shapiro step with increase of applied microwave frequency was revealed. Observation of the sub-harmonic detector response and the fractional n=1/2 Shapiro steps are specific for the presence of the second harmonic in current-phase relation.

The novel type of thin-film bicrystal Josephson junctions made from metal-oxide superconductors with the asymmetrically inclined electrode is offered and fabricated. The first investigations of electron transport and dynamic microwave characteristics of those junctions have been carried out.