D. Agassi et Jr. Cullen, CURRENT-PHASE RELATION IN AN INTERMEDIATELY COUPLED SUPERCONDUCTOR-SUPERCONDUCTOR JUNCTION, Physical review. B, Condensed matter, 54(14), 1996, pp. 10112-10120
The limiting configuration of the S-N-S' junction in which the N-layer
thickness is of the order of the screening length in metals (approxim
ate to 1-2 Angstrom) may serve as a model for a clean S-S' interface.
This configuration supports one N-layer-attached gap state (manifold),
with eigenenergy close to and less than the smaller superconducting g
ap. This gap state is interpreted as the excitation associated with br
eakage of interface-attached Cooper pairs, comprised of one electron f
rom each side of the thin-N layer. The corresponding junction's curren
t-phase (psi) relation is calculated. The gap state is shown to contri
bute an appreciable ''sin(2 psi)'' correction term to the ''sin(psi)''
Josephson term. The corresponding maximum-current magnetic-flux depen
dence of a two-junction configuration (SQUID) is predicted to have zer
os shifted from their common loci at half-integer flux quanta.