Browsing by Author "Chang, VCY"

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Andreev-level tunneling in a ballistic double superconductor normal-metal-superconductor junction
(1997-03-01)
Chang, VCY
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
The supercurrent in a mesoscopic, ballistic, and symmetrically stacked, double superconductor-normal-metal-superconductor (SNS) junction is studied. Our focus is to establish the features in the supercurrent that are associated with the Andreev-level tunneling between the two SNS junctions. Both the junction geometry dependence and the gap function dependence are studied and are analyzed in terms of the quasiparticle scattering processes. The quasiparticle scattering is worked out analytically by solving the Bogoliubov-de Gennes equation within the Andreev approximation. The current is calculated, following Beenakker and van Houten [Phys. Rev. Lett. 66, 3056 (1991)], and the result is found to differ from that of the transmission approach. Such discrepancy between the two approaches, however, is found to disappear for a single SNS junction. In our result, the currents in the two normal regions are checked to be the same.
Critical current of a ballistic asymmetric SNSNS junction
(1998)
Chang, VCY
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
The critical current through an asymmetrically stacked, double superconductor-normal-metal-superconduct (SNS) junction is studied in the mesoscopic and ballistic regime. The asymmetry in such SNSNS junctions excludes any intuitive choice for the phase phi(2) of the middle superconductor S-2. We propose to determine phi(2) from the condition that the currents in the two normal regions are the same. The Bogoliubov-de Gennes equation is solved analytically within the Andreev approximation. In the long S2 limit, we find that the current-phase relation (CPR) can be constructed directly from the CPRs of the two constituent SNS junctions, of which the longer SNS junction imposes a cutoff to the critical current. In short S-2 cases, the cutoff feature remains, but the cutoff value is affected by the Andreev-level tunneling. The dependencies of the SNSNS critical current on the length of S-2, and the asymmetry in the configuration are studied. (C) 1998 Elsevier Science Ltd. All rights reserved.
Current-phase relation of a ballistic asymmetric double superconductor-normal-metal-superconductor junction
(1998-09-01)
Chang, VCY
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
The zero-temperature current-phase (I-phi) relation (CPR) of a mesoscopic, ballistic, and asymmetrically stacked, double superconductor-normal-metal-superconductor (SNS) junction is studied. Here phi is the phase difference between the two superconducting end-electrodes. The lack of configuration symmetry in such asymmetric SNSNS junctions forbids a simple choice of values for the phase phi(2), of the middle superconductor. We propose to determine the values of phi(2), by equating the currents in the two normal regions. Two features in the CPR are found. First, the CPR of the asymmetric junction has a cutoff feature, whose origin is best demonstrated in the long middle superconductor (large L-2) cases, when the critical current of the double SNS junction is bounded by the SNS junction that has the longer normal region. This cutoff feature is more pronounced for longer L-2 and for higher degree of junction asymmetry. Second, in regions other than the cutoff region, the CPR of the asymmetric junction deviates only within a few percent from the CPR of the symmetric junction which has the same total length L-Total in the normal regions and the same L-2. This is in contrast with the greater sensitivity the CPR has to the changes in L-Total or L-2. (C) 1998 Elsevier Science B.V. All rights reserved.