Dresden 2006 – scientific programme

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TT: Tiefe Temperaturen

TT 5: Transport: Nanoelectronics I - Quantum Dots, Wires, Point Contacts - Part 1

TT 5.9: Talk

Monday, March 27, 2006, 12:00–12:15, HSZ 304

Coulomb blockade (CB) and Andreev reflection (AR) in a series of two quantum-point contacts (QPCs) — •Ursula Schröter, Olivier Schecker, and Elke Scheer — FB Physik, Universität Konstanz, 78457 Konstanz

Merging a Green’s functions method and a rate equation technique we developed a model to calculate current-voltage characteristics (IVs) of a series of two QPCs in the normal and the superconducting state. Between the two junctions is a bulk-like metallic island of, however, such small capacitance that the potential shifts considerably with each single excess charge. Our method is valid for arbitrary transmissions of the transport channels in both contacts and multiple AR are included. Interaction between transport processes in both junctions is assumed via the island charging only, but our approach allows extension to maintaining coherence over the whole system.

Based on wave-function overlaps our calculations for QPCs predict qualitatively different behavior from models relying purely on energy conservation and typically used to describe tunnel diodes, even in the low transmission regime. Different conditions have to be fulfilled for particular transport processes to occur. These multiple conditions involve the island charging energy and, in the superconducting state, the gap of the material. Steps in the IVs can be explained in an intuitive energy-level picture. Furthermore we find that multiple AR in the first junction are not necessarily suppressed due to CB by the presence of a second junction of low transmission.