Normal Metal Island Research Articles

Proximity-induced Cooper-pair in a very small normal-metal island

We analyzed the electronic states in a very small normal-metal (N)/insulator (I)/superconductor (S) tunneling junction, taking into account the charging effect generated with the charge transfer through the insulator. This junction has the microscopic (quasiparticle) and the macroscopic (Cooper-pair) aspects at the same time even if the relevant energies (eV, k B T) are much lower than the superconducting gap energy in the S electrode. By using the expression with Andreev reflection and Cooperon for the proximity effect, both the aspects (micro/macro) can be treated consistently. The results show that if the charging energy E C is comparable to or smaller than the Thouless energy of the N region, a proximity-induced gap appears in the quasiparticle density of states and a Cooper-pair exists in the N region. However, the Cooper-pair does not oscillate between N and S regions. This behavior is quite different from the Cooper-pair in the single Cooper-pair box.

Proximity-induced Josephson-quasiparticle process in a single-electron transistor

We have performed experiments in a superconductor--normal-metal--superconductor single-electron transistor in which there is an extra superconducting strip partially overlapping the normal-metal island in good metal-to-metal contact. The superconducting proximity effect gives rise to current peaks at voltages below the quasiparticle threshold. We interpret these peaks in terms of the Josephson-quasiparticle process and discuss their connection with the proximity-induced energy gap in the normal-metal island.