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arXiv:cond-mat/0602576 · Physical Review Letters

Spin-Dependent Quasiparticle Transport in Aluminum Single-Electron Transistors

Andrew J. Ferguson, Søren E. S. Andresen, R. Brenner, R. G. Clark

Published Aug 25, 2006• 7 citations
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Abstract

We investigate the effect of Zeeman splitting on quasiparticle transport in normal-superconducting-normal (NSN) aluminum single-electron transistors (SETs). In the above-gap transport, the interplay of Coulomb blockade and Zeeman splitting leads to spin-dependence of the sequential tunneling. This creates regimes where either one or both spin species can tunnel onto or off the island. At lower biases, spin-dependence of the single quasiparticle state is studied, and operation of the device as a bipolar spin filter is suggested.

DOI
10.1103/physrevlett.97.086602

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We measured the transport properties and charge noise of superconducting single-electron transistors at the temperature of 0.3 K. The devices were fabricated with different ratios between charging and Josephson energies in order to explore their behavior in different experimental situations. We show that, in spite of the substantial thermal fluctuation, it is possible to extract from the data the values of the device parameters and information on the various tunneling mechanisms. Furthermore, we measured the device noise at different bias points, corresponding to regions with different dominant tunneling mechanisms, namely in the supercurrent, in the Josephson quasiparticle, and in the sequential tunneling regions.
We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor-superconductor-normal-metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal-metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that these processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a rate equation approach in combination with the full counting statistics of multiple Andreev reflection.
We propose a novel method for suppression of quasiparticle poisoning in Al Coulomb blockade devices. The method is based on creation of a proper energy gap profile along the device. In contrast to the previously used techniques, the energy gap is controlled by the film thickness. Our transport measurements confirm that the quasiparticle poisoning is suppressed and clear 2e periodicity is observed only when the island is made much thinner than the leads. This result is consistent with the existing model and provides a simple method to suppress quasiparticle poisoning.
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