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arXiv:cond-mat/0308253 · Physical Review Letters

Nonequilibrium Quasiparticles and 2 e Periodicity in Single-Cooper-Pair Transistors

José Aumentado, Mark W. Keller, John M. Martinis, Michel Devoret

Published Feb 13, 2004• 250 citations
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Abstract

We have fabricated single-Cooper-pair transistors in which the spatial profile of the superconducting gap energy was controlled by oxygen doping. The profile dramatically affects the switching current vs gate voltage curve of the transistor, changing its period from 1e to 2e. A model based on nonequilibrium quasiparticles in the leads explains our results, including the observation that even devices with a clean 2e period are "poisoned" by small numbers of these quasiparticles.

DOI
10.1103/physrevlett.92.066802

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We have fabricated single-Cooper-pair transistors in which the spatial profile of the superconducting gap energy was controlled by oxygen doping. The profile dramatically affects the switching current vs gate voltage curve of the transistor, changing its period from 1e to 2e. A model based on nonequilibrium quasiparticles in the leads explains our results, including the observation that even devices with a clean 2e period are "poisoned" by small numbers of these quasiparticles.
We present experiments on single Cooper-pair transistors made of two different superconducting materials. We chose Ti and Al to create an energy gap profile such that the island has a higher gap than the leads, thereby acting as a barrier to quasiparticle tunneling. Our transport measurements demonstrate that quasiparticle poisoning is suppressed in all our TiAlTi structures (higher gap for the island) with clear 2e periodicity observed, whereas full quasiparticle poisoning is observed in all AlTiAl devices (higher gap for the leads) with e periodicity.
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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We have measured the current as a function of gate charge Q in superconducting Coulomb blockade electrometers with charging energies Ec{\mathit{E}}_{\mathit{c}}Ec​\ensuremath{\ll}\ensuremath{\Delta}, the superconducting gap. We find a large pair current which exhibits a clear transition from 2e to e periodicity at about 250 mK. To explain our data, we propose an equilibrium model in which the current is due entirely to Cooper pairs. The periodicity change results from rapid shifts in the instantaneous effective Q, caused by fluctuations in quasiparticle number. We find good agreement, and predict the loss of 2e periodic current for large Ec{\mathit{E}}_{\mathit{c}}Ec​.
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