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IEEE Transactions on Applied Superconductivity

Understanding the behavior of superconducting Coulomb-blockade electrometers

D. Song, A. Amar, Christopher J. Lobb, F. C. Wellstood

Published Jun 1, 1997• 0 citations

Abstract

We report measurements and quantum mechanical simulations of the behavior of superconducting two-junction Coulomb-blockade electrometers biased near the superconducting gap. Our quantum model allows coherent superpositions of states with different number of charges on the island. It predicts maximum current modulation /spl delta/I for devices with tunneling resistance R/sub /spl Sigma///spl ap/64/spl pi//spl Delta//spl planck//sup 2/C/sub /spl Sigma///sup 2//(e/sup 6//spl tau/), where /spl tau/ is the time for which the system maintains quantum coherence, C/sub /spl Sigma// is the total island capacitance and /spl Delta/ is the superconducting energy gap. For comparison, we built Al-AlO/sub x/-Al electrometers with R/sub /spl Sigma// from 61 k/spl Omega/ to 1.7 M/spl Omega/ and C/sub /spl Sigma// ranging from 0.9 fF to 8 fF. We find good agreement for /spl tau//spl ap/0.35 nsec.

DOI: 10.1109/77.622157

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We have measured the current as a function of gate charge Q in superconducting Coulomb blockade electrometers with charging energies

{\mathit{E}}_{\mathit{c}}

\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

{\mathit{E}}_{\mathit{c}}

.