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arXiv:cond-mat/9604021 · Applied Physics Letters

Charge sensitivity of superconducting single-electron transistor

Alexander N. Korotkov

Published Oct 21, 1996• 31 citations
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Abstract

It is shown that the noise-limited charge sensitivity of a single-electron transistor using superconductors (of either SISIS- or NISIN-type) operating near the threshold of quasiparticle tunneling, can be considerably higher than that of a similar transistor made of normal metals or semiconductors. The reason is that the superconducting energy gap, in contrast to the Coulomb blockade, is not smeared by the finite temperature. We also discuss the increase of the maximum operation temperature due to superconductivity and the peaklike features on the I–V curve of SISIS structures.

DOI
10.1063/1.117710

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It is shown that the noise-limited charge sensitivity of a single-electron transistor using superconductors (of either SISIS- or NISIN-type) operating near the threshold of quasiparticle tunneling, can be considerably higher than that of a similar transistor made of normal metals or semiconductors. The reason is that the superconducting energy gap, in contrast to the Coulomb blockade, is not smeared by the finite temperature. We also discuss the increase of the maximum operation temperature due to superconductivity and the peaklike features on the I–V curve of SISIS structures.
We have investigated the effects of quantum fluctuations of quasiparticles on the operation of superconducting radio-frequency single-electron transistors (rf-SETs) for large values of the quasiparticle cotunneling parameter alpha = 8EJ/Ec, where EJ and Ec are the Josephson and charging energies. We find that, for alpha > 1, subgap rf-SET operation is still feasible despite quantum fluctuations that wash out quasiparticle tunneling thresholds. Surprisingly, such rf-SETs show linearity and signal-to-noise ratio superior to those obtained when quantum fluctuations are weak, while still demonstrating excellent charge sensitivity.
Assuming the absence of 1/f noise, we have calculated the charge sensitivity (CS) of a capacitive-coupled double-junction single electron tunneling transistor (SETT) with a superconducting grain, operating at low temperature and low bias voltage such that the tunneling current is dominated by the Andreev process. In this case the effect of co-tunneling is suppressed without using the complicated fabrication method. We have used a realistic sample to demonstrate the procedure of optimizing the CS of a SETT with respect to relevant physical parameters and structure parameters. With the presently available fabrication technology, our theoretical analysis can serve as a guide for increasing the CS of a SETT beyond 10−6 e/√Hz.
We develop a microscopic theory for charge transfer through a single-electron transistor containing a superconducting grain. Both tunnel current and shot noise are shown to be sensitive to the parity of the electron number on the grain. In particular, tunneling may cross over from pair channels to two distinct single-particle channels at finite voltage. The two corresponding threshold voltages are sensitive to the circuit parameters. While it is difficult to identify them from the I-V curve, they can be determined from the normalized zero-frequency noise and the finite-frequency noise spectrum.
We analyze charge tunneling statistics and current noise in a superconducting single-electron transistor in a regime where the Josephson-quasiparticle cycle is the dominant mechanism of transport. Due to the interplay between Coulomb blockade and Josephson coherence, the probability distribution for tunneling events strongly deviates from a Poissonian and displays a pronounced even-odd asymmetry in the number of transmitted charges. The interplay between charging and coherence is reflected also in the zero-frequency current noise which is significantly quenched when the quasiparticle tunneling rates are comparable to the coherent Cooper pair oscillation frequency. Furthermore, the finite frequency spectrum shows a strong enhancement near the resonant transition frequency for Josephson tunneling.
We analyze charge tunneling statistics and current noise in a superconducting single-electron transistor in a regime where the Josephson-quasiparticle cycle is the dominant mechanism of transport. Due to the interplay between Coulomb blockade and Josephson coherence, the probability distribution for tunneling events strongly deviates from a Poissonian and displays a pronounced even-odd asymmetry in the number of transmitted charges. The interplay between charging and coherence is reflected also in the zero-frequency current noise which is significantly quenched when the quasiparticle tunneling rates are comparable to the coherent Cooper pair oscillation frequency. Furthermore, the finite frequency spectrum shows a strong enhancement near the resonant transition frequency for Josephson tunneling.
Here, we report the operation of a superconducting single-electron transistor (S-SET) as a fast and nearly quantum-limited charge-sensitive amplifier. By operating the S-SET such that Cooper pair tunneling is resonant across both of its tunnel junctions, we are able to achieve an excellent charge sensitivity of δq = 1.6 × 10-6e/√(Hz), while also minimizing back action. We find that near its optimal operating point, the S-SET can have both a very low noise temperature Tn≈ 50 mK and a large power gain ASET>;>; 1. These results indicate that our S-SET operates within a factor of 2 of the quantum limit.