Long-term charge offset and glassy dynamics in SET transistors

We report long-term measurements of the charge offset Q/sub 0/ in SET (single-electron tunneling) transistors, made of Al/AlO/sub x//Al tunnel junctions. In one case, we saw a Q/sub 0/ which was constant (within 0.1 e) over a twelve-day period, except for one excursion of short interval. In most cases, we see a transient (since cooldown) relaxation of the rate of wandering of Q/sub 0/ over one to two weeks, which is very reminiscent of the nonequilibrium heat evolution observed in glasses. We propose that this mechanism drives both the initial high level of noise in SET transistors, as well as the high error rate in SET pumps.