Measurement of the I-V characteristics of superconducting dipoles : automatic compensation of low frequency drift

Abstract
Superconducting microbridges and Josephson junctions showing RSJ-like I-V characteristics have potential applications through their current I dependences, at fixed bias voltage V, versus various parameters such as temperature (T), magnetic field (B) or incident optical power (P)... The main problem associated with both low values of voltage (µV-mV range) and dynamic resistance Rd ([MATH] range) was solved earlier ; however, the measurement system still suffers from excessive 1/f noise due to static biasing conditions if neither the incoming signals nor the preamplifier can be chopped. We have overcome this difficulty through the use of a periodic sampling of four points of the I-V characteristic taking advantage of the odd symmetry of this characteristic. In each period, two of the samples occur in the superconducting state and give the zero voltage reference of the measurement system. This allows us to automatically compensate for the low frequency drifts occuring at preamplifier level. The two other periodic samples, opposite in sign, must appear symmetric with respect to the zero voltage reference at the preamplifier output. This constraint is also used to automatically compensate for the low frequency drifts of the ac square signal which controls the I-V operating point. For example, a high Tc superconducting microbridge, used as a temperature sensor, has an equivalent low frequency drift of 0.4 mK_pp and 6 mK_pp respectively with and without the automatic control.