High sensitivity double relaxation oscillation superconducting quantum interference devices with large transfer from flux to voltage

Abstract

Double relaxation oscillation superconducting quantum interference devices (SQUIDs) (DROSs) based on Nb/AlOx tunnel junctions have been fabricated and characterized. The estimated relaxation frequencies range from 400 MHz up to 14 GHz. Transfer coefficients from flux to voltage of 2 up to 7 mV/φ0 have been obtained. Both the intrinsic flux noise and the performance in a flux-locked loop with direct-voltage readout have been determined. Special attention is paid to the effect of damping resistors on the sensitivity of DROSs. The intrinsic sensitivity improves with increasing relaxation frequency, leveling off to a value of 13h at relaxation frequencies above 2–3 GHz for SQUID inductances of about 30 pH. This sensitivity is very close to the theoretical maximum sensitivity of 6h of a comparable standard type dc SQUID. In a flux-locked loop based on direct-voltage readout, a noise level of 0.55 μφ0/√Hz corresponding to an energy sensitivity of 34h has been obtained for a DROS with a SQUID inductance of 29 pH.