Low-noise thin-film TlBaCaCuO dc SQUIDs operated at 77 K

We have made a series of single-level dc superconducting quantum interference devices (SQUIDs) from 4-μm-thick TlBaCaCuO films with large grain sizes and operated them in liquid nitrogen. Although device characteristics could not be precisely controlled, some devices had white-noise levels that approached thermally limited noise above ∼1000 Hz. In addition, devices with 5 and 80 pH loop inductances had 1/ f noise levels at 10 Hz of 2×10-29 and 5×10-29 J/Hz, respectively. The noise levels at these frequencies are comparable to commercial rf SQUIDs operating in liquid helium, but the hysteresis of the voltage-flux characteristic of the high Tc SQUIDs remains large.