Single-layer 2nd-order SQUID gradiometer and fabrication of YBa_2Cu_3O_7 nanobridges as theJosephson elements

摘要

We have studied fabrication of second-order SQUID gradiometers from single-layer of high-T_c. film. The gradiometer contains three parallel-connected pickup coils which are directly coupled to a step-edge junction SQUID. In the study of short-baseline devices, we achieved an unshielded noise of 0.84 pT/cm~2/Hz~(1/2) at 1 Hz for a well-balanced gradiometer. As Josephson elements of lone-baseline devices we made submicron YBCO bridges by using a focused ion beam method. In temperature-dependent critical currents, I_C(T), and normal state resistances, R_N(T), the bridge showed an SIS-type behavior, which is believed to be due to naturally formed grain boundaries crossing the bridges.