Observation of quantum interference effects and SQUID operation in a bulk sample of YBa2Cu3Oy at 77 and 4.2 K

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Abstract
We have studied the response of a bulk sample of YBa2Cu3Oy to an applied static magnetic field B and a 20‐MHz rf field. At both 77 and 4.2 K we found clear evidence of behavior within the sample that was characteristic of a superconducting quantum interference device. At 77 K, the absorption of rf power was a periodic function of B, characterized by several superposed periods in the range 8×10−8 to 9×10−6 T. At 4.2 K only one period, of 4.8×10−7 T, was evident. By applying flux feedback we could lock to minima in the response, and in this mode of operation as a magnetometer the field spectral density S1/2B was 1.5×10−10 T Hz−1/2 at 77 K and 5.8×10−10 T Hz−1/2 at 4.2 K.