Improving the sensitivity of ahigh-Tc SQUID at MHz frequency using a normal metal transformer

Abstract

Superconducting quantum interference devices (SQUIDs) can be used to detectthe signals of nuclear quadrupole resonance (NQR). The NQR frequencies ofsome interesting materials are in the order of MHz. However, the sensitivity of ahigh-Tc SQUID is normally not enough to detect the weak NQR signals. To improve the sensitivity of ahigh-Tc SQUID at MHz frequency, we used a transformer made of normal copper wire.The transformer was composed of a pickup coil, an input coil and a capacitor.The pickup coil was used to detect the magnetic field; the input coil was used tocouple the field to the SQUID; and the capacitor was used to create a resonantfrequency. By using the normal metal transformer, the magnetic field resolution of thehigh-Tc dc SQUID was improved by about 38.8 times (from 220 to5.67 fT Hz−1/2) at 954 kHz.