Preparation of high-quality factor dielectric resonators for low-noise YBa2Cu3O7-δ radio frequency superconducting quantum interference devices

Tank circuit (resonator) is essential for the operation of the radio frequency (rf) superconducting quantum interference device (SQUID) and the quality factor of the tank circuit has significant impact on the sensitivity of the rf SQUID. For high transition temperature (high-T-c) rf SQUID, dielectric resonator is presently one of the most practical tank circuits. By growing YBa2Cu3O7-delta (YBCO) superconducting thin film on (100) SrTiO3 substrate with a pulsed-laser deposition technique and then patterning the YBCO thin film into flux concentrator with a washer structure, a series of dielectric resonators with unloaded quality factor Q(0) ranging from 200 to 6000 has been prepared. Surface morphology and electrical transport measurements have been carried out to investigate the dependence of Q(0) on the property of the YBCO thin film, which suggests that it is effective to boost the Q(0) of the resonator through improvements in both the crystalline quality and the superconductivity of the thin film. By integrating the resonators with the same SQUID chip, a set of YBCO rf SQUIDs has been assembled to explore the influence of Q(0) of the resonator on the performance of the rf SQUID. It is observed that the white noise of the device decreases substantially with the increase of Q(0), in accordance with the theoretical suggestion. Moreover, as Q(0) rises, the low frequency noise of the device is shown to reduce as well, revealing the importance of the high-Q(0) resonator in depressing the noise of the high-T-c rf SQUID over the entire frequency range. With the resonator of Q(0) = 6000, the YBCO rf SQUID at 77 K exhibits a field sensitivity of 50 fT/Hz(1/2) in the white noise region, 58 fT/Hz(1/2) and 110 fT/Hz(1/2) at 10 Hz and 1 Hz respectively, suggesting its potential in applications such as magnetoencephalography in biomagnetism which have critical requirements on field sensitivity of the high-T-c SQUID especially at low frequencies.