Operation of high Tc dc-SQUIDs in dc and ac bias flux-locked loop

We have fabricated YBCO bicrystal junction SQUIDs having various inductance values and measured their noise power spectra in dc and ac bias FLL operations. In the devices, in which low-frequency noise was small, the ac current bias was effective to reduce the noise further. However in other devices apparent flux noise was dominant, which could not be reduced by the ac bias operation. These devices operating at temperatures near 77 K exhibited significant thermal rounding of the I-V characteristic. It is suggested that the thermal rounding contributes to the observed reduced voltage modulation, which is much lower than the value expected from the inductance of the SQUID