Signal properties of radio frequency SQUID with a finite amplitude of the second harmonic in the current-phase relationship

The signal properties of radio frequency (rf) SQUIDs with a finite amplitude of the second harmonic in the current-phase relationship have been theoretically and experimentally investigated. Both amplitude as well as phase detection has been considered in order to readout the SQUID's output signal. We observed peculiarities of the rf SQUID response of a SQUID with YBCO 45 degree grain boundary Josephson junction. We found that the current-phase relationship contains a finite amplitude of the second harmonic. Comparison of the theoretical and experimental results is given. It is shown that the sign of the amplitude of the second harmonic can be determined from the shape of experimental curves, and the value of this amplitude is defined by the positions of extrema of these curves.