Experimental study of amplitude-frequency characteristics of high-transition-temperature radio frequency superconducting quantum interference devices

We measured the amplitude-frequency characteristics of radio frequency supe rconducting quantum interference devices (rf SQUIDs) over a temperature ran ge between 65 and 79 K. Using the expressions derived from the recently dev eloped rf SQUID theory, valid also at large thermal fluctuations, we determ ined from these data the basic parameters of high-transition-temperature su perconductor (HTS) rf SQUIDs. These parameters were: (a) the high-frequency coupling coefficient between the rf SQUID and the tank circuit resonator, k, (b) the SQUIDs hysteretic parameter, beta, (c) the critical current of t he Josephson junction, I-c, (d) its normal resistance, R-n, and (e) its noi se parameter, Gamma. We found a good agreement with the values of beta (I-c ) and R-n determined directly after destructively opening the SQUID loop. I n accordance with the theoretical predictions, our experimental results sho w that at large thermal fluctuation levels (T congruent to 77 K), rf SQUIDs with large loop inductance operate in nonhysteretic mode up to beta values exceeding 3. Furthermore, we have shown that the optimal energy sensitivit y is attained in the nonhysteretic mode at a value of beta distinctly highe r than 1. A quantitative comparison of white noise predicted by the theory with that obtained from the experiment showed a reasonable agreement. We al so discussed the contribution of the phase information to the SQUID's signa l and noise at optimum operation conditions, when a mixer was used as a sig nal detector. (C) 2000 American Institute of Physics. [S0021-8979(00)08018- X].