Theory of rf SQUIDS operating at 77 K

A comprehensive analytical theory of nonhysteretic rf SQUIDs operating in the adiabatic mode at elevated temperatures, around and above 77 K, is presented. When β≪1 (β is the hysteresis parameter) the theory is applicable also for rf SQUIDs operating in the nonadiabatic mode. In contrast to previous theories which are applicable only if βΓ≪1 (the case of small thermal fluctuations — 4 K), where Γ is the noise parameter, the present theory is valid for βΓ around unity or higher (the case of high thermal fluctuations, 77 K). A good qualitatively agreement with experimental data has been found. Based on this analytical investigation, the superiority at 77 K of rf SQUID over its counterpart, the dc SQUID, is theoretically predicted.