Theory of the voltage-current characteristic of high Tc DC SQUIDs and its experimental verification

A new theory for the voltage-current characteristics of high T-c DC SQUIDs is developed. The theory is based on an analytical solution of the two-dimensional Fokker-Planck equation. which describes the dynamical behavior of DC SQUIDs in the presence of large thermal noise. For high T, DC SQUIDs. which operate at 77 K, with L > 100 pH (L is the geometrical inductance of the DC SQUID loop), the theory is valid for any values of the reduced inductance beta = 2LI(C)/Phi(0) and noise parameter Gamma = 2pik(B)T/1(C) Phi(0) (Phi(0) is the flux quantum. I-C is the critical current of one Josephson junction. and k(B) is the Boltzmann constant). Hence. the theory is applicable for most of practical high T-c DC SQUIDs with Gamma approximate to 0.1-1. beta greater than or equal to 1. We made extensive studies to compare our theory with 70 high T-c DC SQUIDs with a sufficiently large range of inductances and critical currents. The results of the comparison are presented. (C) 2001 Elsevier Science B.V. All rights reserved.