PHASE-TRANSITION OF FRUSTRATED 2-DIMENSIONAL JOSEPHSON JUNCTION ARRAYS

We have investigated the resistive behavior of frustrated Josephson tunnel junction arrays. The transitions near f = 1/2, at f = 1 and at a nonspecial value of f = 0.38 are studied in detail with linear and nonlinear resistance measurements. The parameter f is the frustration index, the applied flux per unit cell of the array normalized to the flux quantum PHI-0 = h/2e. The transition at f = 1/2 looks similar to the zero-field Kosterlitz-Thouless transition, including a universal jump in the nonlinear resistance exponent. Compared to f = 0, the transition is shifted to much lower temperatures. Near f = 1/2, below the transition temperature, single vortex crossings dominate the resistance. The transition at f = 1 is qualitively the same as the f = 0 Kosterlitz-Thouless transition, but small deviations are found. For f = 0.38, there is no experimental evidence for a special phase transition; over the whole temperature range, the resistance decreases exponentially with an energy barrier close to two in units of the Josephson coupling energy.