Phase transition of frustrated two-dimensional Josephson junction arrays

We have investigated the resistive behavior of frustrated Josephson tunnel junction arrays. The transitions nearf=1/2, atf=1 and at a nonspecial value off=0.38 are studied in detail with linear and nonlinear resistance measurements. The parameterf is the frustration index, the applied flux per unit cell of the array normalized to the flux quantum ₀=h/2e. The transition atf=1/2 looks similar to the zero-field Kosterlitz-Thouless transition, including a universal jump in the nonlinear resistance exponent. Compared tof=0, the transition is shifted to much lower temperatures. Nearf=1/2, below the transition temperature, single vortex crossings dominate the resistance. The transition atf=1 is qualitively the same as thef=0 Kosterlitz-Thouless transition, but small deviations are found. Forf=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.