Phase transitions of Josephson-tunnel-junction arrays at zero and full frustration

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Abstract
We have fabricated and studied square two-dimensional arrays of Josephson oxide tunnel junctions. Remarkable structure is observed in the longitudinal and transverse resistance as a function of a perpendicular magnetic field. The linear and nonlinear resistance have been measured for f=0 and f=(1/2), where f is the flux in one cell normalized to the flux quantum. At f=0, for the nonlinear resistance, quantitative agreement is found with predictions for a Kosterlitz-Thouless transition, including the universal jump. At f=(1/2) a different phase transition occurs, showing a jump with a nonuniversal value.