Phase Transitions of 2D Josephson Tunnel Junction Arrays

We have studied square 2D arrays of Josephson tunnel junctions at different values of the normalized magnetic flux per cell f. At f=0 Kosterlitz-Thouless behaviour is observed in the linear as well as in the non-linear resistance. For low nonzero values of f the linear resistance shows thermally activated behaviour at low temperatures with f-dependent activation energy. At f=l/2 the phase transition is different from that at arbitrary or zero field