  We report the results of ac sheet conductance measurements performed on fully
frustrated square arrays of Josephson junctions whose coupling energy is
periodically modulated in one of the principal lattice directions. Such systems
are predicted to exhibit two distinct transitions: a low-temperature Ising-like
transition triggered by the proliferation of domain walls and a
high-temperature transition driven by the vortex unbinding mechanism of the
Beresinskii-Kosterlitz-Thouless (BKT) theory. Both the superfluid and
dissipative components of the conductance are found to exhibit features which
unambiguously demonstrate the existence of a double transition whose properties
are consistent with the Ising-BKT scenario.
