The structure and the degenracy of the ground state of a fully-frustrated
XY-model are investigated for the case of a dice lattice geometry.
The results are applicable for the description of Josephson junction arrays
and thin superconducting wire networks in the external magnetic field providing
half-integer number of flux quanta per plaquette. The mechanisms of disordering
of vortex pattern in such systems are briefly discussed.Comment: 10 pages, 3 figure