Orientational order in weakly pinned Abrikosov flux lattices is studied, taking into account two heretofore neglected effects: dislocations and orientational couplings to the underlying lattice. Without orientational couplings, arbitrarily weak pinning destroys long-ranged orientational order for all spatial dimensions d<4. Orientational couplings stabilize long-ranged orientational order. For fields along an axis of fourfold symmetry, sixfold (hexatic) orientational order is described by a random-field Ising model, and so does not occur in d=2 (thin films) but does in d=3 (bulk).

• Received 4 December 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.2523