  We derive the the long-wavelength elastic theory for the quantum Hall smectic
state starting from the Hartree-Fock approximation. Dislocations in this state
lead to an effective nematic model for $T>0$, which undergoes a disclination
unbinding transition from a phase with algebraic orientational order into an
isotropic phase. We obtain transition temperatures which are in qualitative
agreement with recent experiments which have observed large anisotropies of the
longitudinal resistivities in half-filled Landau levels, lending credence to
the liquid crystal interpretation of experiments.

Mesoscale and Nanoscale Physics;Soft Condensed Matter;Statistical Mechanics