  The hexatic phase is an intermediate stage in the melting process of a 2D
crystal due to topological defects. Recently, this exotic phase was
experimentally identified in the vortex lattice of 2D weakly disordered
superconducting MoGe by scanning tunneling microscopic measurements. Here we
study this vortex state by the Nernst effect, which is an effective and
sensitive tool to detect vortex motion, especially in the superconducting
fluctuation regime. We find a surprising Nernst sign reversal at the melting
transition of the hexatic phase. We propose that they are a consequence of
vortex dislocations in the hexatic state which diffuse preferably from the cold
to hot.
