Melting of the 2D vortex lattice in thin <i>a</i>-MoGe films

The vortex lattice in a 2D type II superconductor is expected to melt via a BKT transition that involves an intermediate liquid phase (“hexatic fluid”), which preserves orientational order. Recent experiments combining transport and STM imaging made it possible to identify the hexatic state in a-MoGe thin films, showing that vortex diffusivity in the hexatic fluid state is strongly reduced with respect to the isotropic vortex liquid. To fully characterize the melting transition, we performed systematic transport measurements on weakly disordered, 22 nm-thick a-MoGe films down to 0.020 K and in magnetic fields up to 18 T. Our results unambiguously show that the persistence of orientational correlations in the hexatic phase significantly slows down the vortex motion, leading to vanishingly small flux-flow resistance. This effect is consistent with numerical simulations.