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We report linear and nonlinear electrical transport in perpendicular magnetic fields of very thin amorphous Mo79Ge2i superconducting films. At low fields, we demonstrate the existence of a barrier to vortex motion away from the sample edges. In regimes where the vortex motion is not substantially affected by the sample edges, we observe manifestly collective activated behavior. We relate the field dependence of the observed activation energies to possible mobile configurations of vortices.

Measurements of the ac penetration depth of a-MoGe films in the presence of a perpendicular magnetic field reveal an anomaly in the ac response of the vortex lattice at a characteristic temperature below the H c i(T) line. The field and frequency dependence of this anomaly is found to be consistent with a Kosterlitz-Thouless-type melting of the two-dimensional vortex lattice. Moreover, we observe a crossover in the frequency dependence which suggests that the vortex lattice remains disordered on long length scales below the melting temperature.PACS numbers: 74.60.Ec, 74.60.GeThe question of the existence of a finite-temperature phase transition within the vortex state of type-II superconductors has received considerable attention since the discovery of the high-T^ superconductors. One possible phase transition in these superconductors, as well as conventional superconductors with reduced dimensions, is the dislocation-mediated Kosterlitz-Thouless (KT) [1] melting transition in the two-dimensional vortex lattice (VL). In conventional superconductors, the possibility of this phase transition was first suggested by Huberman and Doniach [2] and by Fisher [3]. At the transition, the solid vortex lattice phase which contains thermally created bound pairs of dislocations, melts as these pairs dissociate at large distances to create free dislocations. However, as Larkin [4] showed long ago, in the solid phase, any amount of disorder will disturb the lattice on long length scales, hence limiting positional correlations to a length scale R c > The consideration of disorder raises two important questions: (1) Can the disordered VL "melt locally" as each correlated volume (vortex bundle) melts internally via a KT-type transition with increasing temperature? (2) Do these correlated volumes freeze into a glasslike state at a lower temperature [5]?To date, there are two reports of experiments that have been interpreted as evidence for KT-type melting of the VL. The vibrating-reed experiments of Gammel, Hebard, and Bishop [6] on amorphous composite In/InO x films showed a large dissipation peak at around the predicted melting temperature. However, recently Brandt [7] has argued that these experimental results are likely related to a size-dependent electromagnetic crossover rather than the melting of the VL. The other case is the resistivity and I-V measurements of Berghuis, van der Slot, and Kes [8] on films of a-NbGe. These experiments were interpreted as a crossover of a pinned VL with a finite critical current at low fields to a melted vortex fluid with flux flow resistance at high fields. The crossover was found to occur close to the expected phase boundary.In this Letter, we present evidence for the existence of a melting transition on short length scales in the disordered VL by studying the behavior of the ac penetration depth of amorphous Mo77Ge23 thin films in the presence of a perpendicular magnetic field. Specifically, we interpret observed abrupt changes in the ac penetration depth as evidence for melting of the two-d...

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