Collective Pinning of a Frozen Vortex Liquid in Ultrathin Superconducting YBa2Cu3O7 Films

The linear dynamic response of the two-dimensional (2D) vortex medium in ultrathin YBa₂Cu₃O₇ films was studied by measuring their ac sheet impedance Z over a broad range of frequencies ω. With decreasing temperature the dissipative component of Z exhibits, at a temperature T^*\(ω\) well above the melting temperature of a 2D vortex crystal, a crossover from a thermally activated regime involving single vortices to a regime where the response has features consistent with a description in terms of a collectively pinned vortex manifold. This suggests the idea of a vortex liquid which, below T^*\(ω\), appears to be frozen at the time scales 1/ω of the experiments.