Magnetic field induced suppression of vortex flow resistance in superconductors with periodic pinning centers

We study vortex flow resistance (VFR) in films of Pb with holes as periodic pinning centers (PPCs) and of Nb with PPCs in the form of Ni dots, as function of temperature, dc current and constant applied magnetic field. The experimental resolution is better than 10−5 of the normal state resistance. At high temperatures near to Tc and high drive currents the resistance shows local minima both at matching fields and zero field. For lower temperatures, however, in a narrow temperature range before the vortex system becomes completely frozen, we observe suppression of the VFR with increasing magnetic field. In the Pb film with PPCs these phenomena show up as a clear zero field resistance excess, which is gradually suppressed by the applied magnetic field. We attribute this unusual feature in the magnetoresistance to thermally excited vortex-antivortex pairs. In the Nb superconducting film with PPCs we observe a gradual suppression of the VFR near integer matching fields up to n = 3 at the lowest accessible dc currents and relevant temperatures. This anomalous behaviour is followed by the observation of negligible VFL for higher fields (H3 < H < H4) and a strong enhancement of VFR for H > H4. We tentatively explain this observation as due to the moving vortex glass.