Brownian motion of vortex-antivortex excitations in very thin films of Bi2Sr2CaCu2O8

Low-frequency magnetic noise and mutual inductance measurements indicate that the electromagnetic response of very thin Bi₂Sr₂CaCu₂O₈ films is dominated by Arrhenius law diffusion of thermally created vortices and antivortices. We explain the measured diffusion coefficient via pinning of the vortex cores on a single type of line defect. The observed superfluid response differs from simple Kosterlitz-Thouless predictions as it is predominantly controlled by vortex-pin rather than vortex-antivortex interactions.