Ing-Jye Hwang and D. Stroud, Department of Physics, The Ohio State University, Columbus, Ohio 43210

We study the vortex number noise S_v(omega) and fluctuation conductivity sigma_1(omega) in two-dimensional Josephson junction arrays (JJA's) at three different applied magnetic fields, corresponding to zero, one-half and 1/24 of a flux quantum per plaquette (f = 0, 1/2, and 1/24). S_v and sigma_1 are obtained by numerically solving the equations for the coupled overdamped resistively-shunted-junction model with Langevin noise to simulate the effects of temperature. In all three cases, we find that S_v(omega) is proportional to omega^(-3/2) at high frequencies omega and flattens out to become frequency-independent at low omega, indicative of vortex diffusion, while sigma_1 is proportional to omega^(-2) at sufficiently high omega and to omega^0 at low frequencies. Both quantities show clear evidence of critical slowing down and a simplified scaling behavior near the normal-to-superconducting transitions at f = 0 and f = 1/2, indicating that the vortex diffusion coefficient is approaching zero and the charge carrier relaxation time is diverging at these temperatures. At f = 1/24, there is no clear phase transition; instead, the vortex diffusion coefficient diminishes continuously as the temperature is lowered towards zero. The critical slowing down of S_v(omega), but not its frequency dependence, is in agreement with recent experiments on the flux noise S_Phi(omega) in Josephson junction arrays, which show a 1/omega frequency dependence. We speculate about some possible reasons for the absence of a 1/omega frequency regime.

PACS numbers: 74.50.+r, 74.40.+k, 64.60.Fr