Monte Carlo Study of the Magnetic Flux Lattice Fluctuations in High-Tc Superconductors

by Beny, Cedric

By allowing to measure the magnetic field distribution inside a material, muon spin rotation experiments have the potential to provide valuable information about microscopic properties of high-temperature superconductors. Nevertheless, information about the intrinsic superconducting properties of the material is masked by random thermal and static fluctuations of the magnetic field which penetrates the material in the form of vortices of quantized magnetic flux. A good understanding of the fluctuations of those vortices is needed for the correct determination of intrinsic properties, notably the coherence length ξ, and the field penetration depth λ. We develop a simulation based on the Metropolis algorithm in order to understand the effect, on the magnetic field distribution, of disorder- and thermally-induced fluctuations of the vortex lattice inside a layered superconductor.

Our model correctly predicts the melting temperatures of the YBa2Cu3O6. 95 (YBCO) superconductor but largely underestimates the observed entropy jump. Also we failed to simulate the high field disordered phase, possibly because of a finite size limitation. In addition, we found our model unable to describe the first-order transition observed in the highly anisotropic Bi2Sr2CaCu2O8+y.

Our model predicts that for YBCO, the effect of thermal fluctuations on the field distribution is indistinguishable from a change in ξ. It also confirms the usual assumption that the effect of static fluctuations at low temperature can be efficiently modeled by convolution of the field distribution with a Gaussian function. However the extraction of ξ at low fields requires a very high resolution of the field distribution because of the low vortex density.