Monte Carlo study of vortex fluctuations in vortex-line phases

The effects of vortex fluctuations on the phase diagram of clean, isotropic, type-II superconductors placed in an external magnetic field H are investigated by Monte Carlo (MC) simulations of a lattice London model. These simulations are carried out at constant H and allow for a finite penetration length lambda. New results supporting previously reported [G. Carneiro, Phys. Rev. Lett. 75, 521 (1995)] evidence for melting of the vortex-line lattice into an entangled vortex liquid, in the region lambda>a(upsilon) (a(upsilon) = mean intervortex spacing), is presented. Fluctuations involving thermal excitation of vortex loops are also investigated. it is found that these fluctuations can be neglected in a temperature region extending from zero to well into the entangled vortex-liquid phase. In this temperature region, the correlations between the field-induced vortices in the entangled vortex liquid are found to be in good qualitative agreement with theoretical predictions based on the analogy between vortex lines and two-dimensional bosons. By comparing MC simulation results with and without an external magnetic field, evidence is found that the entangled vortex liquid evolves smoothly into the normal state as the temperature increases.