MONTE-CARLO STUDY OF VORTEX FLUCTUATIONS IN VORTEX-LINE PHASES

The effects of vortex fluctuations on the phase diagram of clean, isot ropic, type-II superconductors placed in an external magnetic field H are investigated by Monte Carlo (MC) simulations of a lattice London m odel. These simulations are carried out at constant H and allow for a finite penetration length lambda. New results supporting previously re ported [G. Carneiro, Phys. Rev. Lett. 75, 521 (1995)] evidence for mel ting of the vortex-line lattice into an entangled vortex liquid, in th e 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 negl ected in a temperature region extending from zero to well into the ent angled vortex-liquid phase. In this temperature region, the correlatio ns between the field-induced vortices in the entangled vortex liquid a re found to be in good qualitative agreement with theoretical predicti ons based on the analogy between vortex lines and two-dimensional boso ns. By comparing MC simulation results with and without an external ma gnetic field, evidence is found that the entangled vortex liquid evolv es smoothly into the normal state as the temperature increases.