FINITE-SIZE EFFECTS AND ANISOTROPIC MELTING OF THE VORTEX SOLID IN HIGH-TEMPERATURE SUPERCONDUCTORS

A mean-field model of anisotropic melting of the vortex solid in high-temperature superconductors is proposed. For a slab sample with dimensions lab lc, where 2lab and 2lc are the average diameter of the ab plane and the c axis thickness, respectively, large thermal fluctuations and finite-size effects may result in anisotropic two-dimensional melting at crossover temperatures TX(H) below the three-dimensional-melting transition TM(H). Thus a quasi-two-dimensionally ordered vortex-liquid phase may exist in TX(H) <T<TM(H). Generally, TX(H) decreases with the decreasing sample thickness, increasing magnetic field, and larger Ginzburg-Landau parameter (== /). In the limit of 1/2Hc2 H<Hc2, the geometric anisotropy plays a more important role in determining TX(H) than the electronic-mass anisotropy. © 1990 The American Physical Society.