Onsager loop transition and first-order flux-line lattice melting in high-Tcsuperconductors

Abstract

Monte Carlo simulations in conjunction with finite-size scaling analysis are used to investigate the $\mathrm{}(H,T)\mathrm{}$-phase diagram in uniaxial anisotropic high-$T_c$ superconductors, both in zero magnetic field $\mathrm{}(B=0\mathrm{})\mathrm{}$ and in intermediate magnetic fields $(0\mathrm{}\ll B\ll B_{c2\mathrm{}})$ for various mass anisotropies. The model we consider is the uniformly frustrated anisotropic Villain model, which is dual to the lattice London model with an infinite London penetration length. The quantities we consider are various helicity moduli, the structure function, the specific heat, and the distribution of closed non-field-induced vortex loops as a function of the loop size. In zero magnetic field, and for all anisotropies considered, we find one single second-order phase transition, mediated by an Onsager vortex-loop unbinding transition, or blowout. This is the superconductor–normal-metal transition. A comparison with numerical simulations and a critical scaling analysis of the zero-field loop transition yields the same exponent of the loop-distribution function at the critical point. In the intermediate magnetic-field regime, we find two anomalies in the specific heat. The first anomaly at a temperature $T_m$ is associated with the melting transition of the flux-line lattice. The Lindemann ratio at the melting is given by $c_L\approx \mathrm{0.24.}$ The second anomaly at a temperature $T_z$ is one where phase coherence in the BCS order parameter across the sample along the field direction is destroyed. We argue that $T_m{=T}_z$ in the thermodynamic and continuum limit. Hence, there is no regime where the flux-line lattice melts into a disentangled flux-line liquid. The loss of phase coherence parallel to the magnetic field in the sample is argued to be due to the proliferation of closed non-field-induced vortex loops on the scale of the magnetic length in the problem, resulting in flux-line cutting and recombination. In the flux-line liquid phase, therefore, flux lines appear no longer to be well-defined entities. Above the melting temperature, the system always exhibits an incoherent vortex-liquid phase characterized by lack of phase coherence in the BCS order parameter parallel to the magnetic field. For increasing anisotropy, we resolve a δ-function peak in the specific heat. A finite-size scaling analysis of the δ-function peak specific-heat anomaly at the melting transition is used to extract the discontinuity of the entropy at the melting transition. This entropy discontinuity is found to increase rapidly with mass anisotropy, at least for not too layered compounds.