On the Nature of the Phase Transition Triggered by Vortex-Like Deffects in the 2D Ginzburg-Landau Model

The two dimensional lattice Ginzburg-Landau hamiltonian is simulated numerically for different values of the coherence length ξ in units of the lattice spacing a, a parameter which controls amplitude fluctuations. The phase diagram on the plane T − ξ is measured. Amplitude fluctuations change dramatically the nature of the phase transition: for values of ξ/a ≃ 1 , instead of the smooth Kosterlitz-Thouless transition there is a first order transition with a discontinuity in the vortex density v and a sharper drop in the helicity modulus Γ. Both observables v and Γ are analyzed in detail at the crossover region between first and second order which occurs for intermediate values of ξ/a. pacs: 74.20.De, 74.60.-w, 64.60.Cn