Phase transitions in two dimensions

Leo P. Kadanoff, M. P. M. den Nijs

Paul Fendley, Oleg Tchernyshyov

P. Pieri, Giancarlo Calvanese Strinati, I. Ţifrea

The critical temperature of a dilute Bose gas is obtained in terms of the associated small gas parameter, by a unifying approach which treats three and two spatial dimensions on equal footing, even though the superfluid phase transition is known to be driven by different mechanisms in three and two dimensions. Good agreement is found with the critical temperature obtained by recent Monte Carlo data in three dimensions and by previous analytic methods in two dimensions, respectively. This approach is expected to apply more generally to systems where the many-body theory is controlled by a small physical parameter in the high-temperature phase.

Morishige Yoneda, Masaaki NIWA, 正三 篠原

We proposed the anisotropic charged layered XY lattice model for phase transition in high Tc superconducting oxide. It is capable of describing from Kosterlitz-Thouless transition in two dimension to standard second-order phase transition in three dimension. This model is investigated using the mean field plus two loop effective energy approach. We find the tricritical point above Tc that is suggest the possibility of first order phase transition.

William R. Schief

Sergey E. Savel'ev, Guillermo Ramı́rez-Santiago

By mapping the hamiltonian of the spin one ferromagnet onto that of the classical spherical model we investigate the possible phase transitions and the phase diagram of the spin one ferromagnet. Similarly to what happens in the spherical model we find no phase transition in one and two dimensions. Nonetheless, for three dimensions we obtain a phase diagram in which the most important and unexpected feature is the existence of a ferromagnetic-paramagnetic transition at low temperatures.