Field induced tricritical behavior in the S=1/2 quasi one-dimensional frustrated Ising antiferromagnet

The results of extensive histogram cluster heat-bath Monte Carlo simulations on the critical behavior of the quasi-one dimensional Ising antiferromagnet on a stacked triangular lattice are presented. A small applied field is shown to induce a crossover from XY universality to mean-field tricritical behavior. Experimental estimates of critical exponents suggest that these two types of phase transitions are observed in S=1 CsNiCl$_3$ and $S=1/2$ CsCoBr$_3$, respectively. The present results demonstrate that this difference can be explained by an unusual staggered magnetic field arising from quantum exchange mixing previously proposed to account for spin excitations in $S=1/2$ quasi-one-dimensional Ising antiferromagnets.