FIRST AND 2ND-ORDER TRANSITION IN FRUSTRATED XY SYSTEMS

The nature of the phase transition for the XY stacked triangular antif erromagnet (STA) is a controversial subject at present. The field theo retical renormalization group (RG) in three dimensions predicts a firs t order transition. This prediction disagrees with Monte-Carlo (MC) si mulations which favor a new universality class or a tricritical transi tion. We simulate by the Monte-Carlo method two models derived from th e STA by imposing the constraint of local rigidity which should have t he same critical behavior as the original model. A strong first order transition is found. Following Zumbach we analyze the second order tra nsition observed in MC studies as due to a fixed point in the complex plane. We review the experimental results in order to clarify the crit ical behavior observed.