CRITICAL-BEHAVIOR OF TB - A PHYSICAL REALIZATION OF A LIFSHITZ POINT

We consider a XY spin system in a hexagonal crystal lattice with latti ce constants a and c and with competing interactions J(1) and - J(2) a long the z direction. The model is in the same universality class as t he ANNNXY one that exhibits an helicoidal structure characterized by a period q(c). Using renormalization-group analysis carried out to firs t order in epsilon = d - 4.5, we show that q(c) proportional to (c/a - (c/a*))(beta q) with an exponent beta(q) characteristic of the Lifsh itz point vicinitzy. Applications to Tb are shown to be in good agreem ent with the experiments.