Tricritical points in systems with random fields

Mean-field theory and renormalization-group arguments are used to show that the phase transition in a system with a random ordering field becomes first order at sufficiently low transition temperature, provided the (symmetric) random-field distribution function has a minimum at zero field. The first-order region is separated from the second-order region by a tricritical point. Both the critical and the tricritical exponents at d>4 dimensions are shown to be the same as for the pure system at d-2 dimensions. The relevance to spin glasses and other systems is discussed. The new tricritical point is very different from all previously studied tricritical points, as it deviates from mean-field theory at d=5, and not at d=3. Although quantitative results are calculated only at d=5- dimensions, the qualitative results are expected to apply at d=3.