New phenomena in the random field Ising model
Laboratoire de Physique Théorique, Ecole Normale Supérieure, 24 rue Lhomond 75231, Paris Cedex 05, France ( Unité propre 701 du Centre national de la recherche scientifique, associée à l'Ecole Normale Supérieure et à l'Université de Paris-Sud.)
2 Service de physique théorique - Saclay, 91190 Gif-sur-Yvette, France
Accepted: 12 August 1998
We reconsider Ising spins in a Gaussian random field within the replica formalism. The corresponding continuum model involves several coupling constants beyond the single one which was considered in the standard theory approach. These terms involve more than one replica, and therefore in a mean-field theory they do not contribute to the zero-replica limit. However, the fluctuations involving those extra terms are singular on the Curie line below eight dimensions, and by the time one reaches the dimension six, it is necessary to keep them in the renormalization group analysis. As a result it is found that there is no stable fixed point of order . Whether this means that there is no expansion in powers of , or that the transition is driven to first order by these fluctuations, is difficult to decide at this level, but it explains the failure of the correspondence.
PACS: 05.50.+q – Lattice theory and statistics; Ising problems / 64.60.Fr – Equilibrium properties near critical points, critical exponents
© EDP Sciences, 1998