begin{center}(séminaire PHYSTAT-SUD)end{center} Many classical geometrically frustrated spin systems have macroscopically degenerate ground states. In a class of three-dimensional examples, the set of degenerate ground-state spin configurations has power-law correlations and constitutes what is known as a Coulomb phase. This power-law phase is stable against small perturbations, but large perturbations can induce a phase transition to an ordered state. For such a transition, one can ask what difference it makes that the order develops from the Coulomb phase rather than from a conventional, paramagnetic phase. I will discuss this question in connection with spin freezing in frustrated magnets, and in relation to ordering transitions not described by a conventional Landau-Ginzburg approach.

Physics Department, Oxford University