title: Unreachable glass transition in dilute dipolar magnet.
abstract: In magnetic systems the combined effects of disorder and frustration may cause the                 moments to freeze into a disordered state at a spin-glass transition. Recent                 experiments have shown that the rare earth compound                     LiHo(0.045)Y(0.955)F(4) freezes, but that the                 transition is unreachable because of dynamics that are 10(7) times                 slower than in ordinary spin-glass materials. This conclusion refutes earlier                 investigations reporting a speed-up of the dynamics into an exotic anti-glass phase                 caused by entanglement of quantum dipoles. Here we present a theory, backed by                 numerical simulations, which describes the material in terms of classical dipoles                 governed by Glauber dynamics. The dipoles freeze and we find that the ultra-slow                 dynamics are caused by rare, strongly ordered clusters, which give rise to a                 previously predicted, but hitherto unobserved, Griffths phase between the                 paramagnetic and spin-glass phases. In addition, the hyperfine interaction creates a                 high energy barrier to flipping the electronic spin, resulting in a clear signature                 in the dynamic correlation function.
authors.affiliation: Department of Theoretical Physics, Royal Institute of                     Technology, SE-106 91                 Stockholm, Sweden.
authors.forename: A
authors.initials: A
authors.lastname: Biltmo
authors.forename: P
authors.initials: P
authors.lastname: Henelius