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Journal of Physics C Solid State Physics

Low-temperature renormalisation group for disordered ferromagnets and spin glasses

Gary S. Grest

Published Nov 28, 1977• 5 citations
Publisher

Abstract

Disordered spin systems with the number of spin components m are studied in dimensions d=2+ epsilon for small epsilon . For dilute quenched random systems, it is found that the system has the same critical behaviour as for a pure ferromagnetic if m>2. For the spin-glass phase, new critical exponents are calculated for m>or=2. The x-y spin glass is found to have a phase, new critical exponents are calculated for m>or=2. The x-y spin glass is found to have a vanishing critical temperature as d approaches 2, in contrast to the ferromagnetic x-y model.

DOI
10.1088/0022-3719/10/22/002

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Disordered spin systems with the number of spin components m are studied in dimensions d=2+ epsilon for small epsilon . For dilute quenched random systems, it is found that the system has the same critical behaviour as for a pure ferromagnetic if m>2. For the spin-glass phase, new critical exponents are calculated for m>or=2. The x-y spin glass is found to have a phase, new critical exponents are calculated for m>or=2. The x-y spin glass is found to have a vanishing critical temperature as d approaches 2, in contrast to the ferromagnetic x-y model.
The Niemeijer-van Leeuwen formulation of the renormalisation group has been used to discuss the quenched dilute Ising model and the Ising spin-glass in two dimensions. The authors chose a five-spin cell which, in the pure case, treats the Ising antiferromagnet equally as well as the ferromagnet. This choice of cell leads to the existence of a spin-glass ground state which was found to be stable for the whole composition range. The spin-glass transition was found to be sharp and the corresponding critical exponents were calculated. For the dilute Ising model the behaviour of the reduced critical temperature T c (p)/T c (1), the percolation concentration and the correlation length exponent were found to be in good agreement with the exact results and series calculations.
Abstract The second‐order phase transition of a weakly disordered system in 2 + ϵ dimensions is investigated by means of the low‐temperature renormalization group technique. It is shown that there are two fixed points: the Migdal‐Polyakov and the unphysical ones. As the Migdal‐Polyakov fixed point is always stable it is concluded that the disordered and the ordered systems have the same critical exponents. These results agree with the Harris hypothesis, which predicts that phase transitions with positive value of α do not occur in the presence of quenched disorder. The effects of the long‐range exchange interaction on the critical behaviour of the quenched system in σ + ϵ dimensions are also discussed. The criteria for the concentration of nonmagnetic impurities in the systems under consideration are established.
Theoretical ideas for deriving singularities of thermodynamical functions at the second-order phase transitions in spin systems with weak quenched disorder are considered. In particular, p-component vector magnets and the two-dimensional Ising model with disorder in spin-spin interactions are studied. Generalisation of the traditional renormalisation-group scheme, which takes into account non-perturbative spin-glass degrees of freedom, is proposed. Low-temperature properties and the phase transition in the Ising systems with quenched random fields are also considered.
We study the critical properties of the weakly disordered p-component random Heisenberg ferromagnet. It is shown that if the specific-heat critical exponent of the pure system is positive, the traditional renormalization group (RG) flows at dimensions D=4-ε, which are usually considered as describing the disorder-induced universal critical behaviour, are unstable with respect to replica-symmetry breaking (RSB) potentials such as those found in spin glasses. It is demonstrated that the RG flows involving RSB potentials lead to fixed points which have a structure known as the one-step RSB, and there exists a whole spectrum of such fixed points. It is argued that spontaneous RSB can occur due to the interactions of the fluctuating fields with the local non-perturbative degrees of freedom coming from the multiple local minima solutions of the mean-field equations. However, it is not clear whether or not RSB occurs for infinitesimally weak disorder. Physical consequences of these conclusions are discussed.
We study the critical properties of the weakly disordered p-component random Heisenberg ferromagnet. It is shown that if the specific-heat critical exponent of the pure system is positive, the traditional renormalization group (RG) flows at dimensions D=4- epsilon , which are usually considered as describing the disorder-induced universal critical behaviour, are unstable with respect to replica-symmetry breaking (RSB) potentials such as those found in spin glasses. It is demonstrated that the RG flows involving RSB potentials lead to fixed points which have a structure known as the one-step RSB, and there exists a whole spectrum of such fixed points. It is argued that spontaneous RSB can occur due to the interactions of the fluctuating fields with the local non-perturbative degrees of freedom coming from the multiple local minima solutions of the mean-field equations. However, it is not clear whether or not RSB occurs for infinitesimally weak disorder. Physical consequences of these conclusions are discussed.
By means of the renormalisation-group approach, static and dynamic critical behaviours are studied for the XY spin system with cubic anisotropy and with quenched random nonmagnetic impurities. The stability of the quenched-random-impurity fixed point in d ( identical to 4- epsilon ) dimensions, the flow diagram of the interactions and the critical exponents are investigated, within the two-loop approximation, in terms of the epsilon 1/2 expansion. It is shown that the transition that appears is a first-order transition.