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Journal of Physics A Mathematical and General

On the renormalisation group transformations for the linear Z(3) model with a magnetic field

J. C. Cressoni, Roberto J. V. dos Santos

Published Feb 1, 1986• 0 citations
Publisher

Abstract

The linear Z(3) spin system in the presence of a magnetic field is studied by a dedecoration renormalisation group (RG) transformation. In order to perform the transformation, non-usual terms must be added to the Hamiltonian. The linear region around a particular T=0 fixed point, where these terms go to zero, is investigated and the 'critical behaviour' of the free energy, correlation function and susceptibility is obtained. The system is solved exactly and the results are compared with those obtained by the RG approach. Magnetisation and susceptibility curves against temperature are also shown.

DOI
10.1088/0305-4470/19/2/019

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The linear Z(3) spin system in the presence of a magnetic field is studied by a dedecoration renormalisation group (RG) transformation. In order to perform the transformation, non-usual terms must be added to the Hamiltonian. The linear region around a particular T=0 fixed point, where these terms go to zero, is investigated and the 'critical behaviour' of the free energy, correlation function and susceptibility is obtained. The system is solved exactly and the results are compared with those obtained by the RG approach. Magnetisation and susceptibility curves against temperature are also shown.
The field theoretical renormalisation group method, which correctly describes the critical as well as the multicritical (at zero temperature) behaviours of the quantum mechanical d-dimensional Ising model in a transverse magnetic field, is proposed. Due to the introduction of the additional renormalisation the singularities associated with zero temperature have been removed. The crossover scaling form of the inverse correlation length and longitudinal susceptibility are considered for the following spatial dimensions d:2<d<3, d=3 and 3<d<4. For 2<d<3, d=3 and 3<d<4 the calculations are performed to the first order in 3-d and 4-d, respectively.
Phase diagram, critical properties and thermodynamic functions of the two-dimensional field-free quantum-spin-1/2 XXZ model has been calculated globally using a numerical renormalisation group theory. The nearest-neighbour spin-spin correlations and entanglement properties, as well as internal energy and specific heat are calculated globally at all temperatures for the whole range of exchange interaction anisotropy, from XY limit to Ising limits, for both antiferromagnetic and ferromagnetic cases. We show that there exists long-range (quasi-long-range) order at low-temperatures, and the low-lying excitations are gapped (gapless) in the Ising-like easy-axis (XY-like easy-plane) regime. Besides, we identify quantum phase transitions at zero-temperature.
The linear renormalization-group transformation is proposed to study critical temperatures and temperature dependence of the thermodynamic values (free energy, specific heat) of the weakly interacting spin chains. The method is examined in two classical systems: the standard Ising model on a rectangular lattice and the Ising spin chains coupled by pair-pair interactions. In the latter case the transformation does not exhibit any nontrivial fixed point. This result agrees with the exactly determined free energy of the system under consideration. The method is also applied to the weakly interacting quantum XY spin chains. The critical temperature as a function of the interchain interactions and the temperature dependence of the specific heat are found.