A cluster Monte Carlo (MC) updating algorithm is presented for the antiferromagnetic
Z(q) model, into which Ising variables are embedded. Using MC simulations combined with the histogram technique, we were able to study the q-state clock model, for
q=5 and
q=7, in
d=2 and
d=3 dimensions. The two-dimensional system exhibits a phase transition at temperature
T=0 for both
q=5 and
q=7 system states. The critical exponents were derived as well as the ground state entropy for
T=0. Our results for the
q=5 and the
q=7 three-dimensional systems show that they exhibit a phase transition at nonzero temperature.
Based on the subgraph expansions for phase transition models, we present a histogram Monte Carlo method to calculate the free energy and other physical quantities for such models as continuous functions of the system parameter. The method does not have any critical slowing down. We then use physical quantities thus obtained in a percolation renormalization group method to calculate critical point, critical exponents, and thermodynamic order parameter. Our method gives quite accurate results and may be applied to many phase transition models.
In this communication, the histogram-reweighting technique is extended to quantum Monte Carlo studies of spin systems. The proposed method is applied to obtain equilibrium critical parameters of the quantum
S=1/2 Heisenberg ferromagnet on simple cubic lattices. Our estimates are
kBTc/J=1.67764(2), U∗=0.4504(1), $\ensuremath{\gamma}=1.397(20),$ $\ensuremath{\beta}=0.360(6),$ and $\ensuremath{\nu}=0.705(10)$, which are in perfect agreement with the best field-theory and high-temperature-series results. The method can readily be applied to general quantum-spin models, standing as an important tool for the study of quantum phase transitions.