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arXiv:math-ph/0104020

A General Method for Obtaining a Lower Bound for the Ground State Entropy Density of the Ising Model With Short Range Interactions

Lawrence Pack, Ram Anand Puri

Published Apr 14, 2001• 0 citations
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Abstract

We present a general method for obtaining a lower bound for the ground state entropy density of the Ising Model with nearest neighbor interactions. Then, using this method, and with a random coupling constant configuration, we obtain a lower bound for the ground state entropy density of the square, triangular, and hexagonal two-dimensional lattices with free, cylindrical, and toroidal boundary conditions.

DOI
10.48550/arxiv.math-ph/0104020

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We present a general method for obtaining a lower bound for the ground state entropy density of the Ising Model with nearest neighbor interactions. Then, using this method, and with a random coupling constant configuration, we obtain a lower bound for the ground state entropy density of the square, triangular, and hexagonal two-dimensional lattices with free, cylindrical, and toroidal boundary conditions.
The ground state entropy density of the antiferromagnetic Ising model on a triangular lattice is considered in the infinite volume limit as a function of boundary conditions on a finite triangular domain. The ground states of this domain do not map to a dimer covering and so cannot be classified into string sectors. A parametrized boundary condition is identified that allows the entropy density to be tuned to values between nondegeneracy and maximal degeneracy. The results are compared to those for a rectangular periodic domain for which the ground states can be classified into string sectors.
The Heisenberg-Ising Hamiltonian $H=\ensuremath{-}\frac{1}{2}{\ensuremath{\alpha}({\ensuremath{\sigma}}_{x}\ensuremath{\sigma}_{x}^{}{}_{}{}^{\ensuremath{'}}+{\ensuremath{\sigma}}_{y}\ensuremath{\sigma}_{y}^{}{}_{}{}^{\ensuremath{'}})+\ensuremath{\epsilon}({\ensuremath{\sigma}}_{z}\ensuremath{\sigma}_{z}^{}{}_{}{}^{\ensuremath{'}})}$ for rectangular one-, two-, or three-dimensional lattices are considered. The sum is over nearest neighbors and $\ensuremath{\Delta}=\frac{\ensuremath{\epsilon}}{\ensuremath{\alpha}}$ measures the anisotropy of the coupling. Upper and lower bounds for the ground-state energy are established and these bounds apply equally well to lattices of one, two, or three dimensions. Furthermore, it is shown that the ground-state energy per nearest-neighbor pair is nondecreasing as the dimension of the lattice (one, two or three) increases.
We present and demonstrate an accurate, reliable, and computationally cheap method for the calculation of free energies in Monte Carlo simulations of lattice models. Even in the critical region it yields good results with comparatively short simulation runs. The method combines upper and lower bounds on the thermodynamic limit entropy density to yield not only an accurate estimate of the free energy but a bound on the possible error as well. The method is demonstrated on the two- and three-dimensional Ising models and the three-dimensional, three-states Potts model.
Variation principles for obtaining lower bounds by means of density matrices are applied to a number of many-body problems. This is done by means of restrictions derived from the study of exacly solvable models. Two classes of problems are considered: (a) obtaining a lower bound to the ground-state energy of a system of particles, and (b) obtaining a lower bound to the Helmholtz free energy. For the first case an exact lower bound is obtained for the ground-state energy of the particle-conserving Bogoliubov Hamiltonian. Of particular significance in the nonzero temperature case is a rigorous lower bound to the free energy of the Ising model with small external field at low temperatures.
The critical values of the ratio of interaction energies at which the ordering of spins in the ground state is indeterminate (and probably Tc=0{T}_{c}=0Tc​=0) are calculated exactly for a variety of Ising lattices with antiferromagnetic next-nearest-neighbor coupling, and for anisotropic nearest-neighbor coupling. Using these results and the exact two-dimensional solutions, the general dependence of the critical point on interaction ratio is sketched. The complex case of the antiferromagnetic fcc is reconsidered.