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Progress of Theoretical Physics

The ST-Transformation Approach to Analytic Solutions of Quantum Systems. II: Transfer-Matrix and Pfaffian Methods

M. Inoue, Masuo Suzuki

Published Mar 1, 1988• 47 citations
Publisher

Abstract

The general formalism to map d-dimensional quantum systems to (d + 1)-dimensional classical systems by the Trotter-like formula and to solve exactly the mapped systems by some classical methods such as the transfer-matrix (including Bethe Ansatz) or Pfaffian methods is applied to find the free energies, ground state energies, excitation gaps and correlations of the transverse Ising chain and the one-dimensional XY-model.

DOI
10.1143/ptp.79.645

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The general formalism to map d-dimensional quantum systems to (d + 1)-dimensional classical systems by the Trotter-like formula and to solve exactly the mapped systems by some classical methods such as the transfer-matrix (including Bethe Ansatz) or Pfaffian methods is applied to find the free energies, ground state energies, excitation gaps and correlations of the transverse Ising chain and the one-dimensional XY-model.
We give a self-contained exposition of the combinatorial solution of quantum mechanical systems of coupled spins on a one-dimensional lattice. Using Trotter formula, we write the partition function as a generating function of a spanning subgraph of a two-dimensional lattice and solve the combinatorial problem by the method of Pfaffians provided the weights satisfy the so-called free fermion condition. The free energy and the ground state energy as a function of the inverse temperature, couplings J and magnetic fields h, for the XY model in a transverse field with period p=1 and 2, is then obtained.
The one-dimensional S=1 quantum spin system has been mapped onto the equivalent two-dimensional Ising model on the basis of the equivalence theorem of Suzuki. The classical representation of the partition function has been expressed in terms of the maximum eigenvalue of the transfer matrix and evaluated by means of the numerically exact transfer-matrix method. The physical meaning of two kinds of correlation functions in RSD and CBD has been made clear in terms of the concept of the maximum eigenstate. Calculations have been made for both ferromagnetic and antiferromagnetic Heisenberg chains with several single-site anisotropy values, both positive and negative. Good agreements are found with some existing numerical results.
We have performed an analytical study of quantum-classical equivalence for quantum XY -spin chains with arbitrary interactions to explore the classical counterpart of the factorizing magnetic fields that drive the system into a separable ground state. We demonstrate that the factorizing line in the parameter space of a quantum model is equivalent to the so-called natural boundary that emerges in mapping the quantum XY -model onto the two-dimensional classical Ising model. As a result, we show that the quantum systems with the non-factorizable ground state could not be mapped onto the classical Ising model. Based on the presented correspondence we suggest a promising method for obtaining the factorizing field of quantum systems through the commutation of the quantum Hamiltonian and the transfer matrix of the classical model.
Trotter's formula is used to construct two-dimensional classical systems equivalent to some one-dimensional quantum-mechanical systems of interest. The finite-temperature properties of the completely anisotropic Heisenberg chain are expressed in terms of an eight-vertex model in which the vertex weights depend on the size of the lattice. Knowledge of only the largest eigenvalue of the transfer matrix of the eight-vertex model is not sufficient to find the free energy of the chain except in the limit of zero temperature, when Baxter's result for the ground-state energy is recovered. We also examine two models with two species of variables each, and point out that by constructing the equivalent classical problem the trace over one set of variables can be performed.
Introduction to Integrable Many-Body Systems II This is the second part of a three-volume introductory course about integrable systems of interacting bodies. The models of interest are quantum spin chains with nearest-neighbor interactions between spin operators, in particular Heisenberg spin-1/2 models. The Ising model in a transverse field, expressible as a quadratic fermion form by using the Jordan-Wigner transformation, is the subject of Sect. 12. The derivation of the coordinate Bethe ansatz for the XXZ Heisenberg chain and the determination of its absolute ground state in various regions of the anisotropy parameter are presented in Sect. 13. The magnetic properties of the ground state are explained in Sect. 14. Sect. 15 concerns excited states and the zero-temperature thermodynamics of the XXZ model. The thermodynamics of the XXZ Heisenberg chain is derived on the basis of the string hypothesis in Sect. 16; the thermodynamic Bethe ansatz equations are analyzed in high-temperature and low-temperature limits. An alternative derivation of the thermodynamics without using strings, leading to a non-linear integral equation determining the free energy, is the subject of Sect. 17. A nontrivial application of the Quantum Inverse Scattering method to the fully anisotropic XYZ Heisenberg chain is described in Sect. 18. Sect. 19 deals with integrable cases of isotropic spin chains with an arbitrary spin.