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Progress of Theoretical Physics

Transfer-Matrix Calculations of the Spin 1/2 Antiferromagnetic XXZ Model on the 4 x2 Triangular Lattice Using the Fractal Decomposition

Naomichi Hatano, Masuo Suzuki

Published Mar 1, 1991• 19 citations
Publisher

Abstract

The fractal decomposition of exponential operators proposed by Suzuki, which is a new category of the generalized Trotter decomposition valid up to higher orders, is examined with transfer-matrix calculations of the spin 1/2 antiferromagnetic XXZ model on the 4 ×2 triangular lattice. The dependence of correction terms on the Trotter number and temperature are studied. This dependence confirms rapid convergence of the fractal decomposition. The negative-sign problem arising in quantum Monte Carlo simulations is also discussed from the present new point of view.

DOI
10.1143/ptp/85.3.481

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The fractal decomposition of exponential operators proposed by Suzuki, which is a new category of the generalized Trotter decomposition valid up to higher orders, is examined with transfer-matrix calculations of the spin 1/2 antiferromagnetic XXZ model on the 4 ×2 triangular lattice. The dependence of correction terms on the Trotter number and temperature are studied. This dependence confirms rapid convergence of the fractal decomposition. The negative-sign problem arising in quantum Monte Carlo simulations is also discussed from the present new point of view.
An improved transfer matrix method is proposed for calculating the partition function of quantum spin systems, which is expressed by the M-th approximant of the Suzuki-Trotter transformation with Trotter number M. The point is to incorporate the symmetry property of the Trotter subsystem in calculation. The size of our transfer matrix is reduced very much. Thermodynamics is described by the maximum eigenvalue of the transfer matrix. Our method is applied to the XY model with spin 1/2. We consider the cases of pair and tri spin decompositions with Trotter number up to 4 (M≤4). We can do our calculation analytically in those cases. In the pair spin decomposition we find results which have been obtained by the Monte Carlo simulation and by a transfer matrix method by computer. We find nice results in the tri spin decomposition. Our result in the case M=4 is as good as the result in M=8 by the previous Monte Carlo simulation on comparing them with the exact result of Katsura.
An improved transfer matrix method which has been proposed by the author is applied to the isotropic Heisenberg chain with spin 1/2. Both ferro- and antiferromagnetic couplings are considered. We first explain how to find the size of transfer matrix in consideration of the symmetry of Trotter subsystem with Trotter number M. We then compute the internal energy, specific heat, magnetization and susceptibility in the presence as well as absence of magnetic field in the cases of the pair and tri spin decompositions with M=2, 4 and 6. Our results are qualitatively explained on the basis of the level schemes of the clusters of decomposition. We find good and rapid convergence to the infinite chain limit of Bonner and Fisher with increasing Trotter number in the tri spin decomposition except at very low temperatures, when we examine the zero field limit. Convergence with Trotter number in the case of finite field is also rapid in either decomposition. However there are interesting discrepancies, though small, between the results of Bonner and Fisher and ours in the tri spin decomposition, which remain as one of future problems to be investigated. Finally we present brief discussion about strategy and tactics for getting improved results by our method.
The Suzuki-Trotter formula has been used to get the mth approximant to the classical representation of the partition function of the one-dimensional N-spin S=½ quantum spin systems. The equivalent two-dimensional (N×2m) Ising model with four-spin interactions has been studied in detail by using the numerically exact transfer-matrix method for T≥0.05 and m ≤8. The convergence properties have been examined in two different representations; checkerboard decomposition (CBD) and real-space decomposition (RSD). The spin correlation functions in RSD converge much faster than those in CBD. The limiting m→∞ behavior has been estimated from the extrapolation formula of the form: E(m)=E( ∞)+ α/ m2. The limiting values of the energy derived from the nearest-neighbor correlation agree with the correct values excellently.
The dynamics of Ising spins on a two-dimensional fractal lattice has been studied by Monte Carlo simulations. Both the limits $L>>\ensuremath{\xi}$ and $L<<\ensuremath{\xi}$ were studied where LLL is the size of the sample and $\ensuremath{\xi}$ the correlation length. Exact transfer matrix calculations were also carried out to check that the correct equilibrium values were reached. In both limits the predicted breakdown of conventional dynamic scaling was obtained. Using finite-size scaling $\ensuremath{\tau}\ensuremath{\sim}{L}^{z}$ for $L<<\ensuremath{\xi}$, an effective temperature-dependent critical exponent zzz was obtained consistent with the recent prediction $z\ensuremath{\propto}\frac{1}{T}$ for small TTT.