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Monte Carlo Methods and Applications

Field-induced Kosterlitz–Thouless transition in critical triangular-lattice antiferromagnets

Chi‐Ok Hwang, Seung‐Yeon Kim

Published Jul 24, 2014• 3 citations
Publisher

Abstract

Abstract In this paper, we directly obtain from Monte Carlo simulations the critical magnetic field H = 0 . 29 ( 3 ) ${H=0.29(3)}$ of the field-induced Kosterlitz–Thouless transition in the critical triangular-lattice antiferromagnet. The Yang–Lee zero approach clearly shows the field-induced Kosterlitz–Thouless transition and the critical magnetic field agrees well with the results from other indirect methods. Also, the reduced zero-field susceptibility gives us the same conclusion. For the investigations, we used the exact and approximate ground densities of states as a function of magnetization by using both the exact enumeration method for small systems (up to 9×9 lattices) and the Wang–Landau Monte Carlo algorithm for large systems (up to 30×30 lattices).

DOI
10.1515/mcma-2013-0027

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Abstract In this paper, we directly obtain from Monte Carlo simulations the critical magnetic field H = 0 . 29 ( 3 ) H=0.29(3){H=0.29(3)}H=0.29(3) of the field-induced Kosterlitz–Thouless transition in the critical triangular-lattice antiferromagnet. The Yang–Lee zero approach clearly shows the field-induced Kosterlitz–Thouless transition and the critical magnetic field agrees well with the results from other indirect methods. Also, the reduced zero-field susceptibility gives us the same conclusion. For the investigations, we used the exact and approximate ground densities of states as a function of magnetization by using both the exact enumeration method for small systems (up to 9×9 lattices) and the Wang–Landau Monte Carlo algorithm for large systems (up to 30×30 lattices).
The field-induced Berezinskii-Kosterlitz-Thouless (BKT) transition in the ground state of the triangular antiferromagnetic Ising model is studied by the level-spectroscopy method. We analyze dimensions of operators around the BKT line, and estimate the BKT point H(c) approximately equal to 0.5229 +/-0.001, which is followed by the level-consistency check to demonstrate the accuracy of our estimate. Further we investigate the anisotropic case to clarify the stability of the field-induced string-density plateau against an incommensurate liquid state by the density-matrix renormalization-group method.
The critical ground state of the antiferromagnetic triangular Ising model is studied with finite-size scaling, with the use of exact techniques and numerical transfer-matrix methods. Exact solution of the model in zero field yields a conformal anomaly equal to 1, and values of critical exponents. For nonzero fields, dominant eigenvalues of the transfer matrix are computed for systems with linear dimensions up to 27 with the use of an efficient coding of spin states as a system of nonintersecting strings. The results are in perfect agreement with the Gaussian-model--Coulomb-gas scenario proposed by Nienhuis et al. for this model. Quantitative agreement is found with the predictions for exponents of spin-wave and vortex operators. Furthermore, we locate the field-induced Kosterlitz-Thouless transition to a long-range-ordered state at a field Hc{\mathit{H}}_{\mathit{c}}Hc​=0.266\ifmmode\pm\else\textpm\fi{}0.010.
We study the superconducting Kosterlitz-Thouless (KT) transition of the attractive Hubbard model on a two-dimensional triangular lattice using auxiliary field quantum Monte Carlo method for system sizes up to $14\ifmmode\times\else\texttimes\fi{}14$ sites. Combining three methods to analyze the numerical data, we find, for the attractive interaction of $U=\ensuremath{-}4t$, that the transition temperature is broadly maximized around the band filling of 1.2<n<1.61.2<n<1.61.2<n<1.6, while it is found to be rather low in the range of n<1n<1n<1 and n>1.8n>1.8n>1.8. We discuss that this band filling dependence may be explained in terms of the band filling dependence of the superfluid density and the effective mass. Our analysis also teaches us that the estimation of the KT transition temperature requires some caution when the density of states near the Fermi level is small and the calculation is restricted to small system sizes.
A Monte Carlo study is carried out on the six-state clock model on the triangular lattice. Physical quantities which characterize the Kosterlitz-Thouless (KT) phase transition are calculated. The temperature dependence of the exponent of the spin-pair correlation function, η( T ), in the KT phase is obtained precisely. The exponential divergence of the correlation length, ξ∼ exp ( a t - b ), with the theoretical value, b =0.5, gives a good fit to the data by the present MC simulations by choosing a =1.6∼1.8, η=0.23∼0.25 at the upper critical temperature T U =1.46∼1.49. However, the choice of critical parameters, a ∼3.1, b ∼0.3 and η∼0.28 at T U ∼1.53 gives the most consistent result by a finite-size scaling analysis on several physical quantities. The lower critical temperature, T L , is estimated to be 1.1.
The critical coarsening dynamics of the spin S =1/2, 3/2 antiferromagnetic Ising model on a triangular lattice is studied by the dynamic Monte Carlo simulation using a heat bath algorithm. The triangular antiferromagnetic Ising (TAI) model possesses an intrinsic geometrical frustration and a large degeneracy of ground state which may affect the equilibrium and non-equilibrium critical behaviors. The S =1/2 TAI has no phase transition at a finite temperature, but it was suggested that the S =3/2 TAI has the Kosterlitz–Thouless (KT)-type phase transition at a finite temperature. We employ a finite size scaling approach for the correlation function from the short-time dynamics of the S =1/2, 3/2 TAI to calculate the values of the static critical exponent η and the dynamic exponent z. For the S =1/2 TAI, our dynamic scaling analysis provides η =0.498±0.006 and z =2.278±0.020 at T =0, agreeing with the previous results. For the S =3/2 TAI, after identifying a KT-transition temperature TKT =0.51±0.01, we find the temperature-dependent η ranging from 0.301±0.008 at T =0.51 to 0.224±0.016 at T =0 along the KT-line whereas the value of z =2.20±0.06 is constant for T≤TKT. It is shown that the spin S =3/2 TAI model and the two-dimensional XY model, sharing the KT-type phase transition, exhibit similar static critical and coarsening dynamics behavior. For both the S =1/2, 3/2 TAI, the value of z (η) is compatible with (larger than) that of the Ising model at Tc and the XY model for T≤TKT in two-dimension. Our results imply that although the quasi-long-range order disappears with ηXY =0 in the two-dimensional XY model at T =0, the S =3/2 TAI still maintains it with η TAI =0.224 due to the effect of a frustration and a high degeneracy of ground state.
The phase transitions and thermodynamic properties of the clock model with q = 5 spin states on a triangular lattice have been investigated using the Wang–Landau Monte Carlo algorithm. The phase transitions have been analyzed with the histogram method and the fourth-order Binder cumulant method. Two Berezinskii–Kosterlitz–Thouless phase transitions are shown to be observed in the ferromagnetic clock model, while a second-order phase transition has been detected in the antiferromagnetic clock model.