Efficient Monte Carlo algorithm in quasi-one-dimensional Ising spin systems

We have developed an efficient Monte Carlo algorithm, which accelerates slow Monte Carlo dynamics in quasi-one-dimensional Ising spin systems. The loop algorithm of the quantum Monte Carlo method is applied to the classical spin models with highly anisotropic exchange interactions. Both correlation time and real CPU time are reduced drastically. The algorithm is demonstrated in the layered triangular-lattice antiferromagnetic Ising model. We have obtained the relation between the transition temperature and the exchange interaction parameters, which modifies the result of the chain-mean-field theory.Comment: 4 pages, 3 figure

Machine learning as an improved estimator for magnetization curve and spin gap

The magnetization process is a very important probe to study magnetic materials, particularly in search of spin-liquid states in quantum spin systems. Regrettably, however, progress of the theoretical analysis has been unsatisfactory, mostly because it is hard to obtain sufficient numerical data to support the theory. Here we propose a machine-learning algorithm that produces the magnetization curve and the spin gap well out of poor numerical data. The plateau magnetization, its critical field and the critical exponent are estimated accurately. One of the hyperparameters identifies by its score whether the spin gap in the thermodynamic limit is zero or finite. After checking the validity for exactly solvable one-dimensional models we apply our algorithm to the kagome antiferromagnet. The magnetization curve that we obtain from the exact-diagonalization data with 36 spins is consistent with the DMRG results with 132 spins. We estimate the spin gap in the thermodynamic limit at a very small but finite value.Comment: 10pages, 4figures. Revised and the algorithm improve

Relaxation of frustration and gap enhancement by the lattice distortion in the Δ\Delta chain

We clarify an instability of the ground state of the $\Delta$ chain against the lattice distortion that increases a strength $(\lambda)$ of a bond in each triangle. It relaxes the frustration and causes a remarkable gap enhancement; only a $6\%$ increase of $\lambda$ causes the gap doubled from the fully-frustrated case $(\lambda=1)$. The lowest excitation is revealed to be a kink-antikink bound state whose correlation length decreases drastically with $\lambda$ increase. The enhancement follows a power law, $\Delta E_{\rm gap}\sim (\lambda-1) + 1.44 (\lambda -1)^{\frac{2}{3}}$, which can be obtained from the exact result of the continuous model. This model describes a spin gap behavior of the delafossite YCuO$_{2.5}$.Comment: 4 pages, REVTex, 6 figures in eps-files uuencode