Tota Nakamura

A Spin-Glass and Chiral-Glass Transition in a ±J Heisenberg Spin-Glass Model in Three Dimensions

The possibility of a simultaneous spin-glass and chiral-glass transition is pointed out in the three-dimensional ± J Heisenberg model. It is investigated by the nonequilibrium relaxation method starting from the paramagnetic state. The finite-time scaling analysis of the relaxation function of the infinite-size system gives the spin-glass transition at T sg / J =0.21 -0.03 +0.01 and the chiral-glass transition at T cg / J =0.22 -0.04 +0.01 : these coincide within the acceptable range of numerical error. The critical exponent of the spin-glass susceptibility obtained is γ sg = 1.9 ±0.4, which is in agreement with the results of experiments on insulating and canonical spin-glass materials.

Vanishing of the negative-sign problem of quantum Monte Carlo simulations in one-dimensional frustrated spin systems

The negative-sign problem in one-dimensional frustrated quantum spin systems is solved. We can remove negative signs of the local Boltzmann weights by using a dimer basis that has the spin-reversal symmetry. Validity of this new basis is checked in a general frustrated double-spin-chain system, namely the J_0-J_1-J_2-J_3 model. The negative sign vanishes perfectly for

Quantum Monte Carlo calculation of the J1-J2 Model

J_0 + J_1 \leq J_3

Reweighting Method for Quantum Monte Carlo Simulations with the Negative-Sign Problem

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THEORETICAL ANALYSIS OF THE EXPERIMENTS ON THE DOUBLE-SPIN-CHAIN COMPOUND KCUCL3

The Neel and collinear order parameters of the J 1 - J 2 model are calculated by the quantum Monte Carlo simulation with the reweighting method. Systems of sizes N =4×4, 4×8 and 8×8 are treated. The thermodynamic quantities obtained by the size extrapolation suggest that the Neel order remains finite for α≤0.4 and vanishes for α>0.6, where α is defined by J 2 / J 1 , and that the collinear order remains for α≥0.7 and vanishes for α<0.6. The present result is consistent with the result of the modified spin-wave calculation, conflicting with that of the naive spin-wave theory.

Numerical study on the ground-state phase diagram of the S=1/2 XXZ ladder model

We propose a new method for quantum Monte Carlo simulations with the negative-sign problem. In the conventional method, one simulates a virtual system whose Boltzmann factors are the absolute values of the original one, and analyzes the data so that the correct expression for physical quantities may be recovered. We simulate a different virtual system with properties closer to the original system and yet without the negative-sign problem. Comparison between the present method and the conventional one is made for the antiferromagnetic Heisenberg model on the 4×h4 square lattice with frustrations ( J 1 - J 2 model). We conclude that the present method is useful in the low-temperature region where the negative-sign problem becomes serious. It is also shown that the method is considerably efficient on the point that quantities for different parameters can be obtained in a single run.

Partial Disordered Phase of Ising Spin System in Distorted Triangular Lattice RbCoBr3

We have analyzed the experimental susceptibility data of

Weak universality of spin-glass transitions in three-dimensional ±J models

{\mathrm{KCuCl}}_{3}

Infinitesimal incommensurate stripe phase in an axial next-nearest-neighbor Ising model in two dimensions

and found that the data are well explained by the double-spin-chain models with strong antiferromagnetic dimerization. Large quantum Monte Carlo calculations were performed in the spin systems with frustration. This was made possible by removing the negative-sign problem with the use of the dimer basis that has spin-reversal symmetry. The numerical data agree with the experimental data within 1% relative errors in the whole temperature region. We also present a theoretical estimate for the dispersion relation and compare it with recent neutron-scattering experiments. Finally, the magnitude of each interaction bond is predicted.

Criticality of the uniform magnetic order in a ferromagnetic- antiferromagnetic random alternating quantum spin chain

The ground-state phase diagram of the S =1/2 X X Z ladder model is investigated by using the density-matrix renormalization-group method. This model is composed of two S =1/2 X X Z chains with ferromagnetic rung couplings (λ) and intra-chain anisotropy (Δ), and continuously changes from the two independent S =1/2 X X Z chains at λ=0 to the S =1 X X Z chain in the limit of λ→∞ . It is shown that there exist edge states at Δ=1 and λ=1 in relation to the four-fold degeneracy of the ground state and that an excitation gap opens nearly at λ=0 for 0.8≤Δ≤1. In addition, it is clarified that the transition between the Haldane phase and the antiferromagnetic phase occurs at λ=1.15 ±0.01 for Δ=1. A possible phase diagram in the λ-Δ plane is proposed, which is consistent with the scaling analysis based on the bosonization method.