arXiv subjects

S. Miyashita

Publications and source records attributed to S. Miyashita.

41 records · Page 3Linked to original sources

arXiv1998

Griffiths-McCoy singularities in the transverse field Ising model on the randomly diluted square lattice

The site-diluted transverse field Ising model in two dimensions is studied with Quantum-Monte-Carlo simulations. Its phase diagram is determined in the transverse field (Gamma) and temperature (T) plane for various (fixed) concentrations (p). The nature of the quantum Griffiths phase at zero temperature is investigated by calculating the distribution of the local zero-frequency susceptibility. It is pointed out that the nature of the Griffiths phase is different for small and large Gamma.

cond-mat.dis-nn↗

arXiv1997

One-dimensional Heisenberg antiferromagnet in which two S=1/2 and two S=1 spins are arranged alternatively

Ground-state and thermodynamic properties of the one-dimensional Heisenberg antiferromagnet in which two S=1/2 and two S=1 spins are arranged alternatively are studied by a quantum Monte Carlo method and by analytical estimates. It is found that the ground state of the system undergoes a second-order phase transition which accompanies the vanishing of the energy gap.

cond-mat.str-el↗

arXiv1995

A novel intermediate phase in S=2 antiferromagnetic chains with uniaxial anisotropy

We study ground state properties of the $S=2$ quantum antiferromagnetic chain with a uniaxially anisotropic Hamiltonian: $ H = \sum_{j} [S_{j} \cdot S_{j+1} + D (S^{z}_{j})^2 ] $ by a Monte Carlo calculation. While it has been reported that a string order parameter vanishes exponentially at the isotropic Heisenberg point ($D=0$), we found it tends to remain finite in the thermodynamic limit around $D=1.2$. This implies that the model for $S=2$ has a novel intermediate phase between the Haldane phase and the large-$D$ phase, as was anticipated from the earlier argument based on the Valence-Bond-Solid picture.

cond-mat↗

arXiv1994

Metastable lifetimes in a kinetic Ising model: Dependence on field and system size

The lifetimes of metastable states in kinetic Ising ferromagnets are studied by droplet theory and Monte Carlo simulation, in order to determine their dependences on applied field and system size. For a wide range of fields, the dominant field dependence is universal for local dynamics and has the form of an exponential in the inverse field, modified by universal and nonuniversal power-law prefactors. Quantitative droplet-theory predictions are numerically verified, and small deviations are shown to depend nonuniversally on the details of the dynamics. We identify four distinct field intervals in which the field dependence and statistical properties of the lifetimes are different. The field marking the crossover between the weak-field regime, in which the decay is dominated by a single droplet, and the intermediate-field regime, in which it is dominated by a finite droplet density, vanishes logarithmically with system size. As a consequence the slow decay characteristic of the former regime may be observable in systems that are macroscopic as far as their equilibrium properties are concerned.

cond-mat↗

arXiv1993

Dilution Effects on Ordering in the ${S=1/2}$ Heisenberg Antiferromagnet on the Square Lattice

The influence of dilution with non--magnetic impurities on the order in the ground state of the $S=1/2$ Heisenberg antiferromagnet is investigated by Quantum Monte Carlo simulations. Data of the spin correlation functions and thermodynamic properties for system sizes up to $L\times L=16\times16$ and for impurity concentrations up to $δ=37.5\%$ are presented. In the low doping regime the correlation length shows similiar dependence on temperature as in the pure case. The staggered magnetization $\langle N_z^2(δ)\rangle$ in the ground state which is the order parameter of the present model is estimated by extrapolating data at low temperatures. Impurity concentration dependence of $\langle N_z^2(δ)\rangle$ is presented. $\langle N_z^2(δ)\rangle$ decreases monotonically with impurity concentration and becomes very small around $δ_c \sim 0.35$ which is much smaller than the classical percolation threshold.

cond-mat↗