arXiv subjects

Kiyomi Okamoto

Publications and source records attributed to Kiyomi Okamoto.

64 records · Page 4Linked to original sources

arXiv1999

Ground state of an $S=1/2$ distorted diamond chain - model of $\rm Cu_3 Cl_6 (H_2 O)_2 \cdot 2H_8 C_4 SO_2$

We study the ground state of the model Hamiltonian of the trimerized $S=1/2$ quantum Heisenberg chain $\rm Cu_3 Cl_6 (H_2 O)_2 \cdot 2H_8 C_4 SO_2$ in which the non-magnetic ground state is observed recently. This model consists of stacked trimers and has three kinds of coupling constants between spins; the intra-trimer coupling constant $J_1$ and the inter-trimer coupling constants $J_2$ and $J_3$. All of these constants are assumed to be antiferromagnetic. By use of the analytical method and physical considerations, we show that there are three phases on the $\tilde J_2 - \tilde J_3$ plane ($\tilde J_2 \equiv J_2/J_1$, $\tilde J_3 \equiv J_3/J_1$), the dimer phase, the spin fluid phase and the ferrimagnetic phase. The dimer phase is caused by the frustration effect. In the dimer phase, there exists the excitation gap between the two-fold degenerate ground state and the first excited state, which explains the non-magnetic ground state observed in $\rm Cu_3 Cl_6 (H_2 O)_2 \cdot 2H_8 C_4 SO_2$. We also obtain the phase diagram on the $ \tilde J_2 - \tilde J_3$ plane from the numerical diagonalization data for finite systems by use of the Lanczos algorithm.

cond-mat.str-el↗

arXiv1999

Quantum Phase Transition of S=1/2 Trimerized XXZ Spin Chain in Magnetic Field

We study the magnetization plateau at a third of the saturation magnetization of the S=1/2 trimerized XXZ spin chain at T=0. The appearance of the plateau depends on the values of the XXZ anisotropy and the magnitude of the trimerization. This plateauful-plateauless transition is a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type, which is difficult to precisely detect from the numerical data. To determine the phase boundary line of this transition precisely, we use the level crossing of low-lying excitations obtained from the numerical diagonalization. We also discuss the S=1/2 ferromagnetic-ferromagnetic-antiferromagnetic chain.

cond-mat.stat-mech↗

arXiv1998

Magnetization plateau and quantum phase transition of the S=1/2 trimerized XXZ spin chain

We study the plateau of the magnetization curve at $M = M_{\rm s}/3$ ($M_{\rm s}$ is the saturation magnetization) of the $S=1/2$ trimerized $XXZ$ spin chain. By examining the level crossing of low-lying excitations obtained from the numerical diagonalization, we precisely determine the phase boundary between the plateau state and the no-plateau state on the $Δ-t$ plane, where $Δ$ denotes the $XXZ$ anisotropy and $t$ the magnitude of the trimerization. This quantum phase transition is of the Berezinskii-Kosterlitz-Thouless type.

cond-mat.stat-mech↗

arXiv1997

Theoretical analysis of the experiments on the double-spin-chain compound -- KCuCl$_3$

We have analyzed the experimental susceptibility data of KCuCl$_3$ 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 for the first time 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 the 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 the recent neutron-scattering experiment. Finally, the magnitude of each interaction bond is predicted.

cond-mat↗

arXiv1997

Haldane and dimer gap in general double spin-chain models

We have obtained an analytic expression for the $k$ dependence of excitation energy gap for an arbitrary double $S=1/2$ spin-chain by using the nonlocal unitary transformation and the variational method. It is checked to explain the gap behavior of various systems, which include the Haldane system and the dimer system in both extreme limits, and also the ladder model and the Majumdar-Ghosh model. The string order parameter, the dimer order parameter, and the local spin value are also calculated in the ground state. The ground-state energy exhibits a great stabilization by an antiferromagnetic bond dimerization, which might be realized in various new compounds. We also mention the relation of the convergence to the Haldane state with the spin-exchange symmetry of the model. The excited state has one domain wall of a local triplet type except in the vicinity of the Majumdar-Ghosh point, where a local triplet is decomposed to two $S=1/2$ free spins moving among the singlet dimers.

cond-mat↗

arXiv1995

Phase Diagram of $S=1$ Bond-Alternating XXZ chains

The phase transitions between the XY, the dimer and the Haldane phases of the spin-1 bond-alternating XXZ chain are studied by the numerical diagonalization. We determine the phase diagram at $T=0$ and also identify the universality class with the level spectroscopy. We find that exactly on $Δ=0$ there is a Berezinskii-Kosterlitz-Thouless transition line which separates the XY and the Haldane phase and there exists a multicritical point of the XY, the dimer and the Haldane phases. We discuss that the critical properties of this model is of the 2-D Ashkin-Teller type reflecting the hidden $Z_2\times Z_2$ symmetry.

cond-mat↗

arXiv1995

Critical Properties of the transition between the Haldane phase and the large-D phase of the spin-1/2 ferromagnetic-antiferromagnetic Heisenberg chain with on-site anisotropy"

We analytically study the ground-state quantum phase transition between the Haldane phase and the large-$D$ (LD) phase of the $S=1/2$ ferromagnetic-antiferromagnetic alternating Heisenberg chain with on-site anisotropy. We transform this model into a generalized version of the alternating antiferromagnetic Heisenberg model with anisotropy. In the transformed model, the competition between the transverse and longitudinal bond alternations yields the Haldane-LD transition. Using the bosonization method, we show that the critical exponents vary continuously on the Haldane-LD boundary. Our scaling relations between critical exponents very well explains the numerical results by Hida.

cond-mat↗

arXiv1995

Dimer Correlarions in Spin-1/2 Alternating XY Chain

We investigate the long-distance asymptotic behavior of the dimer correlations in the spin-1/2 alternating $XY$chain both at T=0 and at sufficiently low-temperatures. The correlations consist of the dimer long-range order part and the exponentially decaying one. Although the dimer long-range order takes the different values depending on the choice of the spin pairs, the behavior in the decaying term is same irrespective of the choice of spin pairs.

cond-mat↗

arXiv1995

Plateau of the Magnetization Curve of the S=1/2 Ferromagnetic-Ferromagnetic-Antiferromagnetic Spin Chain

I analytically study the plateau of the magnetization curve at $M/M_{\rm S} = 1/3$ (where $M_{\rm S}$ is the saturation magnetization) of the one-dimensional $S=1/2$ trimerized Heisenberg spin system with ferromagnetic ($J_{\rm F}$)-ferromagnetic ($J_{\rm F}$)-antiferromagnetic ($J_{\rm A}$) interactions at $T=0$. I use the bosonization technique for the fermion representation of the spin Hamiltonian through the Jordan-Wigner transformation. The plateau appears when $γ\equiv J_{\rm F}/J_{\rm A} \allowbreak < γ_{\rm C}$, and vanishes when $γ> γ_{\rm C}$, where the critical value $γ_{\rm C}$ is estimated as $γ_{\rm C} = 5 \sim 6$. The behavior of the width of the plateau near $γ_{\rm C}$ is of the Kosterlitz-Thouless type. The present theory well explains the numerical result by Hida.

cond-mat↗

arXiv1995

Critical Properties of the XXZ chain in External Staggered Magnetic Field

We comment on the recent work of Alcaraz and Malvezzi [1995 {\it J. Phys. A: Math. Gen.} {\bf 19} 1521] for the critical properties of the $S=1/2$ $XXZ$ chain in staggered magnetic field. The method of determining the phase boundary from the finite-size numerical data is also discussed.

cond-mat↗