arXiv subjects

Akira Kawaguchi

Publications and source records attributed to Akira Kawaguchi.

8 recordsLinked to original sources

arXiv2005

Singular behavior of an entangled state for a one-dimensional quantum spin system

We studied the entangled state for a one-dimensional $S=1/2$ antiferromagnetic quantum spin chain in a transverse field. We calculate the ground state using the density matrix renormalization group and discuss how the entangled state changes around a quantum phase transition (QPT) point. By analyzing concurrence $C(ρ)$ for two-qubit density matrix $ρ$ after the Lewenstein-Sanpera decomposition, $ρ=Λρ_s + (1-Λ) ρ_e $, where $ρ_s$ is a separable density matrix and $ρ_e$ is a pure entangled state obtained by a linear combination of Bell states, we find singular behaviors both in $C(ρ_e)$ and $1-Λ$ at the QPT point. $C(ρ_e)$ includes the effects of quantum fluctuations, which manifest the competition between the antiferromagnetic spin fluctuation and the effect of transverse field in the transverse Ising model. The quantum fluctuation shows the singular maximum at the QPT point as expected from the general picture of phase transition. In contrast, $1-Λ$ reveals the singular minimum at QPT point.

quant-ph↗

arXiv2003

Quantum Phase Transitions in the One-Dimensional S=1 Spin-Orbital Model: Implications for Cubic Vanadates

We investigate ground-state properties and quantum phase transitions in the one-dimensional S=1 spin-orbital model relevant to cubic vanadates. Using the density matrix renormalization group, we compute the ground-state energy, the magnetization and the correlation functions for different values of the Hund's coupling $J_H$ and the external magnetic field. It is found that the magnetization jumps at a certain critical field, which is a hallmark of the field-induced first-order phase transition. The phase transition driven by $J_H$ is also of first order. We also consider how the lattice-induced ferro-type interaction between orbitals modifies the phase diagram, and discuss the results in a context of the first-order phase transition observed in YVO$_3$ at 77K.

cond-mat.str-el↗

arXiv2002

Hole-doping effects on a frustrated spin ladder

Hole-doping effects are investigated on the {\it t-J} ladder model with the linked-tetrahedra structure. We discuss how a metal-insulator transition occurs upon hole doping with particular emphasis on the effects of geometrical frustration. By computing the electron density and the spin correlation function by the density matrix renormalization group, we show that strong frustration triggers a first-order transition to a metallic phase, when holes are doped into the plaquette-singlet phase. By examining spin excitations in a metallic case in detail, we discuss whether the spin-gap phase persists upon hole doping according to the strength of frustration. It is further shown that the lowest excited state in a spin-gap metallic phase can be described in two independent quasiparticles.

cond-mat.str-el↗

arXiv2002

Dynamical Properties of One-Dimensional Multicomponent Quantum Liquids in Metallic Phase

We investigate low-energy dynamical properties of one-dimensional multicomponent quantum liquids with the short-range interaction as well as the $1/x$-type long-range interaction. By calculating the single-particle spectrum and the dynamical spin susceptibility by means of the bosonization method, we discuss how the orbital degeneracy and the band splitting affect the dynamical response functions. The effect of the long-range interaction is also addressed. Although the long-range interaction suppresses charge fluctuations, it effectively enhances spin fluctuations via the formation of the Wigner crystal.

cond-mat.str-el↗

arXiv2002

First-Order Phase Transitions in Frustrated Spin Systems

We give a short review of our recent works on the first-order quantum phase transitions in frustrated spin chains with orthogonal-dimer structure. When the ratio of the competing antiferromagnetic exchange couplings is varied, a first-order transition occurs between the dimer phase and the plaquette phase, which is accompanied by the discontinuity in the spin excitation gap. We further show that strong frustration triggers the phase transitions in a magnetic field, which exhibit plateaus and jumps in the magnetization curve. The hole-doping effect is also addressed for the orthogonal-dimer chain with linked-tetrahedra structure. It is found that the competing antiferromagnetic interactions result in a first-order metal-insulator transition upon hole doping.

cond-mat.str-el↗

arXiv2002

Magnetization process for a quasi-one-dimensional S=1 antiferromagnet

We investigate the magnetization process for a quasi-one-dimensional S=1 antiferromagnet with bond alternation. By combining the density matrix renormalization group method with the interchain mean-field theory, we discuss how the interchain coupling affects the magnetization curve. It is found that the width of the magnetization plateau is considerably reduced upon introducing the interchain coupling. We obtain the phase diagram in a magnetic field. The effect of single-ion anisotropy is also addressed.

cond-mat.str-el↗

arXiv2001

Magnetic Properties for the One-Dimensional Multicomponent Spin-Gap System

Magnetic properties for the one-dimensional multicomponent quantum spin system with the excitation gap are studied based on the integrable spin model introduced by Bariev {\it et al}. By exactly computing the magnetization, we show how the characteristic structure with plateaus and cusps appears in the magnetization process. To study low-energy dynamics of the system, we apply the finite-size scaling analysis to the excitation spectrum, and thereby evaluate the power-law exponent as well as the enhancement factor for the low-temperature NMR relaxation rate $1/T_1$. We discuss the critical properties of $1/T_1$ around plateaus and cusps in the magnetization curve.

cond-mat.str-el↗

arXiv2001

NMR Relaxation Rate for 1D Multicomponent Spin-Orbital Systems

The NMR relaxation rate $1/T_{1}$ is studied for one-dimensional multicomponent spin-orbital systems. By combining the bosonization techniques and the exact solution of the SU($n$) model, we evaluate the power-law exponent and the enhancement factor for $1/T_1$ at low temperatures. We discuss how the band splitting affects the relaxation rate, and find that $1/T_{1}$ may be enhanced around the critical value of the band splitting. The crossover behavior in $1/T_1$ around the critical point is discussed in terms of the low-frequency dynamical spin susceptibility. The effect of the hole-doping is also addressed.

cond-mat.str-el↗