Hiroaki Onishi

Magnetic Excitations of Spin Nematic State in Frustrated Ferromagnetic Chain

By exploiting density-matrix renormalization group techniques, we investigate the dynamical spin structure factor of a spin-1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions in an applied magnetic field. In a field-induced spin nematic regime, we find gapless longitudinal and gapped transverse spin excitation spectra, in accordance with quasi-long-ranged longitudinal and short-ranged transverse spin correlations, respectively. The gapless point coincides with the dominant longitudinal spin correlation, whereas the gap position exhibits a characteristic field dependence contradicting the dominant transverse spin correlation.

Quantum Narrowing Effect in a Spin–Peierls System with Quantum Lattice Fluctuation

We investigate a one-dimensional S =1/2 antiferromagnetic Heisenberg model coupled to quantum lattice vibration using a quantum Monte Carlo method. We study the ground-state lattice fluctuation whe...

Temperature Dependence of Spin and Bond Ordering in a Spin-Peierls System

We investigate thermodynamic properties of a one-dimensional S =1/2 antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum Monte Carlo method. In particular we study how sp...

Quantum Monte Carlo Study on Magnetization Processes

A quantum Monte Carlo method combining update of the loop algorithm with the global flip of the world line is proposed as an efficient method for studying the magnetization process in an external field, which has been difficult because of inefficiency of the update of the total magnetization. The method is demonstrated in the one dimensional antiferromagnetic Heisenberg model and the trimer model. We attempted various other Monte Carlo algorithms to study systems in the external field and compared their efficiency.

Spin-Peierls transition of the first order in S = 1 antiferromagnetic Heisenberg chains

We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum Monte Carlo method. Investigating the ground state energy of the static bond-alternating chain, we find that the instability to a dimerized chain depends on the value of the spin-phonon coupling, unlike the case of S=1/2. The spin state is the dimer state or the uniform Haldane state depending on whether the lattice distorts or not, respectively. At an intermediate value of the spin-phonon coupling, we find the first-order transition between the two states. We also find the coexistence of the two states.

First-order spin-Peierls transition in S=1 antiferromagnetic Heisenberg chains

Abstract We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum Monte Carlo method. The instability to a dimerized chain depends on the value of the spin–phonon coupling, unlike the case of S=1/2. We find the first-order transition between the uniform Haldane state and the dimerized state. Two non-magnetic states coexist in the chain and a domain-wall structure is observed. In the coexistent state, we find that the length of the uniform Haldane region shrinks, which causes an asymmetrical bond distortion in the dimerized region.

Doping control of realization of an extended Nagaoka ferromagnetic state from the Mott state

Inspired by the Nagaoka ferromagnetism, we propose an itinerant model to study the transition between the Mott singlet state and a ferromagnetic state by emulating a doping process in finite lattices. In the Nagaoka ferromagnetism, the total spin of the system takes the maximum value when an electron is removed from the half-filled system. To incorporate a procedure of the electron removal, our model contains extra sites as a reservoir of electrons, and the chemical potential of the reservoir controls the distribution of electrons. As a function of the chemical potential, the system exhibits ground-state phase transitions among various values of the total spin, including a saturated ferromagnetic state due to the Nagaoka mechanism at finite hole density. We discuss the nature of the ferromagnetism by measuring various physical quantities, such as the distribution of electrons, the spin correlation functions, the magnetization process in the magnetic field, and also the entanglement entropy.

Spin-Peierls Transition in S = 1 Antiferromagnetic Heisenberg Chains

We investigate a one-dimensional S = 1 antiferromagnetic Heisenberg model coupled to lattice distortion by a quantum Monte Carlo method. The instability to a dimerized chain in the ground state depends on the value of the elastic constant, unlike the case of S =1 /2. We find the first-order transition between a uniform Haldane state and a dimerized state. The two non-magnetic states coexist in the ground state and a domain-wall structure is observed.