坂井 徹

The magnetization process of the S = 3/2 antiferromagnetic Heisenberg chain with the single-ion anisotropy D at T = 0 is investigated by the exact diagonalization of finite clusters and finite-size scaling analyses. It is found that a magnetization plateau appears at m = 1/2 for D > 0.93 +/- 0.01. The magnetization curve of the infinite system is also presented for D = 0 and 3.0. (C) 1998 Elsevier Science B.V. All rights reserved.

The magnetization process of the S=3/2 antiferromagnetic Heisenberg chain with single-ion anisotropy D at T=0 is investigated by the exact diagonalization of finite clusters and finite-size scaling analyses. It is found that a magnetization plateau appears at m=1/2 for D>D-c=0.93+/-0.01. The phase transition with respect to D at D, is revealed to be of the Kosterlitz-Thouless type. The magnetization curve of the infinite system is also presented for some values of D.

Dynamical properties of 2D antiferromagnets with hole doping are investigated to elucidate the effects of short-range local magnetic order on the temperature dependence of the dynamical magnetic susceptibility. We show the pseudogap-like behavior of the temperature dependence of the NMR relaxation rate. We also discuss the implications of the results in relation to the observed spin-gap-like behavior of low-doped copper oxide high-Tc superconductors.

The ground-state magnetization process of the (Formula presented) antiferromagnetic chain with the easy-axis single-ion anisotropy described by negative (Formula presented) is investigated. It is numerically found that a phase transition between two different gapless phases occurs at an intermediate magnetic field between the starting and saturation points of the magnetization for (Formula presented) The transition is similar to the spin flopping, but it is second order and not accompanied with any significant anomalous behaviors in the magnetization curve. We also present the phase diagrams in the (Formula presented) and (Formula presented) planes which reveal a possible re-entrant transition. © 1998 The American Physical Society.

Static and dynamic structure factors of Heisenberg ferrimagnetic spin chains are numerically investigated. There exist two distinct branches of elementary excitations, which exhibit ferromagnetic and antiferromagnetic aspects. The ferromagnetic feature is smeared out with the increase of temperature, whereas the antiferromagnetic one persists up to higher temperatures. The scattering intensity is remarkably large at lower boundaries of the ferromagnetic and antiferromagnetic spectra. All these observations are consistent with the ferromagnetic-to-antiferromagnetic crossover in the thermal behavior which has recently been reported. © 1998, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

The ground-state magnetization curve around the critical magnetic field (Formula presented) of quantum spin chains with a spin gap is investigated. We propose a size scaling method to estimate the critical exponent δ defined as (Formula presented) from finite cluster calculation. The applications of the method to the (Formula presented) antiferromagnetic chain and (Formula presented) bond-alternating chain lead to a common conclusion that (Formula presented) The same result is derived for both edges of the magnetization plateau of the (Formula presented) antiferromagnetic chain with single-ion anisotropy. © 1998 The American Physical Society.

The magnetization process of the (Formula presented)=(Formula presented) antiferromagnetic Heisenberg chain with single-ion anisotropy (Formula presented) at (Formula presented) is investigated by the exact diagonalization of finite clusters and finite-size scaling analyses. It is found that a magnetization plateau appears at (Formula presented)(Formula presented) for (Formula presented). The phase transition with respect to (Formula presented) at (Formula presented) is revealed to be of the Kosterlitz-Thouless type. The magnetization curve of the infinite system is also presented for some values of (Formula presented). © 1998 The American Physical Society.

The S=1/2 bond alternating Heisenberg spin chain in a magnetic field is studied by the numerical diagonalization and the finite size scaling based on the conformal field theory. It is found that the magnetic state (0<m<1) is gapless (the central charge c=1) and described by the Luttinger liquid, which corresponds to a spinless Fermion system with repulsive interaction for antiferromagnetic-antiferromagnetic alternating chains, but attractive one for ferromagnetic-antiferromagnetic chains. The critical exponents delta and eta are determined at the critical fields. The results suggest that the phase transitions belong to the same universality class as the S=1 antiferromagnetic chain for both alternating cases.

We propose a theory for the metamagnetism of intermetallic alloy compounds often observed near the paramagnetic side of the critical concentration of the appearance of ferromagnetism. The present treatment assumes that the magnetization process is described in terms of a spin fluctuation spectrum, in clear contrast with conventional approaches based on the Hartree-Fock approximation assuming the fine structure of the density of states curve. © 1995.

The ESR experiment provides us with much information on the ground and excited states of Haldane systems in a magnetic field. In this paper recent experimental results are reported on two typical Haldane materials, NENP and NINO. Then a theoretical interpretation is given on the spectrum it is shown in particular how the ESR spectrum is related to the nature of the ground and excited states, which change with the applied magnetic field. © 1995.

The antiferromagnetic stability of the strongly correlated half-metallic f-electron system is investigated bearing UNiSn in mind. As the result, we show that the hybridization between f and conduction electrons stabilizes the metallic antiferromagnetism supporting the perturbation result obtained previously. © 1995.

The bond alternating Heisenberg spin chain (S = sol1 2) in a magnetic field is studied by the numerical diagonalization and the finite size scaling based on the conformal field theory. It is found that the magnetic state is described by the Luttinger liquid. The asymptotic behavior of the magnetization curve is determined. © 1995.

We propose a phenomenological picture for the mechanism of the metamagnetic transition often observed in intermetallic alloy compounds near the paramagnetic side of the ferromagnetic instabilities such as YCo2 with Al doping. The present treatment assumes the magnetization process is described in terms of the spin fluctuation spectrum, in clear contrast with the previous approaches based on the Hartree-Fock approximation assuming the fine structure of the density of states curve around the Fermi energy.

We study the spin-1 antiferromagnetic Heisenberg chain under the three boundary conditions, which are the free, periodic and antiperiodic boundary conditions, by numerical diagonalization up to 16 sites. We checked that the lowest excitation caused by the domain wall, which is imposed by the boundary condition and breaks the long-range string order, yields the same energy gap under all the boundary conditions in the infinite length limit. It is also found that the ground state and the first excited state have the same density (about 3%) of domain walls due to the quantum fluctuation which reduces the string order parameter about 10% smaller than that of the vbs model in the ground state. The analysis of the mean distance between two domain walls suggests that domain walls due to the quantum fluctuation form a bound state in pairs and does not break the long-range string order. © 1994, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

In order to explain some mysteries in ESR experiments on NENP, we study the S=1 antiferromagnetic chain with a transverse staggered magnetic field. The staggered field is caused by the crystal structure of NENP and makes the ESR absorption possible from the ground state to the lowest excited state with k=π. In addition it removes the phase transition, which was predicted for the Heisenberg model without the staggered field. Numerical diagonalization study on this model qualitatively agrees with the NMR, ESR and heat capacity measurements. In particular, it explains well the field dependence of the energy gap, the intensity of ESR and the staggered moment. It is also found that the standard theory of ESR is valid above the "critical field" corresponding to a minimum of the energy gap, if the sublattice moments are suitably reinterpreted. © 1994, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

a microscopic model is proposed to describe magnetic properties of half-metallic f-electron systems. We explicitly take into account the energy gap in the conduction band and the intra-atomic exchange interaction between localized f-electrons and conduction electrons. Based on the model Hamiltonian, the temperature dependence of the magnetic properties are studied assuming the ferromagnetic ground state. The relative stability of ferromagnetic state against antiferromagnetic state is also discussed by perturbation expansion in terms of the hybridization between the conduction and the localized f-electron states. Some implications are given for UNiSn, which shows the transition from paramagnetic semiconductor to antiferromagnetic metal by lowering temperature. © 1994, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

A theoretical explanation is given on some anomalies in the ESR absorption of NENP, which corresponds to the transition from the ground state to the lowest excited state. It is pointed out that a transverse staggered field, which is caused by a staggered tilting of the principal axes of the local environment of Ni2+ ions, is important to understand the ESR absorption. © 1994.

We study the S=1 antiferromagnetic Heisenberg chain with the anisotropies E Tij [(Sj)2-(Sj)2] and D (Sx)2 in a magnetic field Halong z-axis, using numerical diagonalization up to JV=14 and the finite-size scaling and phenomenological renormalization. We give the phase diagrams in the HE and HD planes repectively at T= 0. It is found that a magnetic field breaking the axial symmetry of the Hamiltonian brings about canted Neel order for Hcl0&lt H0&lt Hc2, if the system has the Haldane gap. The two transitions at Hcl and Hcl are revealed to belong to the same universality class as the two-dimensional Ising model. In addition it is found that even if the system is in the large-D phase {D0&gt DC-1), the same transitions occur in the region where the value of D is about 1.0 -1.5. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

It is known that the one-dimensional S = 1 antiferromagnet has an energy gap and therefore has no magnetization up to a finite field H(c). We find that the magnetization appears nonlinearly above H(c). So the field derivative dm/dH diverges at H(c). This explains the cusp in dm/dH observed experimentally in the quasi-one-dimensional S = 1 material Ni(C2H8N2)2NO2ClO4.

It is known that the one-dimensional S=1 antiferromagnet has an energy gap and therefore has no magnetization up to a finite field Hc. We find that the magnetization appears nonlinearly above Hc. So the field derivative dm/dH diverges at Hc. This explains the cusp in dm/dH observed experimentally in the quasi-one-dimensional S=1 material Ni(C2H8N2)2NO2ClO4. © 1991 The American Physical Society.

The shape of the magnetization curve of the S= 1 Heisenberg antiferromagnet at zero temperature is discussed. It is found that the magnetization behaves as [formula omitted] at the beginning of magnetization as a function of external field h. The transverse correlation function [formula omitted] of this system decays algebraically at zero temperature when it is partially magnetized by the field to the z-direction [formula omitted]. The exponent η is 0.3∼0.5. This exponent can be calculated by the Lut-tinger liquid assumption and diagonalization results of finite chains. This is in contrast to the exponential decay of this chain in zero field. © 1991, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

A one-dimensional S=1 Heisenberg antiferromagnet in a magnetic field H (z axis) at T=0 is studied by numerical diagonalizations up to N=16. We give the magnetization curve at the thermodynamic limit, and derive an anomaly at Hc1 (=), where is the Haldane gap. It is also found that the transverse spin correlation has the asymptotic form S0xSrx(-1)rr-, and the transverse staggered susceptibility stxx diverges between Hc1 and Hc2 (=4). The exponent has a minimum (0.3) at magnetization m1/3 and 0.5 at Hc1 and Hc2. If the system is quasi-one-dimensional, even small interchain couplings can create a canted Néel order, within a mean-field approximation for interchain interactions. This is consistent with a recent NMR measurement for Ni(C2H8N2)2NO2(ClO4) (NENP) at low temperature. © 1991 The American Physical Society.

The 5= 1 antiferromagnetic Heisenberg chain in a magnetic field along the z-axis at T=0 is studied by numerical diagonalizations up to N= 16 and an analysis of size corrections. We check that the system obeys the conformal field theory with the central charge c= 1 in a magnetic state (0&lt m &lt 1). We investigate the asymptotic form of the transverse and parallel spin correlations and give the exponents η and η, defined by [formula omitted] and [formula omitted]. We determine 77=1/2 and ηz = 2 at m = 0 and 1. In addition, we check the relation ηηz=1, which is consistent with the Luttinger liquid concept. In terms of the concept, the anomalies of magnetization at m – 0 and 1 are discussed. © 1991, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

The ground state of 1D S = 1 XXZ model is studied. It is found that its correlation function corresponds to the modified Bessel function at lambda = 1, and the same transition as 2D Ising model occurs at lambda-c > 1. Quasi-1D interchain interactions also yield an order-disorder transition.

A 1D S=1 Heisenberg antiferromagnet in a magnetic field H(→z-axis) at T=0 is studied by numerical diagonalizations. It is found that a phase transition exists at Hc1(=Δ), where Δ is the Haldane gap, and the transverse staggered susceptibility χst xx diverges and the spin correlation is asymptotically 〈S0 xSr x〉~(-1)rr-η between Hc1 and Hc2(=4). If the system is quasi-lD, even small interchain interactions can make canting Néel order within a mean field approximation for interchain interactions. This is consistent with a recent NMR measurement for NENP at low temperature. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

A quasi-one-dimensional S=1 Heisenberg antiferromagnet with a single-ion anisotropy Dtsumj(Sjz)2 is investigated. By treatment of interchain interactions with coupling constant J as a mean field, this system has been revealed to have a disordered ground state due to an effect of the Haldane gap, if J is small enough. By use of this approximation and application of the finite-size-scaling technique to a chain, the ground-state phase diagram in the JD plane is presented. This analysis leads to the prediction that Ni(C2H8N2)2NO2(ClO4) has no Néel order even at T=0. In addition it is found that the phase transition with respect to D (&lt 0) for a chain belongs to the two-dimensional Ising-model universality class, which agrees with Haldanes conjecture. © 1990 The American Physical Society.

The S=1 isotropic antiferromagnetic Heisenberg chain is studied by exact diagonalizations using the Lanczös algorithm. Energy gaps, structure factors at k, and staggered susceptibilities at T=0 are calculated for finite rings up to N=16, and extrapolated to an infinite system using Shanks transformation. The estimated energy gap is 0.411 0.001, which agrees with the result of Monte Carlo calculations by Nightingale and Blöte. Further, it is found that a finite-size correction decays exponentially and the decay constant corresponds with the correlation length, which is about 5. © 1990 The American Physical Society.

We investigate the Heisenberg antiferromagnets on the square lattice with different coupling constants in two directions of bonds such that the ratio is J(=J±/J//). For S=1/2 and S=1, we estimate the critical value Jc such that Néel order exists for Jc&lt J&lt 1, while not for 0&lt J&lt Jc. The results of the spin wave theory suggest that Jc is very small (Jc&lt 10-1) and a disordered phase exists only in quasi-one-dimensional regions. Then we treat interchain couplings as a mean field. By this approximation Jc is derived from Jc=(2xst)-l wherexst is the staggered susceptibility of a chain. We estimatexst for finite chains by the exact diagonalization of the hamiltonian and extrapolate to infinite systems. Our estimations are Jc=0 for S= 1/2 and Jc&gt 0.025 for S=1. © 1989, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.