Atsuhiro Kitazawa

Phase diagram of S=12 two-leg XXZ spin-ladder systems

Research Institute for Applied Mechanics,Kyushu University, Kasuga, Fukuoka 816-858, Japan(Dated: February 2, 2008)We investigate the ground state phase diagram of the S=1/2 two-leg XXZ spin lad-der system with an isotropic interchain coupling. In this model, there is the Berezinskii-Kosterlitz-Thouless transition which occurs at the XY-Haldane and the XY-rung singletphase boundaries. It was difficult to determine the transition line using traditional meth-ods. We overcome this difficulty using the level spectroscopy method combined with thetwisted boundary condition method, and we check the consistency. We find out that thephase boundary between XY phase and Haldane phase lies on the ∆ = 0 line. And weshow that there exist two different XY phases, which we can distinguish investigating a XXcorrelation function.

Magnetization plateau and quantum phase transition of the S = ½ trimerized XXZ spin chain

We study the plateau of the magnetization curve at M = Ms/3 (Ms is the saturation magnetization) of the S = ½ 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 -t plane, where denotes the XXZ anisotropy and t the magnitude of the trimerization. This quantum phase transition is of the Berezinskii-Kosterlitz-Thouless type.

Breakdown of the magnetization plateau of the S = 1/2 ferromagnetic-ferromagnetic-antiferromagnetic trimerized spin chain with anisotropy

We study the breakdown of the magnetization plateau at the magnetization M = MS/3 (MS is the saturation magnetization) of the S = 1/2 anisotropic spin chain with ferromagnetic-ferromagnetic-antiferromagnetic interactions. We consider the model with the isotropic ferromagnetic (trimer) coupling JF, and anisotropic antiferromagnetic coupling (Jx = Jy = JAF and Jz = JAF). For the limit of large JF/JAF, the model is equivalent to the S = 3/2 XXZ chain with the exchange anisotropy . There is a phase transition between the plateau (small-) and the no-plateau (large-) regions. This phase transition is of the Berezinskii-Kosterlitz-Thouless type, and we determine the phase boundary from the numerical diagonalization data. For = 1, in particular, the phase transition between the plateau and the no-plateau regions occurs at the point c = 15.4.

Magnetization-plateau state of the S = 3 / 2 spin chain with single-ion anisotropy

We reexamine the numerical study of the magnetized state of the

An SU(2) symmetry of the one-dimensional spin-1 XY model

S=3/2

Phase Transitions of S = 3/2 and S = 2 XXZ Spin Chains with Bond Alternation

spin chain with single-ion anisotropy

Instability of the one-dimensional extended Hubbard model in a magnetic field

D(g0)

BIFURCATION AT THE C=3/2 TAKHTAJAN-BABUJIAN POINT TO THE C= 1 CRITICAL LINE

for the magnetization

Quantum phase transition of trimerized XXZ spin chain in magnetic field

{M=M}_{S}/3,

Spin-gap phases in Tomonaga-Luttinger liquids

where