Spin nematic liquid of one-dimensional quantum spin systems in magnetic field

The spin nematic state is one of interesting topics in the field of the condensed matter physics. Most theoretical and numerical studies which predicted the spin nematic order so far were based on the spin frustration or the biquadratic interaction. Now we propose another mechanism of the spin nematic phase based on the easy-axis anisotropy and the external magnetic field. When the magnetic field is applied along the easy-axis anisotropy of conventional antiferromagnets, the spin flop transition with the magnetization jump occurs and the canted Neel order is realized. In some one-dimensional systems, however, instead of this first-order transition, two second-order transitions appear and the two-magnon bound state appears as the intermediate phase[1]. This phase corresponds to the Tomonaga-Luttinger liquid phase where the SDW spin correlation along the magnetic field and the nematic spin correlation perpendicular to the field are quasi-long-range orders. The recent study on several one-dimensional systems using the numerical diagonalization of finite-size clusters and the conformal field theory indicated that the nematic spin correlation dominant region appears at higher external field. The phase diagrams with respect to the easy-axis anisotropy and the magnetization are obtained for the following systems: the S=1/2 spin ladder with the anisotropic ferromagnetic rung coupling[2], the S=1/2 ferromagnetic and antiferromagnetic bond-alternating chain[3], the S=1/2 delta chain, the S=1/2 distorted diamond chain, the S=1 antiferromagnetic chain with the easy-axis anisotropy[4]. The possibility of these spin nematic liquid phase realized in some realistic materials is discussed.