Extended String Order Parameter in the (S=1,1⁄2) Mixed Spin Chain

Abstract

We have found a set of order parameters that characterize the plateau state of the (S=1,1⁄2) spin chain system. The order parameters are regarded as extensions of the string order parameter of the S=1 antiferromagnetic Heisenberg spin chain. The extension is made by mapping the S_z=1⁄2 (S_z=−1⁄2) state of an S=1⁄2 spin onto the S_z=0 (S_z=−1) state of an S=1 spin. Numerical data indicate that, when single-ion anisotropy D is introduced, all the order parameters decrease continuously and vanish in the large-D region. Our estimate of the critical value D_c for the phase transition agrees well with the phase boundary determined by a previous level spectroscopy analysis. We present estimates of three critical exponents that are associated with the order parameters. A scaling relation between the exponents holds well in the present system.