Spin correlation functions and susceptibilities in the easy-plane XXZ chain
Abstract
We present a Green's-function theory of magnetic short-range order in the S=1/2 easy-plane XXZ chain based on the projection method for the dynamic spin susceptibility and a decoupling of three-spin operator products introducing vertex parameters. The longitudinal and transverse static susceptibilities and two-point correlation functions of arbitrary range are calculated self-consistently for all wavenumbers, temperatures, and anisotropy parameters -1≤ ≤ 1. In the easy-plane ferromagnetic region ( < 0), the longitudinal correlators of spins at distance n change sign at a finite temperature T0(n,), in reasonable agreement with recent data obtained by finite-chain diagonalizations. The temperature dependence of the uniform static susceptibilities exhibits a maximum which is explained as an effect of magnetic short-range order which decreases with increasing temperature.
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