CORRELATION AMPLITUDE FOR THE S=1 2 XXZ SPIN CHAIN IN THE CRITICAL REGION - NUMERICAL RENORMALIZATION-GROUP STUDY OF AN OPEN-CHAIN/

The density-matrix renormalization-group technique is used to calculat e the spin-correlation functions [(SjSkx)-S-x] and [(SjSkz)-S-z] of th e one-dimensional S = 1/2 XXZ model in the gapless regime. The numeric al results for open chains of 200 spins are analyzed by comparing them with correlation functions calculated from a low-energy field theory. This gives precise estimates of the amplitudes of the correlation fun ctions in the thermodynamic limit. The exact amplitude recently conjec tured by Lukyanov and Zamolodchikov and the logarithmic correction in the Heisenberg model are confirmed numerically. [S0163-1829(98)51326-4 ].