Antiferromagnetic vs Spin-Peierls State in 1-D Heisenberg Antiferromagnets

Abstract

We have treated the 1-D Heisenberg antiferromagnet in a mean-field theory, and determined a phase diagram showing the regions for the antiferromagnetic state, the spin-Peierls state and the paramagnetic state. When a spin-phonon coupling constant is greater than 1.08, the antiferromagnetic state is stable below a Néel temperature T_N=0.568 J⁄k_B. When it is less than 1.08, the spin-Peierls state is stable below a transition temperature T_P, which is higher than T_N and increases as the coupling constant decreases.