Bond-order wave phase, spin solitons, and thermodynamics of a frustrated linear spin-½ Heisenberg antiferromagnet

Abstract

The linear spin-½ Heisenberg antiferromagnet with exchanges J₁ and J₂ between first and second neighbors has a bond-order wave (BOW) phase that starts at the fluid-dimer transition at J₂/J₁=0.2411 and is particularly simple at J₂/J₁=½. The BOW phase has a doubly degenerate singlet ground state, broken inversion symmetry, and a finite-energy gap E_m to the lowest-triplet state. The interval 0.4<J₂/J₁<1.0 has large E_m and small finite-size corrections. Exact solutions are presented up to N=28 spins with either periodic or open boundary conditions and for thermodynamics up to N=18. The elementary excitations of the BOW phase with large E_m are topological spin-½ solitons that separate BOWs with opposite phase in a regular array of spins. The molar spin susceptibility χ_M(T) is exponentially small for T«E_m and increases nearly linearly with T to a broad maximum. J₁ and J₂ spin chains approximate the magnetic properties of the BOW phase of Hubbard-type models and provide a starting point for modeling alkali-tetracyanoquinodimethane salts.