paper

Bond-order wave phase, spin solitons and thermodynamics of a frustrated linear spin-1/2 Heisenberg antiferromagnet

arXiv:1006.5052 · doi:10.1103/PhysRevB.81.054413

Abstract

The linear spin-1/2 Heisenberg antiferromagnet with exchanges $J_{1}$ , $J_{2}$ between first and second neighbors has a bond-order wave (BOW) phase that starts at the fluid-dimer transition at $J_{2} / J_{1} = 0.2411$ and is particularly simple at $J_{2} / J_{1} = 1/2$ . 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 $0.4 < J_{2} / J_{1} < 1.0$ 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-1/2 solitons that separate BOWs with opposite phase in a regular array of spins. The molar spin susceptibility $I ¨ _{M} (T)$ is exponentially small for $T ≪ E_{m}$ and increases nearly linearly with $T$ to a broad maximum. $J_{1}$ , $J_{2}$ spin chains approximate the magnetic properties of the BOW phase of Hubbard-type models and provide a starting point for modeling alkali-TCNQ salts.

10 pages, 12 figures