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

TL;DR

This paper investigates the bond-order wave phase, spin solitons, and thermodynamic properties of a frustrated linear spin-1/2 Heisenberg antiferromagnet, revealing exact solutions, topological excitations, and magnetic behavior relevant to certain salts.

Contribution

It provides exact solutions and detailed analysis of the BOW phase, including elementary excitations and thermodynamics, for the first time in this model.

Findings

BOW phase starts at J2/J1=0.2411 and is simple at 1/2

Elementary excitations are topological spin-1/2 solitons

Spin susceptibility is exponentially small at low T and peaks broadly

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 $\chi_M(T)$ is exponentially small for $T \ll E_m$ 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.