Thermodynamics of Integrable Chains with Alternating Spins

TL;DR

This paper analyzes the thermodynamic properties of an integrable quantum spin chain with alternating spins of 1/2 and 1, revealing gapless excitations and conformal invariance under specific conditions.

Contribution

It introduces a two-parameter family of integrable Hamiltonians for alternating spin chains and characterizes their low-temperature thermodynamics and conformal properties.

Findings

Model exhibits two gapless excitations in the antiferromagnetic case.

At equal parameters, the model is conformally invariant with central charge 2.

Special parameter choices lead to infinitely many solutions with lowest energy.

Abstract

We consider a two-parameter $(\bar c, \tilde c)$ family of quantum integrable Hamiltonians for a chain of alternating spins of spin $s=1/2$ and $s=1$. We determine the thermodynamics for low-temperature $T$ and small external magnetic field $H$, with $T << H$. In the antiferromagnetic $(\bar c > 0, \tilde c > 0)$ case, the model has two gapless excitations. In particular, for $\bar c = \tilde c$, the model is conformally invariant and has central charge $c_{vir} = 2$. When one of these parameters is zero, the Bethe Ansatz equations admit an infinite number of solutions with lowest energy.