Phase Diagram of the 1/2-1/2-1-1 Spin Chain by the Nonlinear Sigma Model

TL;DR

This paper investigates the phase diagram of a mixed spin chain with spins 1/2 and 1 arranged as 1/2-1/2-1-1, identifying phase boundaries and disordered phases using the nonlinear sigma model.

Contribution

It maps a mixed spin chain to the nonlinear sigma model to determine phase boundaries and characterize disordered phases.

Findings

Identified two gapless boundaries in the phase diagram.

Described phases in terms of singlet clusters.

Mapped the spin chain to the nonlinear sigma model.

Abstract

We examine a periodic mixed spin chain with spin magnitudes 1/2 and 1 which are arrayed as 1/2-1/2-1-1. The three independent parameters are ratios of the four exchange couplings. We determine phase boundaries in the parameter space by using the gapless condition which was previously derived by mapping a general inhomogeneous spin chain to the nonlinear sigma model. We find two gapless boundaries separating three disordered phases. The features of the phases are explained in terms of singlet clusters.