Phase Diagram of $S=1$ Bond-Alternating XXZ chains

TL;DR

This paper maps the phase diagram of spin-1 bond-alternating XXZ chains, identifying phase transitions, universality classes, and multicritical points using numerical methods, revealing a Berezinskii-Kosterlitz-Thouless transition and Ashkin-Teller criticality.

Contribution

It provides the first detailed phase diagram and critical analysis of the spin-1 bond-alternating XXZ chain, including the identification of a multicritical point and the nature of phase transitions.

Findings

Existence of a BKT transition line at Δ=0 separating XY and Haldane phases

Identification of a multicritical point where XY, dimer, and Haldane phases meet

Critical properties of the model are of the 2-D Ashkin-Teller type

Abstract

The phase transitions between the XY, the dimer and the Haldane phases of the spin-1 bond-alternating XXZ chain are studied by the numerical diagonalization. We determine the phase diagram at $T=0$ and also identify the universality class with the level spectroscopy. We find that exactly on $\Delta=0$ there is a Berezinskii-Kosterlitz-Thouless transition line which separates the XY and the Haldane phase and there exists a multicritical point of the XY, the dimer and the Haldane phases. We discuss that the critical properties of this model is of the 2-D Ashkin-Teller type reflecting the hidden $Z_2\times Z_2$ symmetry.