Ground-state phase diagram of a spin-1/2 frustrated ferromagnetic XXZ chain: Haldane dimer phase and gapped/gapless chiral phases

TL;DR

This paper maps the ground-state phase diagram of a frustrated spin-1/2 XXZ chain, revealing novel phases like the Haldane dimer and various chiral, dimer, and Neel phases, using advanced numerical and theoretical methods.

Contribution

It uncovers the Haldane dimer phase with ferromagnetic dimer order and explores the rich phase structure with anisotropy, advancing understanding of frustrated quantum spin chains.

Findings

Identification of the Haldane dimer phase with ferromagnetic dimer order.

Discovery of gapless vector chiral phase with gapless excitations.

Existence of gapped phases with coexisting orders between chiral and other phases.

Abstract

The ground-state phase diagram of a spin-1/2 XXZ chain with competing ferromagnetic nearest-neighbor (J_1<0) and antiferromagnetic second-neighbor (J_2>0) exchange couplings is studied by means of the infinite time evolving block decimation algorithm and effective field theories. For the SU(2)-symmetric (Heisenberg) case, we show that the nonmagnetic phase in the range -4<J_1/J_2<0 has a small but finite ferromagnetic dimer order. We argue that this spontaneous dimer order is associated with effective spin-1 degrees of freedom on dimerized bonds, which collectively form a valence bond solid state as in the spin-1 antiferromagnetic Heisenberg chain (the Haldane spin chain). We thus call this phase the Haldane dimer phase. With easy-plane anisotropy, the model exhibits a variety of phases including the vector chiral phase with gapless excitations and the even-parity dimer and Neel phases with gapped excitations, in addition to the Haldane dimer phase. Furthermore, we show the existence of gapped phases with coexisting orders in narrow regions that intervene between the gapless chiral phase and any one of Haldane dimer, even-parity dimer, and Neel phases. Possible implications for quasi-one-dimensional edge-sharing cuprates are discussed.