Two Coupled Chains are Simpler than One: Field-induced Chirality in a Frustrated Quantum Spin Ladder

TL;DR

This study combines NMR experiments and numerical calculations to reveal a field-induced chiral magnetic phase in a frustrated spin ladder, showing that coupling between chains simplifies the phase diagram and induces classical-like behavior in a quantum system.

Contribution

It demonstrates how moderate rung coupling stabilizes a chiral phase in a frustrated spin ladder, providing new insights into field-induced magnetism and chirality in quantum spin systems.

Findings

Field-induced spiral magnetic structure observed in BiCu2PO6.

Moderate rung coupling simplifies the phase diagram below half-saturation.

Chiral phase and its response to Dzyaloshinskii-Moriya interactions follow classical expectations.

Abstract

Although the frustrated spin chain (zigzag chain) is a Drosophila of frustrated magnetism, the understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is incomplete. We address this problem through nuclear magnetic resonance (NMR) experiments on $\text{BiCu}_2\text{PO}_6$ in magnetic fields up to 45 T, revealing a field-induced spiral magnetic structure. Conjointly, we present advanced numerical calculations showing that even moderate rung coupling dramatically simplifies the phase diagram below half-saturation magnetization by stabilizing a field-induced chiral phase. Surprisingly for a one-dimensional model, this phase and its response to Dzyaloshinskii-Moriya (DM) interactions adhere to classical expectations. While explaining the behavior at the highest accessible magnetic fields, our results imply a different origin for the solitonic phases occurring at lower fields in $\text{BiCu}_2\text{PO}_6$. An exciting possibility is that the known, DM-mediated coupling between chirality and crystal lattice gives rise to a new kind of spin-Peierls instability.