Emergent spin-$1$ Haldane gap and ferroelectricity in a frustrated spin-$1/2$ ladder

TL;DR

This study reveals that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ exhibits a nonmagnetic ground state with a Haldane spin gap and ferroelectricity induced by spin chirality, combining experimental and theoretical insights.

Contribution

It demonstrates the emergence of a spin-1 Haldane gap and ferroelectricity in a frustrated spin-1/2 ladder, a novel phenomenon in quantum magnetism.

Findings

Identification of a nonmagnetic tetramer ground state.

Observation of a Haldane spin gap and spin excitations.

Ferroelectricity induced by a tiny magnetic field.

Abstract

We report experimental and theoretical evidence that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a novel class of ferroelectricity induced by a vector spin chirality order.