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

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.

main text (6 pages, 5 figures) + supplemental material (10 pages, 8 figures), to appear in Phys. Rev. B (R)