Correlation between spin helicity and electric polarization vector in quantum-spin chain magnet LiCu$_2$O$_2$

TL;DR

This study uses polarized neutron scattering to explore the relationship between spin helicity and electric polarization in the quantum-spin chain magnet LiCu$_2$O$_2$, confirming the spin-current model and revealing significant quantum fluctuations.

Contribution

It provides experimental evidence linking spin helicity and electric polarization in a quantum-spin system, supporting the applicability of the spin-current model to such materials.

Findings

Reversal of spin chirality correlates with electric polarization reversal.

Large quantum fluctuations affect the classical-spin structure expectations.

Confirmation of the spin-current model in a quantum $S=1/2$ chain magnet.

Abstract

Measurements of polarized neutron scattering were performed on a $S=1/2$ chain multiferroic LiCu$_2$O$_2$. In the ferroelectric ground state with the spontaneous polarization along the c-axis, the existence of transverse spiral spin component in the $bc$-plane was confirmed. When the direction of electric polarization is reversed, the vector spin chirality as defined by ${\bf C}_{ij} = {\bf S}_i \times {\bf S}_j$ ($i$ and $j$ being the neighboring spin sites) is observed to be reversed, indicating that the spin-current model or the inverse Dzyaloshinskii-Moriya mechanism is applicable even to this $e_{\mathrm{g}}$-electron quantum-spin system. Differential scattering intensity of polarized neutrons shows a large discrepancy from that expected for the classical-spin $bc$-cycloidal structure, implying the effect of large quantum fluctuation.