Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, $\gamma$-Cu2(OD)3Cl, a proposed valence bond solid (VBS)

TL;DR

This study uses inelastic neutron scattering to investigate the magnetic excitations of clinoatacamite, a frustrated quantum magnet proposed to host a valence bond solid state, clarifying its magnetic structure and exchange interactions.

Contribution

It provides detailed inelastic neutron scattering data and revises the understanding of the magnetic structure and exchange interactions in clinoatacamite.

Findings

Revealed complex magnetic excitation spectra.

Disproved the quasi-2D kagomé model for this material.

Supported the view of a distorted pyrochlore magnetic structure.

Abstract

The frustrated magnet clinoatacamite, $\gamma$-Cu$_2$(OH)$_3$Cl, is attracting a lot of interest after suggestions that at low temperature it forms an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised Cu$^{2+}$ ($S=1/2$) spins.\cite{Lee_clinoatacamite} Key to the arguments surrounding this proposal were suggestions that the kagom\'e planes in the magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing the system to behave as a quasi-2-dimensional magnet. This was reasoned from the near 95$^\circ$ angles made at the bridging oxygens that mediate exchange between the Cu ions that link the kagom\'e planes. Recent work pointed out that this exchange model is inappropriate for $\gamma$-Cu$_2$(OH)$_3$Cl, where the oxygen is present as a $\mu_3$-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron powder diffraction to show that the low temperature magnetic structure ($T<6$ K) was canted and involved significant spin ordering on all the Cu$^{2+}$ spins, which is incompatible with the interpretation of simultaneous VBS and N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted pyrochlore magnet. In this report we show detailed inelastic neutron scattering spectra and revisit the responses of this frustrated quantum magnet.