Evidence for a parity broken monoclinic ground state in the S = 1/2 Kagome antiferromagnet Herbertsmithite

Nearest neighbor interacting S = ½ spins on the ideal Kagomé lattice have been predicted to form a variety of novel quantum entangled states, including quantum spin-liquid (SL) and valence bond solid phases. However, in real materials, the presence of additional perturbative spin interactions may greatly expand the variety of entangled states, which recent theoretical analyses have shown are identifiable through the spontaneous loss of particular discrete point group symmetries. Here we comprehensively resolve the ground state point group symmetries of the prototypical Kagomé SL candidate ZnCu₃(OH)₆Cl₂ (Herbertsmithite) using a combination of optical ellipsometry and wavelength-dependent multi-harmonic optical polarimetry. We uncover a subtle parity breaking monoclinic structural distortion well above the nearest neighbor exchange energy scale. Surprisingly, the parity-breaking order parameter is dramatically enhanced upon cooling and closely tracks the build-up of nearest neighbor spin correlations, suggesting that it is energetically favored by the SL state. The refined low temperature symmetry group greatly restricts the number of viable ground states, and, in the perturbative limit, points toward the formation of a nematic SL ground state, a SL analogue of a liquid crystal.