quantum spin ice*

A correlated liquid state was reported in the cerium stannate pyrochlore Ce₂Sn₂O₇ [1]. Its true nature remained elusive, but recent works on Ce₂Sn₂O₇ [2] and Ce₂Zr₂O₇ [3-4] have further investigated the case of cerium pyrochlores based on degrees of freedom having both magnetic dipole and magnetic octupole components. Although these works vary in their details, they [2,4-6] agree towards a quantum spin ice (QSI) based on a manifold of ice-rule correlated octupoles. Earlier theoretical works [7-8] had conceptualised this octupolar QSI, where quantum dynamics is endowed by weaker couplings between other components of the ‘dipole-octupole’ pseudo-spins.

This talk reviews the findings reported on Ce₂Sn₂O₇ [1-2], puts these in perspective with results on other cerium pyrochlores [3-6], and presents new experimental data that further hint at these materials being genuine representatives of a QSI – the model 3D quantum spin liquid. We present the octupolar diffuse scattering observed in thermal neutron scattering experiments on Ce₂Sn₂O₇ and the excitations measured in low-energy neutron spectroscopy [2]. Using neutron backscattering spectroscopy, which provides micro-eV energy resolution, we find a gapped spectrum showing characteristic features that match theory predictions [9-10] for pair production and propagation of fractional matter excitations coupled to a background gauge field. This experimental result corroborates the regime of strong light-matter interaction predicted for the emergent universe in a QSI [11].

[1] Sibille et al., Phys. Rev. Lett. 115, 097202 (2015) [2] Sibille et al., Nature Phys. 16, 546-552 (2020) [3] Gaudet et al., Phys. Rev. Lett. 122, 187201 (2019) [4] Gao et al., Nature Phys. 15, 1052-1057 (2019) [5] Bhardwaj et al., npj Quantum Materials 7, 51 (2022) [6] Smith et al., Phys. Rev. X 12, 021015 (2022)] [7] Huang, Chen & Hermele, Phys. Rev. Lett. 112, 167203 (2014) [8] Li & Chen, Phys. Rev. B 95, 041106 (2017) [9] Udagawa & Moessner, Phys. Rev. Lett. 122, 117201 (2019) [10] Morampudi, Wilczek & Laumann, Phys. Rev. Lett. 124, 097204 (2020) [11] Pace, Morampudi, Moessner & Laumann, Phys. Rev. Lett. 127, 117205 (2021)