Single Crystal Diffuse Neutron Scattering Study of the Dipole-Octupole Quantum Spin Ice Candidate Ce_2Zr_2O_7: No Apparent Octupolar Correlations above T = 0.05 K

The insulating magnetic pyrochlore Ce_2Zr_2O_7 has attracted much attention as a quantum spin ice candidate with dipole-octupole character that permits spin ice phases based not only on magnetic dipole moments but also allows for even-more-exotic octupole-based spin ice phases. This work reports low-temperature neutron diffraction measurements on single crystal Ce_2Zr_2O_7 with Q-coverage both at low Q where the magnetic form factor for dipoles is near maximal and at high Q covering the region where the magnetic form factor for Ce^3+ octupoles is near maximal. This study was motivated by recent powder neutron diffraction studies of other Ce-based dipole-octupole pyrochlores, Ce_2Sn_2O_7 and Ce_2Hf_2O_7, which each showed temperature-dependent diffuse diffraction at high Q that was interpreted as arising from octupolar correlations. Our measurements use an optimized single crystal diffuse scattering instrument that allows us to screen against strong single crystal Bragg scattering in Ce_2Zr_2O_7. The temperature-difference neutron diffraction reveals a low-Q peak consistent with dipolar spin ice correlations. For larger Q, the temperature-difference neutron diffraction shows an alternation between positive and negative net intensity. These features are qualitatively consistent with the corresponding numerical-linked-cluster (NLC) calculations using pseudospin interaction parameters reported for Ce_2Zr_2O_7, Ce_2Sn_2O_7, or Ce_2Hf_2O_7. Importantly, neither the measured data nor any of the NLC calculations show increased scattering at high Q resulting from octupolar correlations. We conclude that at the lowest attainable temperatures for our measurement (T = 0.05 K), octupolar correlations are not present in Ce_2Zr_2O_7 on the level of our observation threshold of ∼ 0.1% of the low-Q dipole scattering.