Single-Crystal Diffuse Neutron Scattering Study of the Dipole-Octupole Quantum Spin-Ice Candidate Ce2⁢Zr2⁢O7: No Apparent Octupolar Correlations Above 𝑇=0.05  K

The insulating magnetic pyrochlore Ce2⁢Zr2⁢O7 has gained attention as a quantum spin-ice candidate with dipole-octupole character that arises from the crystal-electric-field ground-state doublet for the Ce3+ Kramers ion. This dipole-octupole character permits both spin-ice phases based on magnetic dipoles and those based on more-exotic octupoles. This work reports low-temperature neutron diffraction measurements on single-crystal Ce2⁢Zr2⁢O7 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 Ce3+ octupoles is near maximal. This study was motivated by recent powder neutron diffraction studies of other Ce-based dipole-octupole pyrochlores, Ce2⁢Sn2⁢O7 and Ce2⁢Hf2⁢O7, which each showed temperature-dependent diffuse diffraction at high 𝑄, interpreted as arising from octupolar correlations. Our measurements use an optimized single-crystal diffuse scattering instrument that allows us to screen against strong Bragg scattering from Ce2⁢Zr2⁢O7. The temperature-difference neutron diffraction reveals a low-𝑄 peak consistent with dipolar spin-ice correlations reported in previous work, and an alternation between positive and negative net intensity at higher 𝑄. These features are consistent with our numerical-linked-cluster calculations using pseudospin interaction parameters previously reported for Ce2⁢Zr2⁢O7, Ce2⁢Sn2⁢O7, and Ce2⁢Hf2⁢O7. Importantly, neither the measured data nor any of the NLC calculations show evidence for increased scattering at high 𝑄 resulting from octupolar correlations. We conclude that at the lowest attainable temperature for our measurements (𝑇 =0.05  K), scattering from octupolar correlations in Ce2⁢Zr2⁢O7 is not present in the neutron diffraction signal on the level of our observation threshold of around 0.1% of the low-𝑄 dipole scattering. We compare these results to those obtained earlier on powder Ce2⁢Sn2⁢O7 and Ce2⁢Hf2⁢O7, and to low-energy inelastic neutron scattering from single-crystal Ce2⁢Zr2⁢O7.