Octupolar correlations and spinon continuum in Ce³⁺ pyrochlores

Rare-earth pyrochlore frustrated magnets have played a central role in the study of three- dimensional quantum spin liquids (QSL) over the last years. Recent experimental studies on Ce₂Sn₂O₇ and Ce₂Zr₂O₇ have further boosted the interest in quantum spin ice (QSI) states. Owing to the crystal electric field in pyrochlores, Ce³⁺ can stabilize a dipole-octupole Kramers ground state doublet. Magnetic measurements do not show any sign of ordering or freezing down to the lowest accessible temperatures but suggest the presence of a correlated ground state.

In Ce₂Sn₂O₇, the octupolar degrees of freedom become dominant at low temperatures under the effect of octupole-octupole couplings. A signature of these correlations could be found in neutron scattering experiments, as a broad quasi-elastic liquid-like hump at high values of momentum transfer. This study unveiled the formation of a quantum liquid of magnetic octupoles, a peculiar U(1) QSL state envisioned by theory. Low-energy time-of-flight and backscattering neutron spectroscopy reveal the presence of a continuum of fractional dipole excitations. The observed density of states displays characteristics, such as its threshold, center and extent, that are in agreement with theoretical predictions.

To further characterize these multipolar interactions, single crystal experiments are highly urged but hindered by the difficulty to synthesis large single crystals of Ce₂Sn₂O₇. Ce₂Hf₂O₇ is another pyrochlore material where trivalent cerium ions can be stabilized. Unlike Ce₂Sn₂O₇, Ce₂Hf₂O₇ can be grown using the travelling solvent floating zone technique. We present results of multiple techniques demonstrating that the octupolar QSI is also the ground state of Ce₂Hf₂O₇.