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The Ce₂X₂O₇ pyrochlores, with X = Zr and Sn, are insulators with Ce³⁺ degrees of freedom decorating a pyrochlore lattice, an archetype for geometrical frustration in three dimensions. High energy neutron spectroscopy reveals that the crystal field ground state originating from J=5/2 Ce³⁺ is comprised exclusively of mJ=±3/2, which gives the resulting S_effective=1/2 degree of freedom both a dipole and and an octupole character [1, 2, 3].  It also imprints a near-neighbour XYZ spin Hamiltonian onto this system, and such interacting Seffective=1/2 degrees of freedom on the pyrochlore lattice are theoertically known to allow for both U(1) spin liquid ground states, and ordered All-In All Out (AIAO) ground states, both of which can have either dipolar or octupolar character.  I will review our recent experimental work on both large single crystals of Ce₂Zr₂O₇ and on small single crystals and powder samples of Ce₂Sn₂O₇.  Our measurements on Ce₂Zr₂O₇ above T=0.06 K show no evidence of an ordered state [4].  Theoretical modelling of heat capacity data, magnetic susceptibility data and neutron scattering allows for estimates of the terms in the near-neighbour XYZ Hamiltonian, and these strongly support a U(1) quantum spin liquid ground state for Ce₂Zr₂O₇ [4].  A similar analysis on related experimental data on Ce₂Sn₂O₇ shows it to display a dipolar spin ice state at T∼ 0.1 K, proximate to an AIAO dipolar Neel ground state [5].  I will review these recent results in the context of related experimental studies [2,6].  

[1] J. Gaudet et al., Phys. Rev. Lett., 122, 187201 (2019).

[2] B. Gao et al, Nat. Phys., 15, 1052 (2019).

[3] R. Sibille et al, Phys. Rev. Lett., 115, 097202 (2015).

[4] E. M. Smith et al., Phys. Rev. X, 12, 021015 (2022).

[5] D. R. Yahne et al., (unpublished, 2022).

[6] R. Sibille et al., Nat. Phys., 16, 546 (2019).