Two-Peak Heat Capacity Accounts for Rln(2) Entropy and Ground State Access in the Dipole-Octupole Pyrochlore...

Magnetic heat capacity measurements of a high-quality single crystal of the dipole-octupole pyrochlore Ce_{2}Hf_{2}O_{7} down to a temperature of T=0.02  K are reported. These show a two-peaked structure, with a Schottky-like peak at T_{1}∼0.065  K, similar to what is observed in its sister Ce pyrochlores Ce_{2}Zr_{2}O_{7} and Ce_{2}Sn_{2}O_{7}. However, a second sharper peak is observed at T_{2}∼0.025  K, signifying the entrance to the ground state. The ground state appears to have gapped excitations, as even the most abrupt extrapolation to C_{P}=0 at T=0  K fully accounts for the Rln(2) entropy associated with the pseudospin-1/2 doublet for Ce^{3+} in this environment. The ground state could be conventionally ordered, although theory predicts a much larger anomaly in C_{P} at much higher temperatures than the measured T_{2} for expectations from an all-in, all-out ground state of the XYZ Hamiltonian for Ce_{2}Hf_{2}O_{7}. The sharp low-temperature peak could also signify a crossover from a classical spin liquid to a quantum spin liquid (QSL). For both scenarios, comparison of the measured C_{P} with NLC calculations suggests that weak interactions beyond the nearest-neighbor XYZ Hamiltonian become relevant below T∼0.25  K. The diffuse magnetic neutron scattering observed from Ce_{2}Hf_{2}O_{7} at low temperatures between T_{2} and T_{1} resembles that observed from Ce_{2}Zr_{2}O_{7}, which is well established as a π-flux quantum spin ice (QSI). Together with the peak in the heat capacity at T_{2}, this diffuse scattering from Ce_{2}Hf_{2}O_{7} is suggestive of a classical spin liquid regime above T_{2} that is distinct from the zero-entropy quantum ground state below T_{2}.