Two-Peak Heat Capacity Accounts for $R\ln(2)$ Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce$_2$Hf$_2$O$_7$

E. M. Smith; A. Fitterman; R. Schäfer; B. Placke; A. Woods; S. Lee; S. H. -Y. Huang; J. Beare; S. Sharma; D. Chatterjee; C. Balz; M. B. Stone; A. I. Kolesnikov; A. R. Wildes; E. Kermarrec; G. M. Luke; O. Benton; R. Moessner; R. Movshovich; A. D. Bianchi; B. D. Gaulin

doi:10.1103/4qxy-l8pg

paper

Two-Peak Heat Capacity Accounts for $R ln (2)$ Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce $_{2}$ Hf $_{2}$ O $_{7}$

arXiv:2501.08327 · doi:10.1103/4qxy-l8pg

Abstract

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 $T_{1} \sim 0.065$ 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 $T_{2} \sim 0.025$ 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 $R ln (2)$ 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 cross-over 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 $T \sim 0.25$ 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 $T_{2}$ .

arXiv commentMain Text (6 pages, 4 figures), Supplemental Material (14 pages, 17 figures)

Two-Peak Heat Capacity Accounts for $R ln (2)$ Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce $_{2}$ Hf $_{2}$ O $_{7}$

Abstract

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Two-Peak Heat Capacity Accounts for Rln(2) Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce2​Hf2​O7​

Abstract

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Two-Peak Heat Capacity Accounts for $R ln (2)$ Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce $_{2}$ Hf $_{2}$ O $_{7}$