Two-Peak Heat Capacity Accounts for $R\ln(2)$ Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce$_2$Hf$_2$O$_7$
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2025
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| author | Smith, E. M. Fitterman, A. Schäfer, R. Placke, B. Woods, A. Lee, S. Huang, S. H. -Y. Beare, J. Sharma, S. Chatterjee, D. Balz, C. Stone, M. B. Kolesnikov, A. I. Wildes, A. R. Kermarrec, E. Luke, G. M. Benton, O. Moessner, R. Movshovich, R. Bianchi, A. D. Gaulin, B. D. |
| author_facet | Smith, E. M. Fitterman, A. Schäfer, R. Placke, B. Woods, A. Lee, S. Huang, S. H. -Y. Beare, J. Sharma, S. Chatterjee, D. Balz, C. Stone, M. B. Kolesnikov, A. I. Wildes, A. R. Kermarrec, E. Luke, G. M. Benton, O. Moessner, R. Movshovich, R. Bianchi, A. D. Gaulin, B. D. |
| contents | 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 $T_2$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2501_08327 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Two-Peak Heat Capacity Accounts for $R\ln(2)$ Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce$_2$Hf$_2$O$_7$ Smith, E. M. Fitterman, A. Schäfer, R. Placke, B. Woods, A. Lee, S. Huang, S. H. -Y. Beare, J. Sharma, S. Chatterjee, D. Balz, C. Stone, M. B. Kolesnikov, A. I. Wildes, A. R. Kermarrec, E. Luke, G. M. Benton, O. Moessner, R. Movshovich, R. Bianchi, A. D. Gaulin, B. D. Strongly Correlated Electrons 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 $T_2$. |
| title | Two-Peak Heat Capacity Accounts for $R\ln(2)$ Entropy and Ground State Access in the Dipole-Octupole Pyrochlore Ce$_2$Hf$_2$O$_7$ |
| topic | Strongly Correlated Electrons |
| url | https://arxiv.org/abs/2501.08327 |