arXiv subjects

Greta L. Chappell

Publications and source records attributed to Greta L. Chappell.

4 recordsLinked to original sources

arXiv2025

Large Kohn anomaly and phonon collapse induced by charge density wave in UPt$_2$Si$_2$

Using high-energy-resolution inelastic x-ray scattering, we observe anomalous softening and damping of the transverse acoustic phonon in UPt$_2$Si$_2$ as the system is cooled towards the charge density wave (CDW) transition temperature, T$_{\rm{CDW}}$. The phonon exhibits a marked Kohn-type anomaly around the CDW wave vector, Q$_{\rm{CDW}}$, and becomes overdamped within a finite momentum range already well above T$_{\rm{CDW}}$. The dispersion anomaly is consistent with potential Fermi surface nesting, which together with the extended phonon collapse indicate strong electron-phonon coupling. The transition temperature estimated from the phonon softening is markedly lower than T$_{\rm{CDW}}$, consistent with a primarily electronic instability rather than a phonon-driven transition. Our results establish UPt$_2$Si$_2$ as a prime example of a strongly correlated electron CDW system with exceptionally strong electron-phonon coupling driving phonon softening and collapse.

cond-mat.str-el↗

arXiv2025

Adaptation of Wallace's Approach to the Specific Heat of Elemental Solids with Significant Intrinsic Anharmonicity, Particularly the Light Actinide Metals

The quasiharmonic approximation is the most common method for modeling the specific heat of solids; however, it fails to capture the effects of intrinsic anharmonicity. In this study, we introduce the "elastic softening approximation," an alternative approach to modeling intrinsic anharmonic effects on thermodynamic quantities, which is grounded in Wallace's thermodynamic framework that tracks entropy changes resulting from the continuous change (e.g., softening) of phonons as a function of temperature. A key finding of our study is a direct correlation between Poisson's ratio and the differential rate of phonon softening at finite frequencies, compared to lower frequencies relevant to elastic moduli measurements. We observe that elemental solids such as $α$-Be, diamond, Al, Cu, In, W, Au, and Pb, which span a wide range of Poisson's ratios and exhibit varying degrees of intrinsic anharmonicity, consistently follow this trend. When applied to $α$-U, $α$-Pu, and $δ$-Pu, our method reveals unusually large anharmonic phonon contributions at elevated temperatures across all three light actinide metals. These findings are attributed to the unique combination of enhanced covalency and softer elastic moduli inherent in the actinides, potentially influenced by their 5f-electron bonding.

cond-mat.mtrl-sci↗

arXiv2024

A noncollinear density functional theory ansatz for the phononic and thermodynamic properties of $α$-Pu

Plutonium's phase diagram is host to complex structures and interactions that make the description of its ground state properties elusive. Using all-electron density functional theory, we study the thermodynamic properties of $α$-Pu. To do this, we build on recent work in the literature by introducing a novel noncollinear magnetic ansatz for $α$-Pu's ground state. The noncollinear ansatz accurately recovers the experimental phonon density of states, heat capacity, and thermal expansion. These new results on $α$-Pu along with recent results on $δ$-Pu demonstrate the efficacy of noncollinear ansatzes for the description of plutonium.

cond-mat.mtrl-sci↗

arXiv2020

Charge density wave with anomalous temperature dependence in UPt2Si2

Using single crystal neutron and x-ray diffraction, we discovered a charge density wave (CDW) below 320 K, which accounts for the long-sought origin of the heat capacity and resistivity anomalies in UPt2Si2. The modulation wavevector, Qmod, is intriguingly similar to the Fermi surface nesting wavevector of URu2Si2. Qmod shows an unusual temperature dependence, shifting from commensurate to incommensurate position upon cooling and becoming locked at ~ (0.42 0 0) near 180 K. Bulk measurements indicate a cross-over toward a correlated coherent state around the same temperature, suggesting an interplay between the CDW and Kondo-lattice-like coherence before coexisting antiferromagnetic order sets in at TN = 35 K.

cond-mat.supr-con↗