arXiv subjects

Roxanne M. Tutchton

Publications and source records attributed to Roxanne M. Tutchton.

6 recordsLinked to original sources

arXiv2026

Gaunt and Breit Two-electron contributions to Mean-field Transformations and Fine Structure Splitting

Materials utilized by novel energy systems are often studied using weakly correlated mean-field theories. However, if these systems incorporate heavy elements, relativistic effects must be included. Therefore a Kramers unrestricted Coupled Cluster with singles and doubles excitation formalism within a molecular mean-field Exact Two-Component framework (X2C-mmf) using a four-component Dirac-Hartree-Fock (DHF) reference state is presented. The exact X2C-mmf transformed normal-order Hamiltonian incorporates all one-electron and two-electron (2e) contributions from the Coulomb, Gaunt, and Breit operators and is used with the Equation of Motion method to calculate the excitation energies of the alkali group of elements. Using this framework, the effects of 2e Gaunt and Breit integrals are studied. Results demonstrate growing contributions from these integrals to the generated X2C-mmf mean-fields and electronic fine structure calculations with increasing atomic number. Overall, this paper outlines the method and effect of a higher level of accuracy within the X2C-mmf approach and lays the foundation for future theoretical development of relativistic calculations within this framework.

cond-mat.mtrl-sci↗

arXiv2026

Temperature dependence of electronic conductivity from ab initio thermal simulation

We present a temperature-dependent extension of the approximate electronic conductivity formula of Hindley and Mott that leverages time-averaged fluctuations of the electronic density of states obtained from ab initio molecular dynamics. By thermally averaging the square of the density of states near the Fermi level, we obtain an estimate of the temperature dependence of the conductivity. This approach termed the thermally-averaged Hindley-Mott (TAHM) method was applied to five representative systems: crystalline aluminum (c-Al), aluminum with a grain boundary (AlGB), a four-layer graphene-aluminum composite (Al-Gr), amorphous silicon (a-Si) and amorphous germanium-antimony-telluride (a-GST). The method reproduces the expected Bloch-Gruneisen decrease in conductivity for c-Al and AlGB. Generally, the reduction (increase) in conductivity for metallic (semiconducting) materials are reproduced. It captures microstructure-induced, thermally activated conduction in multilayer Al-Gr, a-Si and a-GST. Overall, the approach provides a computationally efficient link between time-dependent electronic structure and temperature-dependent transport, offering a simple and approximate tool for exploring electronic conductivity trends in complex and disordered materials.

cond-mat.mtrl-sci↗

arXiv2026

Atomistic modeling of uranium monocarbide with a machine learning interatomic potential

Uranium monocarbide (UC) is an advanced ceramic fuel candidate due to its superior uranium density and thermal conductivity compared to traditional fuels. To accurately model UC at reactor operating conditions, we developed a machine learning interatomic potential (MLIP) using an active learning procedure to generate a comprehensive training dataset capturing diverse atomic configurations. The resulting MLIP predicts structural, elastic, thermophysical properties, defect formation energies, and diffusion behaviors, aligning well with experimental and theoretical benchmarks. This work significantly advances computational methods to explore UC, enabling efficient large-scale and long-time molecular dynamics simulations essential for reactor fuel qualification.

cond-mat.mtrl-sci↗

arXiv2025

First-Principles Study of the Fermi Surface Topology of CeCu$_{2}$Si$_{2}$

Since the discovery of heavy-fermion superconductivity in CeCu$_{2}$Si$_{2}$, the material has attracted great interest particularly with regard to the nature of the superconducting pairing and its mechanism. Consequently, it is essential to better understand the electronic Fermi surface topology and its role in strong antiferromagnetic fluctuations. The standard density functional theory method is insufficient to model the interplay of strong onsite Coulomb repulsion in localized 4{\it f}-electrons and their hybridization with itinerant ligand-orbital electrons. We have performed electronic ground state calculations on CeCu$_{2}$Si$_{2}$ using the Gutzwiller wavefunction approximation. The Gutzwiller approximation captures the quasiparticle band renormalization from the strong onsite Coulomb repulsion. We have performed an analysis of this effect on the electronic structure and the Fermi surface topology by varying the interaction strength and taking into account the crystal-field splitting. Using the de Haas van Alphen effect, the extremal Fermi surface cross-sectional areas were calculated to quantify the effects of quasiparticle mass renormalization on the Fermi surface. Our results confirm two Fermi surface sheets corresponding to the heavy (488m$_{e}$) and light (4.35m$_{e}$) quasiparticles, which is in close agreement with experimental measurements as well as the renormalized band method.

cond-mat.str-el↗

arXiv2025

Toward machine learning interatomic potentials for modeling uranium mononitride

Uranium mononitride (UN) is a promising accident-tolerant fuel because of its high fissile density and high thermal conductivity. In this study, we developed the first machine learning interatomic potentials for reliable atomic-scale modeling of UN at finite temperatures. We constructed a training set using density functional theory (DFT) calculations that was enriched through an active learning procedure, and two neural network potentials were generated. Both potentials successfully reproduce key thermophysical properties of interest, such as temperature-dependent lattice parameter, specific heat capacity, and bulk modulus. We also evaluated the energy of stoichiometric defect reactions and defect migration barriers and found close agreement with DFT predictions, demonstrating that our potentials can be used for modeling defects in UN. Additional tests provide evidence that our potentials are reliable for simulating diffusion, noble gas impurities, and radiation damage.

cond-mat.mtrl-sci↗

arXiv2020

Hybridization effect on the X-ray absorption spectra for actinide materials: Application to PuB$_4$

Studying the local moment and 5$f$-electron occupations sheds insight into the electronic behavior in actinide materials. X-ray absorption spectroscopy (XAS) has been a powerful tool to reveal the valence electronic structure when assisted with theoretical calculations. However, the analysis currently taken in the community on the branching ratio of the XAS spectra generally does not account for the hybridization effects between local $f$-orbitals and conduction states. In this paper, we discuss an approach which employs the DFT+Gutzwiller rotationally-invariant slave boson (DFT+GRISB) method to obtain a local Hamiltonian for the single-impurity Anderson model (SIAM), and calculates the XAS spectra by the exact diagonalization (ED) method. A customized numerical routine was implemented for the ED XAS part of the calculation. By applying this technique to the recently discovered 5$f$-electron topological Kondo insulator PuB$_4$, we determined the signature of 5$f$-electronic correlation effects in the theoretical X-ray spectra. We found that the Pu 5$f$-6$d$ hybridization effect provides an extra channel to mix the $j=5/2$ and $7/2$ orbitals in the 5$f$ valence. As a consequence, the resulting electron occupation number and spin-orbit coupling strength deviate from the intermediate coupling regime.

cond-mat.str-el↗