arXiv subjects

Casey Eichstaedt

Publications and source records attributed to Casey Eichstaedt.

4 recordsLinked to original sources

arXiv2025

Quasiparticle Description of Angle-Resolved Photoemission Spectroscopy for SrCuO2

SrCuO2 has long been considered a near-archetypal realization of a quasi one dimensional (1D) system of interacting electrons with short-range interactions. Within this framework, experimental observations - interpreted through the lens of the 1D Hubbard model-suggest that electron and hole excitations decay into two types of (unphysical) collective bosonic modes: spinons, which carry the spin degree of freedom, and holons, which carry the charge degree of freedom. This model, known as spin-charge separation, is most directly evidenced by angle-resolved photoemission spectroscopy (ARPES), where a photo-induced hole decays into a continuum of these excitations. Here we present an alternative perspective grounded in first-principles, self-consistent, and parameter-free many-body perturbation theory. In this revised quasiparticle framework, ARPES can be understood as a one-body effect arising from mild disorder in a long-range antiferromagnetic ground state. the emergence of the so-called spinon branch arises naturally from spin disorder, the anomalous line widths are accurately captured, and we provide a compelling explanation for the spectral weight observed at the non-magnetic zone boundary. This reinterpretation provides a unified explanation for key experimental signatures previously attributed to spin-charge separation, including features observed in optical conductivity. Additionally, we show that SrCuO2 exhibits a nontrivial interchain coupling that significantly influences both its one-particle and two-particle spectral functions. By comparing the spectral features of SrCuO2 with those of La2CuO4, we argue that SrCuO2 shares notable similarities with the two-dimensional cuprates - both being rooted in a common CuO4 plaquette-based molecular orbital framework.

cond-mat.supr-con↗

arXiv2024

Electron-Phonon Coupling in Many-Body Perturbation Theory: Developments within the Quasiparticle Self-Consistent GW approximation and LMTO Formalism

The calculation of electron-phonon (e-ph) coupling from first principles is a topic of great interest in materials science, offering a robust, non-empirical framework to understand and predict a wide range of physical phenomena. While significant progress has been made using the Kohn-Sham framework of density functional theory (KS-DFT), it is increasingly evident that standard approximations in KS-DFT often fall short of providing accurate results. These shortcomings are frequently linked to the non-local nature of the exchange-correlation potential, prompting the development of advanced methodologies within DFT and many-body perturbation theory. Despite these efforts, a highly reliable and efficient first-principles approach to accurately capture e-ph interactions remains elusive. To address this challenge, we introduce a novel field-theoretical methodology that integrates the foundational work of Baym and Hedin with the Quasiparticle Self-Consistent GW (QSGW) approximation, implemented within the Questaal electronic structure suite. Our approach, based on a response function framework, ensures that Pulay-like incomplete-basis-set corrections are not required to account for changes in basis functions, paving the way for a high-fidelity description of e-ph coupling.

cond-mat.mtrl-sci↗

arXiv2023

Experimental Evidence for Non-spherical Magnetic Form Factor in Ru$^{3+}$

The Mott insulator $α$-RuCl$_3$ has generated great interest in the community due to its possible field-induced Kitaev quantum spin liquid state. Despite enormous effort spent trying to obtain the form of the low energy Hamiltonian, there is currently no agreed upon set of parameters which is able to explain all of the data. A key piece of missing information lies in the determination of the magnetic form factor of Ru$^{3+}$, particularly for a true quantitative treatment of inelastic neutron scattering data. Here we present the experimentally derived magnetic form factor of Ru$^{3+}$ in the low spin 4$d^5$ state using polarized neutron diffraction within the paramagnetic regime on high quality single crystals of $α$-RuCl$_3$. We observe strong evidence of an anisotropic form factor, expected of the spin-orbit coupled $j_{\textrm{eff}} = \frac{1}{2}$ ground state. We model the static magnetization density in increasing complexity from simple isotropic cases, to a multipolar expansion, and finally \emph{ab initio} calculations of the generalized $j_{\textrm{eff}} = \frac{1}{2}$ ground state. Comparison of both single ion models and inclusion of Cl$^-$ anions support the presence of hybridization of Ru$^{3+}$ with the surrounding Cl$^{-}$ ligands.

cond-mat.str-el↗

arXiv2019

Deriving models for the Kitaev spin-liquid candidate material $α$-RuCl$_3$ from first principles

We use the constrained random phase approximation (cRPA) to derive from first principles the Ru-$t_{2g}$ Wannier function based model for the Kitaev spin-liquid candidate material $α$-RuCl$_3$. We find the non-local Coulomb repulsion to be sizable compared to the local one. In addition we obtain the contribution to the Hamiltonian from the spin-orbit coupling and find it to also contain non-negligible non-local terms. We invoke strong coupling perturbation theory to investigate the influence of these non-local elements of the Coulomb repulsion and the spin-orbit coupling on the magnetic interactions. We find that the non-local Coulomb repulsions cause a strong enhancement of the magnetic interactions, which deviate from experimental fits reported in the literature. Our results contribute to the understanding and design of quantum spin liquid materials via first principles calculations.

cond-mat.str-el↗