arXiv subjects

Chung Xu

Publications and source records attributed to Chung Xu.

3 recordsLinked to original sources

arXiv2026

Antiferroquadrupolar Order in Altermagnetic CoF$_2$

Altermagnets host non-relativistic spin-split electronic states whose microscopic origin is theoretically linked to a hidden charge order, yet experimental studies of this charge order are limited. We therefore investigate charge order within CoF$_2$, a $d$-wave altermagnetic compound with a rutile crystal structure and $\Gamma$-point antiferromagnetism. Combining resonant elastic X-ray scattering, symmetry analysis and ab initio calculations, we directly observe charge ordering and identify it as antiferroquadrupolar in nature. Via electronic structure calculations, we show that the experimentally observed antiferroquadrupolar order gives rise to the characteristic altermagnetic spin-splitting, thereby establishing empirical evidence for the decomposition of the altermagnetic order parameter into magnetic and charge degrees of freedom. We hence demonstrate that antiferroquadrupolar order is the microscopic origin of altermagnetism in CoF$_2$, with implications to the wider family of rutile altermagnets. Furthermore, our approach is applicable to studying altermagnetism in general, having demonstrated that resonant elastic X-ray scattering can serve as a direct probe of the charge multipoles that underpin spin-split electronic states in these materials.

cond-mat.str-el↗

arXiv2025

Ab initio X-ray Near-Edge Spectroscopy of Sodium-Based Multi-Alkali Antimonides

Multi-alkali antimonides (MAAs) are promising materials for vacuum electron sources. While sodium-based MAAs have demonstrated superior characteristics for ultrabright electron sources, their synthesis remains challenging, often resulting in mixed stoichiometries and polycrystalline domains. To address this complexity and guide the characterization of experimentally grown photocathodes, we present a comprehensive theoretical study of the X-ray near-edge spectroscopy (XANES) of four ternary MAAs: cubic Na$_2$KSb and hexagonal NaK$_2$Sb, representing the experimentally known phase of each stoichiometry, as well as hexagonal Na$_2$KSb and cubic NaK$_2$Sb, two computationally predicted polymorphs. Employing state-of-the-art ab initio methods based on all-electron density-functional theory and the solution of the Bethe-Salpeter equation (BSE), we compute and analyze the XANES at the sodium and potassium K-edges, potassium L$_{2,3}$-edge, and antimony K and L$_2$-edges. Our analysis reveals distinct spectral fingerprints for the experimentally known phases, cubic Na$_2$KSb and hexagonal NaK$_2$Sb, particularly at the sodium K-edge and potassium L$_{2,3}$-edge, providing useful indications for their identification in complex samples. We further investigate the role of excitonic effects by comparing BSE spectra and their counterparts obtained in the independent-particle approximation, highlighting their significant influence on the near-edge features, especially for shallower core levels. Our findings offer a useful theoretical benchmark for the experimental characterization and diagnostics of sodium-based MAA photocathodes, complementing experiments on resolved phases and providing the spectral fingerprints of computationally predicted phases that could emerge in polycrystalline samples.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and optical properties of computationally predicted Na-K-Sb crystals

Thanks to their favorable electronic and optical properties, sodium-potassium-antimonides are an emerging class of crystals used as photocathodes in particle accelerators. The persisting challenges related to the synthesis and characterization of these materials demand support from theory and make the study of computationally predicted polymorphs particularly relevant to identifying the structure and composition of the samples. Using first-principles methods based on density-functional theory and many-body perturbation theory, the electronic and optical properties of cubic NaK$_{2}$Sb and hexagonal Na$_{2}$KSb are studied. Both systems, most commonly found in the hexagonal and cubic phase, respectively, exhibit an indirect fundamental gap that is energetically very close to the direct band gap at $\Gamma$ of magnitude 0.81 eV for NaK$_{2}$Sb and 0.70 eV for Na$_{2}$KSb. In the band structure of both materials, Sb $p$-states dominate the valence region with minor contributions from the alkali $p$-states, while the alkali $s$-states mainly contribute at lower energies. The optical spectra of both crystals are not subject to sizeable excitonic effects, except for a redshift of the excitation energies of the 50-100 meV and some redistribution of the oscillator strength beyond the lowest-energy peak in the near-infrared region. Our results indicate that computationally predicted cubic NaK$_{2}$Sb and hexagonal Na$_{2}$KSb have favorable characteristics as photocathodes and, as such, their presence in polycrystalline samples is not detrimental for these applications.

cond-mat.mtrl-sci↗