arXiv subjects

Keisuke Higashi

Publications and source records attributed to Keisuke Higashi.

4 recordsLinked to original sources

arXiv2024

Atomic multiplet and charge-transfer screening effects in 1$s$ and 2$p$ core-level X-ray photoelectron spectra of early 3$d$ transition-metal oxides

We present a comparative analysis of 1$s$ and 2$p$ core-level hard X-ray photoelectron spectroscopy (HAXPES) spectra in metallic VO$_2$ and CrO$_2$. Even though the V 1$s$ and 2$p$ spectra in VO$_2$ display similar line shapes except the absence or presence of a spin-orbit coupling splitting, the Cr 1$s$ and 2$p$ spectra exhibit distinct main-line shapes. The experimental HAXPES spectra are analyzed by the Anderson impurity model based on the density functional theory + dynamical mean-field theory and a conventional MO$_6$ cluster model. We elucidate the complex interplay between formation of the intra-atomic multiplet and charge transfer effect on the chemical bonding followed by the 1$s$ and 2$p$ core electron excitations. We demonstrate the advantage of the 1$s$ excitations to the routinely-employed 2$p$ excitations for distinguishing between metal-ligand and metal-metal charge transfer contributions in early 3$d$ transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2023

Single- and Multimagnon Dynamics in Antiferromagnetic $α$-Fe$_2$O$_3$ Thin Films

Understanding the spin dynamics in antiferromagnetic (AFM) thin films is fundamental for designing novel devices based on AFM magnon transport. Here, we study the magnon dynamics in thin films of AFM $S=5/2$ $α$-Fe$_2$O$_3$ by combining resonant inelastic x-ray scattering, Anderson impurity model plus dynamical mean-field theory, and Heisenberg spin model. Below 100 meV, we observe the thickness-independent (down to 15 nm) acoustic single-magnon mode. At higher energies (100-500 meV), an unexpected sequence of equally spaced, optical modes is resolved and ascribed to $ΔS_z = 1$, 2, 3, 4, and 5 magnetic excitations corresponding to multiple, noninteracting magnons. Our study unveils the energy, character, and momentum-dependence of single and multimagnons in $α$-Fe$_2$O$_3$ thin films, with impact on AFM magnon transport and its related phenomena. From a broader perspective, we generalize the use of L-edge resonant inelastic x-ray scattering as a multispin-excitation probe up to $ΔS_z = 2S$. Our analysis identifies the spin-orbital mixing in the valence shell as the key element for accessing excitations beyond $ΔS_z = 1$, and up to, e.g., $ΔS_z = 5$. At the same time, we elucidate the novel origin of the spin excitations beyond the $ΔS_z = 2$, emphasizing the key role played by the crystal lattice as a reservoir of angular momentum that complements the quanta carried by the absorbed and emitted photons.

cond-mat.str-el↗

arXiv2023

Satellites in the Ti~1$s$ core level spectra of SrTiO$_3$ and TiO$_2$

Satellites in core level spectra of photoelectron spectroscopy (PES) can provide crucial information on the electronic structure and chemical bonding in materials, particular in transition metal oxides. This paper explores satellites of the Ti 1$s$ and 2$p$ core level spectra of SrTiO$_3$ and TiO$_2$. Conventionally, soft x-ray PES (SXPS) probes the Ti 2$p$ core level; however, it is not ideal to fully capture satellite features due to its inherent spin-orbit-splitting (SOS). Here, hard x-ray PES(HAXPES) provides access to the Ti 1$s$ spectrum instead, which allows us to study intrinsic charge responses upon core-hole creation without the complication from SOS and with favorable intrinsic linewidths. The experimental spectra are theoretically analyzed by two impurity models, including an Anderson impurity model (AIM) built on local density approximation (LDA) and dynamical mean-field theory (DMFT), and a conventional TiO$_6$ cluster model. The theoretical results emphasize the importance of explicit inclusion of higher-order Ti-O charge-transfer processes beyond the nearest-neighboring Ti-O bond to simulate the core level spectra of SrTiO$_3$ and TiO$_2$. The AIM approach with continuous bath orbitals provided by LDA+DMFT represents the experimental spectra well. Crucially, with the aid of the LDA+DMFT method, this paper provides a robust prescription of how to use the computationally cheap cluster model in fitting analyses of core level spectra.

cond-mat.mtrl-sci↗

arXiv2021

Core-Level X-Ray Spectroscopy of Infinite-Layer Nickelate: LDA+DMFT Study

Motivated by recent core-level x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) experiments for the newly-discovered superconducting infinite-layer nickelate, we investigate the core-level spectra of the parent compounds NdNiO$_2$ and LaNiO$_2$ using the combination of local density approximation and dynamical mean-field theory (LDA+DMFT). Adjusting a charge-transfer energy to match the experimental spectra, we determine the optimal model parameters and discuss the nature of the NdNiO$_2$ ground state. We find that self-doping from the Nd 5$d$ states in the vicinity of the Fermi energy prohibits opening of Mott-Hubbard gap in NdNiO$_2$. The present Ni $L_3$ XAS and RIXS calculation for LaNiO$_2$ cannot explain the difference to NdNiO$_2$ spectra.

cond-mat.str-el↗