arXiv subjects

Atsushi Hariki

Publications and source records attributed to Atsushi Hariki.

At least 19 recordsLinked to original sources

arXiv2026

Charge-Transfer Electronic Structure of NiX$_2$ (X = S, Se)

We investigate the electronic structures of NiS$_2$ and NiSe$_2$ using density functional theory combined with dynamical mean-field theory (DFT+DMFT). A realistic electronic structure within DFT+DMFT was determined by optimizing the double-counting correction to reproduce experimental valence-band photoemission spectra. The validity of the present model is further confirmed by its successful description of the Ni 2$p$ core-level photoemission and Ni $L$-edge x-ray absorption spectra of NiS$_2$. Our results reveal a smaller charge-transfer energy than previously assumed, resulting in substantial ligand-to-Ni charge transfer and a reduced Ni local moment. We clarify how the relative position and interaction between the Ni upper Hubbard band and the antibonding chalcogen-dimer states shape the evolution of the low-energy electronic structure across the NiS$_{2-x}$Se$_x$ series.

cond-mat.mtrl-sci↗

arXiv2026

Probing La-based nickelates with Ni 1$s$ core-level photoelectron spectroscopy

We present a comparative Ni core level photoemission study of La$_3$Ni$_2$O$_7$, Nd$_3$Ni$_2$O$_7$, and LaNiO$_3$ using both the Ni $2p$ and the Ni $1s$. We address the challenges in analyzing the widely investigated Ni $2p$ spectra arising from the substantial overlap in energy of the Ni $2p$ with the La $3d$. We show that on the other hand the deep Ni $1s$ core level does provide a clean view on the intrinsic electronic excitations and we highlight its potential to resolve detailed differences in the electronic structure within the strongly correlated Ruddlesden-Popper series La$_{n+1}$Ni$_n$O$_{3n+1}$.

cond-mat.str-el↗

arXiv2026

UCd$_{11}$: A strongly localized 5$f^3$ material

UCd$_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\rm N}$ = 5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5$f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd$_{11}$ adopts the formal U$^{3+}$ 5$f^3$ configuration, while core-level photoemission spectroscopy (PES) data of UCd$_{11}$ reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd$_{11}$, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd$_{11}$ is a highly localized uranium 5$f^3$ system. Furthermore, core-level spectra obtained from a DFT+DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5$f$ behavior.

cond-mat.str-el↗

arXiv2026

X-ray magnetic circular dichroism evidence of intrinsic $d$-wave altermagnetism in rutile-structure NiF$_2$

We present the x-ray magnetic circular dichroism (XMCD) at the Ni $L_{2,3}$-edge as an evidence of the $d$-wave altermagnetism in rutile-structure NiF$_2$. Sizable XMCD signal is observed in excellent agreement with theoretical simulations. Owing to a considerable net magnetization due to spin canting, the XMCD spectrum consists of an altermagnetic signal as well as a non-negligible ferromagnetic contribution. We verify experimentally that the XMCD spectrum can be written as a sum of contributions from altermagnetism and weak ferromagnetism. Two experimental methods to isolate the ferromagnetic contribution are shown to yield essentially the same result. These are dependence of XMCD on applied magnetic fields below the Néel temperature and the XMCD measured in applied field above the Néel temperature. Our results demonstrate the utility of XMCD as a probe for altermagnetic materials with the coexisting weak ferromagnetism induced by the relativistic spin-orbit coupling.

cond-mat.str-el↗

arXiv2025

UTe$_2$: a narrow band superconductor

We investigate the nature of the 5$f$ electrons in the unconventional odd-parity superconductor UTe$_2$, focusing on the degree of covalency, localization versus itinerancy, and dominant electronic configuration. This is achieved using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT) calculations. A key aspect of our approach is the material-specific tuning of the double-counting correction parameter, $μ_{\rm dc}$, within the DFT+DMFT part. This tuning is guided by the energy dependence of photo-ionization cross-sections in valence band photoelectron spectroscopy. The reliability of the parameters is confirmed by the accurate reproduction of the angle-resolved valence-band photoemission spectra and the U 4$f$ core-level data. The DFT+DMFT model reveals that in UTe$_2$ U 5$f^n$ configurations with n=1 to 4 contribute to the ground state, with the 5$f^2$ configuration being most prevalent and an average 5$f$ shell fillings close to 2.5. The model further suggests that the 5$f$ electrons form narrow bands and that charge fluctuations due to degeneracy play a role in addition to coherent valence dynamics arising from hybridization with the conduction bath. Additionally, the significance of the U 6$d$ states in UTe$_2$ is discussed.

cond-mat.str-el↗

arXiv2025

Valency, charge-transfer, and orbital-dependent correlation in bilayer nickelates Nd3Ni2O7

We examine the bulk electronic structure of Nd3Ni2O7 using Ni 2p core-level hard x-ray photoemission spectroscopy combined with density functional theory + dynamical mean-field theory. Our results reveal a large deviation of the Ni 3d occupation from the formal Ni2.5+ valency, highlighting the importance of the charge-transfer from oxygen ligands. We find that the dominant d8 configuration is accompanied by nearly equal contributions from d7 and d9 states, exhibiting an unusual valence state among Ni-based oxides. Finally, we discuss the Ni dx2-y2 and dz2 orbital-dependent hybridization, correlation and local spin dynamics.

cond-mat.str-el↗

arXiv2024

Metal-insulator transition in CaCu$_3$Fe$_4$O$_{12}$

We study structurally-triggered metal-insulator transition in CaCu$_3$Fe$_4$O$_{12}$ by means of local density approximation (LDA) +$U$ and LDA+dynamical mean-field theory (DMFT). The ferrimagnetic insulating phase is essentially the same within both approaches. While LDA+$U$ describes the metal-insulator transition as a Peierls-like instability driven by Fermi surface nesting in the magnetically ordered phase, LDA+DMFT allows also the site-selective Mott transition without magnetic ordering as well as smooth crossover between the two pictures. We point out similarities and differences to rare-earth nickelates.

cond-mat.str-el↗

arXiv2024

Quantifying the U $5f$ covalence and degree of localization in U intermetallics

A procedure for quantifying the U $5f$ electrons' covalence and degree of localization in U intermetallic compounds is presented. To this end, bulk sensitive hard and soft x-ray photoelectron spectroscopy were utilized in combination with density-functional theory (DFT) plus dynamical mean-field theory (DMFT) calculations. The energy dependence of the photoionization cross-sections allows the disentanglement of the U\,$5f$ contribution to the valence band from the various other atomic subshells so that the computational parameters in the DFT\,+\,DMFT can be reliably determined. Applying this method to UGa$_2$ and UB$_2$ as model compounds from opposite ends of the (de)localization range, we have achieved excellent simulations of the valence band and core-level spectra. The width in the distribution of atomic U\,$5f$ configurations contributing to the ground state, as obtained from the calculations, quantifies the correlated nature and degree of localization of the U\,5$f$. The findings permit answering the longstanding question why different spectroscopic techniques give seemingly different numbers for the U 5$f$ valence in intermetallic U compounds.

cond-mat.str-el↗

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↗

arXiv2024

Mechanism of intermetallic charge transfer and bond disproportionation in BiNiO$_3$ and PbNiO$_3$ revealed by hard x-ray photoemission spectroscopy

Perovskites with Bi or Pb on the A-site host a number of interesting and yet to be understood phenomena such as negative thermal expansion in BiNiO$_3$. We employ hard x-ray photoemission spectroscopy of Ni 2$p$ core-level as well as valence band to probe the electronic structure of BiNiO$_3$ and PbNiO$_3$. The experimental results supported by theoretical calculations using dynamical mean-field theory reveal essentially identical electronic structure of the Ni-O subsystem typical of Ni$^{2+}$ charge-transfer insulators. The two materials are distinguished by filling of the Bi(Pb)-O antibonding states in the vicinity of the Fermi level, which is responsible for the Bi disproportionation in BiNiO$_3$ at ambient pressure and absence of similar behavior in PbNiO$_3$. The present experiments provide evidence for this conclusion by revealing the presence/absence of Bi/Pb $6s$ states at the top of the valence band in the two materials.

cond-mat.str-el↗

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

Valence skipping, internal doping and site-selective Mott transition in PbCoO$_3$ under pressure

We present a computational study of PbCoO$_3$ at ambient and elevated pressure. We employ the static and dynamic treatment of local correlation in form of density functional theory + $U$ (DFT+$U$) and + dynamical mean-field theory (DFT+DMFT). Our results capture the experimentally observed crystal structures and identify the unsaturated Pb $6s$ - O $2p$ bonds as the driving force beyond the complex physics of PbCoO$_3$. We provide a geometrical analysis of the structural distortions and we discuss their implications, in particular, the internal doping, which triggers transition between phases with and without local moments and a site selective Mott transition in the low-pressure phase.

cond-mat.str-el↗

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↗

arXiv2021

Damping of spinful excitons in LaCoO$_3$ by thermal fluctuations: Theory and experiment

We present Co $L_3$-edge resonant inelastic x-ray scattering (RIXS) of bulk LaCoO$_3$ across the thermally-induced spin-state crossover around 100~K. Owing to a high energy resolution of 25~meV, we observe unambiguously the dispersion of the intermediate-spin (IS) excitations in the low temperature regime. Approaching the intermediate temperature regime, the IS excitations are damped and the bandwidth is reduced. The observed behavior can be well described by a model of mobile IS excitons with strong attractive interaction, which we solve using dynamical mean-field theory for hard-core bosons. Our results provide a detailed mechanism of how high-spin (HS) and IS excitations interact to establish the physical properties of cobaltite perovskites.

cond-mat.str-el↗

arXiv2020

Revealing the site selective local electronic structure of Co$_{3}$O$_{4}$ using $2p3d$ resonant inelastic X-ray scattering

We investigate mixed-valence oxide Co$_3$O$_4$ using Co $2p3d$ resonant inelastic X-ray scattering (RIXS). By setting resonant edges at Co$^{2+}$ and Co$^{3+}$ ions, the $dd$ excitations on the two Co sites are probed selectively, providing detailed information on the local electronic structure of Co$_3$O$_4$. The $2p3d$ RIXS result reveals the $^4$T$_{2}$ excited state of tetrahedral Co$^{2+}$ site at 0.5 eV beyond the discriminative power of optical absorption spectroscopies. Additionally, the $^3$T$_{2g}$ excited stated at 1.3 eV is uniquely identified for the octahedral Co$^{3+}$ site. Guided by cluster multiplet simulations, the ground-state character of the Co$^{2+}$ and Co$^{3+}$ site is determined to be high-spin $^4$A$_{2}$(T$_d$) and low-spin $^1$A$_{1g}$(O$_h$), respectively. This indicates that only the Co$^{2+}$ site is magnetically active site at low-temperatures in Co$_3$O$_4$. The ligand-to-metal charge transfer analysis suggests a formation of a strong covalent bonding between Co and O ions at the Co$^{3+}$ site, while Co$^{2+}$ is rather ionic.

cond-mat.mtrl-sci↗

arXiv2020

X-ray spectroscopy of rare-earth nickelate LuNiO$_3$: LDA+DMFT study

We present a computational study of resonant inelastic x-ray scattering (RIXS) and x-ray absorption in a representative rare-earth nickelate LuNiO$_3$. We study the changes in the spectra across the metal-insulator/site-disproportionation transition. In particular, we address the question of site-selectivity of the two methods in the disproportionated insulating phase and the signature of metal-insulator transition in the fluorescence-like feature of the RIXS spectra. To this end we use the local density approximation + dynamical mean-field theory (LDA+DMFT) approach combined with configuration integration method to compute the core-level spectra.

cond-mat.str-el↗

arXiv2019

LDA+DMFT approach to resonant inelastic x-ray scattering in correlated materials

We present a computational study of $L$-edge resonant inelastic x-ray scattering (RIXS) in correlated 3$d$ transition-metal oxides using an $ab$ $initio$ method based on local density approximation + dynamical mean-field theory (DMFT). The present method, building on Anderson impurity model with an optimized continuum bath within DMFT, is an extension of the cluster model to include unbound electron-hole pair excitations as well as material-specific charge-transfer excitations with less empirical parameters. We find a good agreement with available experimental data. The relationship between correlated bands and fluorescence-like feature in the RIXS spectra is discussed.

cond-mat.str-el↗