arXiv subjects

Takashi Mizokawa

Publications and source records attributed to Takashi Mizokawa.

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↗

arXiv2025

Yb 4f-Ta 5d Hybridization and Valence Evolution in Tetragonal Tungsten Bronze Ba(3-x)YbxTa5O15

Here we investigate the electronic structure of the tetragonal tungsten bronze Ba$_{3-x}$Yb$_x$Ta$_{5}$O$_{15}$ by making use of hard x-ray photoemission spectroscopy. The core level spectroscopy shows that the substitution with Yb ions in the series first occurs on the compact S1 site. For $x\leq1$, Yb is found to be dominantly Yb$^{2+}$ with a small mixing of Yb$^{3+}$, while for $x>1$, a significant increase of Yb$^{3+}$ is found, suggesting not only that site S2 favours Yb$^{3+}$, but also that their presence affects also the valency of the ions in site S1. The valence band spectra shows a relatively deep Yb$^{2+}$ doublet, but at the same time indications of a Ta~$5d$-Yb~$4f$ interaction are found, suggesting the presence of Yb~$4f$ carriers at the Fermi level through this hybridization. Our results thus point towards an exotic form of $d$-$f$ electronic interplay that together with the structural degrees of freedom can result in the unusual trends observed in the physical properties of Ba$_{3-x}$Yb$_x$Ta$_{5}$O$_{15}$.

cond-mat.mtrl-sci↗

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

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↗

arXiv2022

Direct observation of multiple conduction-band minima in high-performance thermoelectric SnSe

We report time- and angle-resolved photoemission spectroscopy on SnSe which currently attracts great interest due to its extremely high thermoelectric performance. Laser-assisted photoemission signals are observed within $\pm$20 fs of the pump pulse arrival. Around 30-50 fs after the photoexcitation, the conduction band minima are not populated by the photoexcited electrons while the valence bands are considerably broadened. In going from 90 fs to 550 fs after the photoexcitation, the photoexcited carriers are decayed into the multiple conduction band minima. The observed conduction bands are consistent with the band structure calculations. The multiple conduction minima suggest possibility of high and anisotropic thermoelectric performance of n-type SnSe single crystal if it is realized.

cond-mat.mtrl-sci↗

arXiv2021

Hybridization-gap Formation and Superconductivity in the Pressure-induced Semimetallic Phase of the Excitonic Insulator Ta$_2$NiSe$_5$

The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps = 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of 0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Ni-valence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc = 8 GPa. Superconductivity with a maximum Tsc = 1.2 K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.

cond-mat.str-el↗

arXiv2021

Observation of Kondo hybridization with an orbital-selective Mott phase in 4d Ca2-xSrxRuO4

The heavy fermion state with Kondo-hybridization (KH), usually manifested in f-electron systems with lanthanide or actinide elements, was recently discovered in several 3d transition metal compounds without f-electrons. However, KH has not yet been observed in 4d/5d transition metal compounds, since more extended 4d/5d orbitals do not usually form flat bands that supply localized electrons appropriate for Kondo pairing. Here, we report a doping- and temperature-dependent angle-resolved photoemission study on 4d Ca2-xSrxRuO4, which shows the signature of KH. We observed a spectral weight transfer in the γ-band, reminiscent of an orbital-selective Mott phase (OSMP). The Mott localized γ-band induces KH with the itinerant \b{eta}-band, resulting in spectral weight suppression around the Fermi level. Our work is the first to demonstrate the evolution of the OSMP with possible KH among 4d electrons, and thereby expands the material boundary of Kondo physics to 4d multi-orbital systems.

cond-mat.str-el↗

arXiv2020

Detecting electron-phonon couplings during photo-induced phase transition

Photo-induced phase transitions have been intensively studied owing to the ability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it has currently been a central issue how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photo-induced phase transitions. Here, we develop a new analysis method, frequency-domain angle-resolved photoemission spectroscopy, to gain precise insight into electron-phonon couplings during photo-induced insulator-to-metal transitions for Ta$_2$NiSe$_5$. We demonstrate that multiple coherent phonons generated by displacive excitations show band-selective coupling to the electrons. Furthermore, we find that the lattice modulation corresponding to the 2 THz phonon mode, where Ta lattice is sheared along the a-axis, is the most relevant for the photo-induced semimetallic state.

cond-mat.str-el↗

arXiv2020

V 3$d$ charge and orbital states in V$_2$OPO$_4$ probed by x-ray absorption spectroscopy

V 3$d$ charge and orbital states in V$_2$OPO$_4$ have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of V$_2$OPO$_4$ is very unique in that the charge transfer between V$^{2+}$ and V$^{3+}$ in face sharing VO$_6$ chains provides negative thermal expansion as reported by Pachoud {\it et al.} [J. Am. Chem. Soc. {\bf 140}, 636 (2018).] The near edge region of O 1$s$ XAS exhibits the three features which can be assigned to transitions to O 2$p$ mixed into the unoccupied V 3$d$ $t_{2g}$ and $e_{g}$ orbitals of V$^{2+}$ and V$^{3+}$. The V 2$p$ XAS line shape can be reproduced by multiplet calculations for a mixed valence state with V$^{2+}$ and V$^{3+}$. The polarization dependence of the O 1$s$ and V 2$p$ XAS spectra indicates V 3$d$ orbital order in which $xy$ and $yz$ (or $zx$) orbitals are occupied at the V$^{3+}$ site in the face sharing chains. The occupied $xy$ orbital is essential for the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites along the chains while the occupied $yz$ (or $zx$) orbital provides the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites between the chains.

cond-mat.str-el↗

arXiv2020

Spin-orbital liquid in Ba$_3$CuSb$_2$O$_9$ stabilized by oxygen holes

Both the Jahn-Teller distortion of Cu$^{2+}$O$_6$ octahedra and magnetic ordering are absent in hexagonal Ba$_3$CuSb$_2$O$_9$ suggesting a Cu 3$d$ spin-orbital liquid state. Here, by means of resonant x-ray scattering and absorption experiment, we show that oxygen 2$p$ holes play crucial role in stabilizing this spin-orbital liquid state. These oxygen holes appear due to the "reaction" Sb$^{5+}$$\rightarrow$Sb$^{3+}$ $+$ two oxygen holes, with these holes being able to attach to Cu ions. The hexagonal phase with oxygen 2$p$ holes exhibits also a novel charge-orbital dynamics which is absent in the orthorhombic phase of Ba$_3$CuSb$_2$O$_9$ with Jahn-Teller distortion and Cu 3$d$ orbital order. The present work opens up a new avenue towards spin-charge-orbital entangled liquid state in transition-metal oxides with small or negative charge transfer energy.

cond-mat.str-el↗

arXiv2018

Photo-induced semimetallic states realised in electron-hole coupled insulators

Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally-friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realization of photo-induced insulator-to-metal transitions in Ta2Ni(Se1-xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic-insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron-hole coupled systems.

cond-mat.mtrl-sci↗

arXiv2018

Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$

Femtosecond time-resolved mid-infrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta$_2$NiSe$_5$ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C = 0.30~\pm~0.08$~mJ/cm$^2$ due to optical absorption saturation. This limits the optical excitation density in Ta$_2$NiSe$_5$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta$_2$NiSe$_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.

cond-mat.str-el↗

arXiv2017

Orbital order and fluctuations in the two-leg ladder materials BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$

The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the Néel temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe$_2$S$_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe$_2$Se$_3$.

cond-mat.supr-con↗

arXiv2017

Ultrafast Electronic Band Gap Control in an Excitonic Insulator

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta$_2$NiSe$_5$ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of $F_{C} = 0.2$ mJ cm$^{-2}$, the band gap $narrows$ transiently, while it is $enhanced$ above $F_{C}$. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta$_2$NiSe$_5$, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta$_2$NiSe$_5$ with light on the femtosecond time scale.

cond-mat.str-el↗

arXiv2014

Synchrotron X-ray Diffraction Study of Structural Phase Transition in Ca10(Ir4As8)(Fe2-xIrxAs2)

We report a structural transition found in Ca10(Ir4As8)(Fe2-xIrxAs2)5, which exhibits superconductivity at 16 K. The c-axis parameter is doubled below a structural transition temperature of approximately 100 K, while the tetragonal symmetry with space group P4/n (No.85) is unchanged at all temperatures measured. Our synchrotron x-ray diffraction study clearly shows iridium ions at a non-coplanar position shift along the z-direction at the structural phase transition. We discuss that the iridium displacements affect superconductivity in Fe2As2 layers.

cond-mat.supr-con↗

arXiv2014

Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeO$_{1-x}$F$_{x}$BiS$_{2}$ revealed by Ce $L_3$-edge x-ray absorption spectroscopy

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on CeO$_{1-x}$F$_x$BiS$_2$, in which the superconductivity of the BiS$_2$ layer and the ferromagnetism of the CeO$_{1-x}$F$_x$ layer are induced by the F-doping, in order to investigate the impact of the F-doping on the local electronic and lattice structures. The Ce $L_3$-edge XAS spectrum of CeOBiS$_2$ exhibits coexistence of $4f^1$ (Ce$^{3+}$) and $4f^0$ (Ce$^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4f^0$ state decreases with the F-doping and completely disappears for $x>0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling channel by the F-doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS$_2$ layer and the ferromagnetic CeO$_{1-x}$F$_x$ layer.

cond-mat.str-el↗

arXiv2013

Superconductivity in Ca10(Ir4As8)(Fe2As2)5 with Square-Planar Coordination of Iridium

We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca10(Ir4As8)(Fe2As2)5. This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first- principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir4As8 layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe2As2, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.

cond-mat.supr-con↗