Takashi Mizokawa

Bio: Takashi Mizokawa is an academic researcher from Waseda University. The author has contributed to research in topics: Angle-resolved photoemission spectroscopy & Photoemission spectroscopy. The author has an hindex of 57, co-authored 400 publications receiving 11697 citations. Previous affiliations of Takashi Mizokawa include Solid State Physics Laboratory & University of Tokyo.

Electronic structure of La1-xSrxMnO3 studied by photoemission and x-ray-absorption spectroscopy

Abstract: The electronic structure of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ has been studied by photoemission and O 1s x-ray-absorption spectroscopy. Spectra of the Mn 2p core levels and the valence bands for ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{SrMnO}}_{3}$ have been analyzed using a configuration-interaction cluster model. The ground state of ${\mathrm{LaMnO}}_{3}$ is found to be mixed ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L states and that of ${\mathrm{SrMnO}}_{3}$ to be heavily mixed ${\mathit{d}}^{3}$ and ${\mathit{d}}^{4}$L states, reflecting their strong covalency. The character of the band gap of ${\mathrm{LaMnO}}_{3}$ is of the p-to-d charge-transfer type while that of ${\mathrm{SrMnO}}_{3}$ has considerable p-p character as well as p-d character. Holes doped into ${\mathrm{LaMnO}}_{3}$ mainly of oxygen p character are coupled antiferromagnetically with the ${\mathit{d}}^{4}$ local moments of the ${\mathrm{Mn}}^{3+}$ ions and become itinerant, thus aligning the Mn moments ferromagnetically. The changes in the electronic structure with carrier doping are not of the rigid band type: By La substitution for ${\mathrm{SrMnO}}_{3}$, the so-called in-gap spectral weight (of ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ symmetry) appears with its peak located 1--2 eV below the Fermi level and grows in intensity with increasing La concentration, while the spectral intensity of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states above the Fermi level decreases, showing a transfer of spectral weight from the unoccupied to the occupied ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states with electron doping. Meanwhile, the intensity at the Fermi level remains low even in the metallic phase (0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.6). The energy shifts of core-level peaks and valence-band features with x suggest a downward shift of the Fermi level with hole doping, but the shift is found to be very small in the metallic phase. The importance of the orbital degeneracy of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ band and possible orbital fluctuations in the ferromagnetic phase are pointed out.

Electronic structure of 3d-transition-metal compounds by analysis of the 2p core-level photoemission spectra

Abstract: The electronic structures of a wide range of transition-metal compounds, including Cu, Ni, Co, Fe, and Mn oxides and sulfides, with metal valences ranging from 2+ to 4+, have been investigated by a cluster-type configuration-interaction analysis of the core-level 2p x-ray photoemission spectra. We show that by including the d-d exchange interaction (retaining only diagonal terms) and an anisotropic metal-ligand hybridization in the model, these spectra can be well reproduced, and so can be used to deduce quantitatively values for the ligand-to-metal charge-transfer energy \ensuremath{\Delta}, the on-site d-d Coulomb repulsion energy U, and the metal-ligand transfer integrals T. Systematics for \ensuremath{\Delta} and U are generally consistent with those found from previous valence-band studies and follow expected chemical trends. By using values of \ensuremath{\Delta} and U found from this model, we show that most of the transition-metal compounds studied in this work can be classified in the charge-transfer regime of the Zaanen-Sawatzky-Allen diagram. A few exceptions to these systematics have been found. Small U values found for pyrite-type ${\mathrm{CoS}}_{2}$ and ${\mathrm{FeS}}_{2}$ and large T values for Mn perovskite oxides, as well as the neglect of other mechanisms such as exciton satellites, may indicate a limitation of the local-cluster model.

Electronic structure of SrFe 4 + O 3 and related Fe perovskite oxides

Abstract: The electronic structure of ${\mathrm{SrFeO}}_{3}$ has been investigated by x-ray photoemission and ultraviolet photoemission spectroscopy. We find that the ground state consists of heavily mixed ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L states, reflecting the large covalency. The Fe 3s core-level splitting, together with a subsequent cluster-model configuration-interaction calculation, shows that a high-spin ${\mathit{t}}_{2\mathit{g}}^{3}$${\mathit{e}}_{\mathit{g}}$ ground state is stabilized. The Fe 2p core levels have been interpreted using a p-d charge-transfer cluster-model calculation. The charge-transfer energy ${\mathrm{\ensuremath{\Delta}}}_{\mathit{e}\mathit{f}\mathit{f}}$, defined with respect to the lowest multiplet levels of the ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L configurations, is negative, which means that a large amount of charge is transferred via Fe-O bonds from the O 2p bands to the metal d orbitals and that the ground state is dominated by the ${\mathit{d}}^{5}$L configuration. This reduces the charge on the ionic sites, leading to only a small chemical shift between the ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{4+}$ compounds. The band-gap energy ${\mathit{E}}_{\mathrm{gap}}$, calculated using the cluster model for the high-spin ${\mathit{d}}^{4}$ configuration, is small due to the small charge-transfer energy and the large exchange stabilization of the adjacent ${\mathit{d}}^{5}$ configuration. This small value for ${\mathit{E}}_{\mathrm{gap}}$ leads to the presence of itinerant d electrons in the periodic lattice, causing metallic conductivity in ${\mathrm{SrFeO}}_{3}$ and charge disproportionation in ${\mathrm{CaFeO}}_{3}$.

Electronic structure and orbital ordering in perovskite-type 3d transition-metal oxides studied by Hartree-Fock band-structure calculations.

Abstract: We have studied transition-metal 3d-oxygen 2p lattice models, where full degeneracy of transition-metal 3d and oxygen 2p orbitals and on-site Coulomb and exchange interactions between 3d electrons are taken into account, by means of a spin- and orbital-unrestricted Hartree-Fock (HF) approximation. The electronic-structure parameters deduced from the cluster-model analyses of the photoemission spectra are used as input. We have applied this method to perovskite-type 3d transition-metal oxides, which exhibit various electrical and magnetic properties. It is shown that the HF results can explain the ground-state properties of insulating oxides. The relationship between spin- and orbital-ordered solutions and the Jahn-Teller-type and ${\mathrm{GdFeO}}_{3}$-type distortions in R${\mathrm{TiO}}_{3}$, R${\mathrm{VO}}_{3}$, R${\mathrm{MnO}}_{3}$, and R${\mathrm{NiO}}_{3}$ (R is a rare earth atom or Y) is extensively studied. Single-particle excitation spectra calculated using Koopmans' theorem give us an approximate but relevant picture on the electronic structure of the perovskite-type 3d transition-metal oxides. As a drawback, the HF calculations tend to overestimate the magnitude of the band gap compared with the experimental results and to predict some paramagnetic metals as antiferromagnetic insulators. \textcopyright{} 1996 The American Physical Society.

Electronic structure of early 3d-transition-metal oxides by analysis of the 2p core-level photoemission spectra.

Abstract: The electronic structures of a wide range of early transition-metal (TM) compounds, including Ti and V oxides with metal valences ranging from 2+ to 5+ and formal d-electron numbers ranging from 0 to 2, have been investigated by a configuration-interaction cluster model analysis of the core-level metal 2p x-ray photoemission spectra (XPS). Inelastic energy-loss backgrounds calculated from experimentally measured electron-energy-loss spectra (EELS) were subtracted from the XPS spectra to remove extrinsic loss features. Parameter values deduced for the charge-transfer energy Delta and the d-d Coulomb repulsion energy U are shown to continue the systematic trends established previously for the late TM compounds, giving support to a charge-transfer mechanism for the satellite structures. The early TM compounds are characterized by a large metal d-ligand p hybridization energy, resulting in strong covalency in these compounds. Values for Delta and U suggest that many early TM compounds should be reclassified as intermediate between the charge-transfer regime and the Mott-Hubbard regime.

Pattern Recognition and Machine Learning

Abstract: Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors

Abstract: Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic interactions that lead to a Curie temperature (T(C)) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T(C) of Ga(1-)(x)Mn(x)As and that of its II-VI counterpart Zn(1-)(x)Mn(x)Te and is used to predict materials with T(C) exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.

Dynamical mean-field theory of strongly correlated fermion systems and the limit of infinite dimensions

Abstract: We review the dynamical mean-field theory of strongly correlated electron systems which is based on a mapping of lattice models onto quantum impurity models subject to a self-consistency condition. This mapping is exact for models of correlated electrons in the limit of large lattice coordination (or infinite spatial dimensions). It extends the standard mean-field construction from classical statistical mechanics to quantum problems. We discuss the physical ideas underlying this theory and its mathematical derivation. Various analytic and numerical techniques that have been developed recently in order to analyze and solve the dynamical mean-field equations are reviewed and compared to each other. The method can be used for the determination of phase diagrams (by comparing the stability of various types of long-range order), and the calculation of thermodynamic properties, one-particle Green's functions, and response functions. We review in detail the recent progress in understanding the Hubbard model and the Mott metal-insulator transition within this approach, including some comparison to experiments on three-dimensional transition-metal oxides. We present an overview of the rapidly developing field of applications of this method to other systems. The present limitations of the approach, and possible extensions of the formalism are finally discussed. Computer programs for the numerical implementation of this method are also provided with this article.