K. Morikawa

Electronic structure and electron-phonon interaction in transition metal oxides with d0 configuration and lightly doped compounds

Abstract While photoemission spectroscopy has been used as a powerful technique to study the electronic structure of solids, it also reflects the dynamical as well as the static properties of the lattice through electronphonon interaction. In this paper, we present the results of photoemission studies on insulating and lightly carrier-doped oxides and discuss their relevance to the lattice properties of the solids. First, the electronic structure of perovskite-type Ti oxides as well as that of V oxides with empty metal 3 d bands is deduced and discussed in relation to recent microscopic theories of ferroelectricity. Next, we point out that some spectral features of these oxides, when lightly doped with electrons, indicate a sign of strong coupling between the doped electrons and lattice distortion and hence the polaronic nature of the charged carriers.

Systematic change of spectral function observed by controlling electron correlation in Ca1-xSrxVO3 with fixed 3d1 configuration.

Abstract By using ultra-violet photoemission spectroscopy (UPS) and soft x-ray absorption spectroscopy (XAS), we have observed a systematic change of the spectral function around the Fermi level in the perovskite-type conductive 3 d 1 system, Ca 1−x Sr x VO 3 . When we decrease the Sr concentration (x) from 1 to 0 in Ca 1−x Sr x VO 3 , UPS and XAS intensities near the Fermi level decrease, and the reduced spectral weight is transferred to high energy part about 2 eV below and above Fermi level. The high energy features are thought to correspond to the remnants of lower and upper Hubbard bands, and the spectral weight within ∼1eV of Fermi level is assigned to the quasiparticle excitations or renormalized 3 d -band. That is, the observed systematic spectral change indicates the opening of Mott-Hubbard gap.

Magnetic and optical properties of CaVO3−y

Abstract The physical properties of CaVO 3−y change from metallic to insulating with increasing oxygen concentration for slightly off-stoichiometric samples. A detailed study of magnetic properties for the off stoichiometric chemical composition has been investigated for Ca x VO 3−y (x=0.97, 1.00 and 1.03) with various oxidation treatments. The best residual resistance ratio (RRR) is obtained for the sample with stoichiometric Ca and V concentration. Ca 0.97 VO 3−y specially shows growing antiferromagnetic-like anomaly at 110K and low temperature enhancement with increasing heat treatment temperature. In addition, optical reflectivity spectra have been measured for these metallic and insulating samples.

Metal-Insulator Transition in Nearly Stoichiometric CaVO3

A metal-to-insulator (MI) transition in perovskite-type vanadium oxide CaVOy in the vicinity of oxygen stoichiometry is reported. In spite of good metallic conductivity for Stoichiometric CaVOy (y=3.00), very slight oxidation (ycong3.05) causes the system to be insulating but does not induce any significant structural changes. This type of MI transition is contrary to that reported on another d1 system LaTiOy, which is a Mott-Hubbard insulator for y=3.00 but becomes metallic by oxidation. We suggest that in CaVOy, extra oxygens primarily induce additional scattering which causes the metallic CaVO3 to be insulating.

Systematic change of spectral functions observed by controlling the electron correlation in Ca1−xSrxVO3

The studies of ultra-violet photoemission spectroscopy and soft X-ray absorption spectroscopy of a perovskite-type 3d 1 Mott-Hubbard system Ca 1−x Sr x VO 3 , which is metallic in the whole composition range (0 < x < 1), are reported. As we control electron-electron interaction of the system by changing the Sr concentration (x), we have observed a systematic change of the spectra, which clearly manifests how the single-particle density of states around Fermi level (E F ) behaves near a Mott transition

Photoemission and inverse-photoemission spectra of quasi-two-dimensional Mo4O11

Electronic structures of Magneli-phase η- and γ-Mo4O11 have been studied by means of ultraviolet photoemission and inverse-photoemission spectroscopies. Resonant photoemission spectra have been also measured in the photon energy range from 32 eV to 90 eV The energy positions of Mo 4d states can be qualitatively explained in terms of a simple band structure model.

Shubnikov-de Haas effect in CaVO3−y

Abstract We have observed magnetoresistance oscillations in CaVO 3−y single crystals below 4.2 K and in fields above 10 Tesla. The oscillation frequency is about 300 Tesla. This frequency corresponds to a surface cross section of 3×10 14 cm -2 in the reciprocal lattice space for cubic CaVO 3 . This is 1% of the cross section of the first Brillouin zone.

PHOTOEMISSION AND INVERSE-PHOTOEMISSION STUDY OF FERROMAGNETIC VALENCE FLUCTUATING SYSTEM CEFE2

Abstract Various electron-spectroscopic results including core-level X-ray photoemission, X-ray absorption, Ce 3d-4f and 4d-4f resonant photoemission, and inverse-photoemission spectroscopy on the valence fluctuating ferromagnet CeFe 2 are presented and analyzed with model calculations using the Anderson-impurity Hamiltonian. Surface effects on the spectra are taken into account in the analysis. The valence-band spectra are also compared with the band-structure density of states calculated using the local-spin-density approximation. The validity of describing the Ce 4f states as a localized impurity level and in the 4f band picture is discussed.

Metal-to-insulator transitions in CaVOy.

Abstract A metal-to-insulator transition in CaVO y by varying the oxygen stoichiometry is reported. Stoichiometric CaVO y (y=3.00) shows good metallic conductivity but slight oxidation (y≅3.05) causes the system to be insulating without any significant structural changes. This behavior is contrary to that seen in another d 1 system LaTiO y , which is a Mott insulator for y=3.00 but becomes metallic by oxidation. We suggest that, in CaVO y , oxygen defects primarily induce additional scattering which causes the metallic CaVO 3 to be insulating rather than add extra carriers.

Spectral weight transfer, mass renormalization and chemical potential shift in Mott-Hubbard systems

Abstract We present the results of systematic photoemission and inverse-photoemission studies of strongly correlated metallic systems which have one-, two- and three-dimensional crystal structures. The spectral weight distribution exhibits characteristic evolution as a function of band width and band filling as well as of mass anisotropy. Mass enhancement is deduced from the weight transfer between the coherent and incoherent parts of the spectral function. Chemical potential shift as a function of band filling exhibits an unusual suppression near Mott transition.