Showing papers by "Shik Shin published in 1997"

Electron Energy-loss Spectroscopy Study of the Metal-insulator Transition in VO2.

Abstract: Electron energy-loss spectra of VO2 have been measured from perfect single crystalline areas of 100–180 nm diameter in the metallic and insulating phases. A sharp peak has been observed at 1.2 eV in the valence electron excitation spectra of the metallic phase but not in those of the insulating phase. We assign the peak to an interband transition or d-d transition by inspecting the dielectric function derived from the present loss function with the help of an energy band diagram already given, although an excitation at about 1 eV already observed by optical measurement was assigned to the excitation of a free-carrier plasmon. The peak due to the O 1s → V 3d(t2g) transition in the O 1s excitation spectra decreased in intensity but increased in the full width at half-maximum (FWHM) at the transition from the metallic phase to the insulating phase. These changes are considered to have been due to the fact that the d\varparallel band splits into the upper unoccupied and lower occupied d\varparallel bands and the upper d\varparallel band rises above the π* band in the insulating phase, but that the d\varparallel band is unsplit and located within the π* band in the metallic phase.

Resonant soft-x-ray emission study in relation to the band structure of cBN

Abstract: The resonant soft-x-ray emission (SXE) and its total photon yield (TPY) spectra were measured at the B 1s and N 1s edges of cubic boron nitride (cBN) using undulator radiation. The band-gap energy was found to be about 6.2 eV, which is in good agreement with other experiments. It was found that the emission from the high symmetry point in the SXE spectrum is enhanced when the same high symmetry point in the TPY spectrum is excited. The line shapes in both the SXE and N 1s TPY spectra were consistent with the calculated partial density of states, though the total bandwidth was not well reproduced. On the other hand, the exciton effect was found to be strong in the B 1s TPY spectra.

Observation of the surface 4f state of CePd 7 sby means of the resonant-inverse-photoemission study at the Ce 4d absorption edge

Abstract: The resonant inverse photoemission study (RIPES) of ${\mathrm{CePd}}_{7}$ , has been carried out at the Ce 4d\ensuremath{\rightarrow}4f absorption edge. The strong resonant enhancement of the 4f cross section enables us to distinguish two 4f components in the empty electronic state near the Fermi level. The incidence-angle dependence of the RIPES indicates a clear difference between ground-state configurations at the bulk and surface. It is found that the former shows a strongly hybridized 4f state, while the latter shows a localized 4f character. The angle dependence of the RIPES of \ensuremath{\alpha}-Ce metal has been also carried out and similar results as those of ${\mathrm{CePd}}_{7}$ were obtained. The RIPES at the Ce 4d\ensuremath{\rightarrow}4f edge is found to be a powerful method to investigate the surface 4f state.

Photoemission study of chain- and layer-type TlS crystals

Abstract: Thallium monosulphide is a mixed-valence compound containing monovalent and trivalent Tl ions. The compound crystallizes in two different structures, the tetragonal TlSe type and the monoclinic type. Photoemission studies were carried out, using a synchrotron photon source, on TlS crystals with both the TlSe- and -type structures. Comparison of the spectra shows that the line shape of the Tl 5d core level is more asymmetric in the TlSe type than the type. The asymmetry reflects the density of states near the Fermi level of the crystals. It was also found that as the incident photon energy increases the intensities of the valence band peaks increase relative to those of the Tl 5d core levels. Comparing the intensity ratios with the calculated photoionization cross section ratios, the characters of the valence bands were discussed.

Ti 2p-, Ti 3p- and O 1s-resonant photoemission studies of Ti2O3

Abstract: The Ti 2 p - and Ti 3 p -resonant photoemission studies of Ti 2 O 3 were carried out as well as O 1 s -resonant photoemission. The Ti 3 d band has a broad band width of about 3 eV, but the strong resonance enhancement was observed at Ti 3 d band just below the Fermi level on the Ti 2 p resonance. The enhancement of the doublet Ti 3 p main line was observed mainly at the higher binding energy. The resonant enhancements of the satellite structures in the Ti 3 d and Ti 3 p band regions were also found. The resonance spectra were analyzed by the cluster model calculation with large hybridization energy, where the satellite was elucidated by the charge transfer satellite.

Application of resonant X-ray emissions for molecular/electronic structure analysis of boron nitrides

Abstract: –B1s-1 transitions were observed in w-BN composed of four-fold boron atoms and in h-BN composed of three-fold boron atoms, when the photon energy of the incident undulator beams coincided with the B1s–B2pπ* absorption energy. However, no resonance was observed in c-BN composed of four-fold boron atoms. The resonant X-ray emission reflects the electronic structure of unoccupied molecular orbitals which strongly depend on the conformnations of the boron atoms. These findings confirm that resonant X-ray emissions can be useful indices for molecular and electronic structure analysis of boron nitrides.

Relaxation process of the excited state and selection rule in the soft X-ray emission of Si and cBN.

Abstract: The soft X-ray emission spectra of Si and cubic boron nitride (cBN) are compared in the Si2p- and B1s-core exciton regions because inversion symmetry is lost and the band gap is extremely large for cBN although both crystal structures are very similar. The background in the soft X-ray emission spectrum of Si is much larger than that for cBN but becomes extremely weak as the excitation energy becomes low and the Raman process becomes dominant compared with the fluorescence process. The background is found to be mainly formed by electron excitation from the valence band to the conduction band. The Raman spectrum is similar to the absorption spectrum in cBN, while those of Si are quite different from each other. It is shown that the centrosymmetry is important in the selection rule of Raman scattering. In resonant Raman scattering the selection rule partly breaks down due to the lattice relaxation process that emits an ungerade phonon in the intermediate state.

Polarization analysis of x-ray absorption at the O 1s absorption edge in PrBa2Cu3O7: Unusual electronic structure in CuO2 plane

Abstract: The polarization dependence of x-ray absorption at the oxygen 1 s absorption edge in untwinned LuBa 2 Cu 3 O 7 and PrBa 2 Cu 3 O 7 single crystal specimens was measured. The hole states of oxygen in the CuO 2 plane in PrBa 2 Cu 3 O 7 locate at higher energy position than those of the oxygen in the CuO 2 in superconducting LuBa 2 Cu 3 O 7 , although the hole states of chain sites remain at the same energy position in both Pr123 and Lu123 samples. Although they are mainly of pσ character rather than of pπ and were attributed as an upper Hubbard band, a possibility was pointed out that a trace of pπ character hole states in the CuO 2 plane exists in Pr123. The formation of Zhang-Rice singlet is suppressed by pπ holes in the CuO 2 plane on account of the strong Coulomb interaction on an oxygen site.