H. Matsuyama

Evidence from angle-resolved resonant photoemission for oxygen-2p nature of the Fermi-liquid states in Bi2CaSr2Cu2O8

It has been generally accepted that a strong on-site Coulomb repulsion of the Cu 3d electrons dominates the electronic structure of the high-transition-temperature (high-Tc) superconductors. The on-site Coulomb repulsion has been evaluated as 6–7 eV, comparable with the valence-band width1,2. This strong correlation is thought to cause the Cu 3d electrons to be localized as in a Mott insulator, and doped holes may be transferred to oxygen sites, as the charge transfer energy is small compared with the correlation energy. These doped holes yield a substantial density of states at the Fermi level, characteristic of metals. There has been great effort to find and characterize the electronic states at the Fermi level, because these states relate directly to the mechanism of the high-Tc superconductivity by providing Cooper pairs below Tc. Here we report the first direct evidence for the dominant oxygen-2p nature of the Fermi-liquid state in the high-Tc superconductor, obtained using the technique of angle-resolved resonant photoemission.

Impurity-state-like nature of Fermi-liquid states in Bi2Sr2CaCu2O8 observed by photoemission and x-ray absorption

Abstract Photoemission and X-ray absorption spectroscopy have been performed on Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8 in which the hole concentration is controlled by the ratio between Ca and Y atoms. It was found that the density of electronic states at the Fermi level gradually increases as the hole concentration with almost no rigid shift of the electronic structure with respect to the Fermi level. This suggests that the electronic states at the Fermi level (Fermi-liquid states) would not be simple one-electron states but be a kind of impurity states produced through the strong hybridization between doped O2p hole orbitals and empty Cu3d orbitals.

Angle-resolved photoemission study of nonsuperconductive Bi2Sr2Ca0.4Y0.6Cu2O8

Abstract A comparative angle-resolved photoemission measurement has been performed on nonsuperconductive Bi 2 Sr 2 Ca 0.4 Y 0.6 Cu 2 O 8 and superconductive Bi 2 Sr 2 CaCu 2 O 8 to study the nature and origin of the electronic states near the Fermi level. It was found that hole-doping does not cause a rigid shift of the density of states relative to the Fermi level, but creates new electronic states in the vicinity of the Fermi level.

Nature and origin of the Fermi-liquid states in Bi2Sr2CaCu2O8 studied by photoemission and inverse photoemission

The nature and origin of the Fermi-liquid states in high-Tc superconductor have been studied by angle-resolved photoemission and inverse photoemission spectroscopies on Bi2Sr2Ca1−xYxCu2O8 (x=0.0 to 0.5). The result shows that impurity-like states or mid-gap states are created in the charge-transfer gap by hole-doping.

Comment on “Is Nd2−xCexCuO4 an electron-superconductor?”

A critical comment was made to the electron energy-loss spectroscopy [Z. Phys. B-Condensed Matter75, 421 (1989)] which questionedn-type superconductivity in Nd2−xCexCuO4. Comparative oxygen-K absorption and oxygen 1s photoemission spectroscopies for Nd2−xCexCuO4 and Bi2Sr2CaCu2O8 have shown that these two superconductors are classified into two different categories.

Angle-Resolved Photoemission and Inverse Photoemission Study of Bi 2 Sr 2 Ca 1-x Y x Cu 2 O 8 (x=0.0 and 0.6)

Angle-resolved photoemission (ARPES) and inverse photoemission (ARIPES) measurements were performed on Bi2Sr2Ca1-xYxCu2O8 (x=0.0 and 0.6). The ARIPES result suggests that there are two bands crossing the Fermi level along the ΓX direction. Comparative ARPES measurement on superconductor (x=0.0) and non-superconductor (x=0.6) indicates that hole-doping creates new electronic states in the vicinity of the Fermi level.

Oxygen K-Absorption Spectroscopy of La2−xSrxCuO4 (x=0.0−0.5), YBa2Cu3O7−δ (δ=0.0−0.7), and Nd2−y,CeyCuO4 (y=0.0−0.2)

Oxygen K-absorption spectra were measured for La-, Y-, and Nd-system high-Tc superconductors. The spectrum of insulator for La- and Y-systems has a small structure about 1 eV above the absorption edge, being assigned to the Cu3d10 state. Hole-doping produces a new structure just at the absorption edge, whose intensity increases with hole concentration. On the other hand, the intensity of the Cu3d10 state gradually decreases. This indicates that the hole-doping brings about a transfer of weight of electronic states from the Cu3d10 states to the mid-gap states at the Fermi level. No discontinuous change was observed in the spectrum between the superconductor and the normal metal for La-system, suggesting that the electronic structure of the high-Tc superconductor is essentially the same as that of normal metal. Oxygen K-absorption spectrum of Nd-system exhibits almost no change against Ce doping, suggesting that the O2p orbital is not directly involved in the high-Tc mechanism for the electron-doped superconductor.

Photoemission study of single crystal BaPb1−xBixO3

We have measured photoemission spectra of single crystal BaPb1−xBixO3 at liquid nitrogen temperature. Clear Fermi edges have been observed in the valence band spectra for metallic compositions. The valence band peak dominated by O 2p states does not show a rigid band shift with x. The gross features in the experimental density of states (DOS) agree well with calculated DOS in the insulating phase rather than in the metallic phase.

Photoemission Study of Single Crystal Bi2Sr2CaCu2O8

Photoemission measurements with synchrotron radiation have been performed on single crystal Bi2Sr2CaCu208. Two energy bands with dispersion of 0.2–0.5 eV were observed in the vicinity of the Fermi level and one of them crosses the Fermi level midway between the center and boundary of the Brillouin zone, giving a clear evidence for existence of a Fermi surface. The Fermi-edge peak exhibits a pronounced enhancement at photon energy of the 0 2s core threshold, meaning a dominant 0 2p nature of the Fermi-edge states. These results indicate existence of the Fermi-liquid states with dominant 0 2p nature in the high-Tc superconductor. The superconductivity could be driven by formation of Cooper-pairs of the 0 2p holes in the Fermi-liquid states, probably through the spin or charge fluctuation.