Photoemission studies of (La~-"Sr")2Cu04and YBa2Cu3067 have revealed that their electronic structure is essentially that of a Mott insulator, i.e. , Cu 3d electrons are localized, and the density of states at the Fermi level is very low. This result combined with the magnetic properties suggest a fluctuation of the local moment due to intersite exchange, which may be consistent with a bipolaron state or a resonating valence-bond state.Recently discovered high-T, superconductivity in the La-Ba-Cu-0 system' and related Cu oxides has stimulated extensive research in these materials. So far, most theoretical studies have been based on the twodimensional character of the electronic energy bands giving a high density of states (DOS) near the Fermi level (EF).Density-functional band-structure calculations have yielded a strongly hybridized Cu3d-02p antibonding band at EF, which leads to a strong electron-phonon coupling for a breathing-type displacement of 0 atoms.On the other hand, Anderson has proposed the resonating-valence-bond or quantum-spin-liquid state, which may occur in frustrated Heisenberg spin systems, and also discussed their unusual magnetic and transport properties in the normal state. ' In the light of the above two extreme points of view, the knowledge of the electronic structure is a key step toward understanding not only the superconducting but also the normal-state properties. This Rapid Communication reports a photoemission study on (La~-"Sr") 2Cu04-s and YBa2CusOs 7. The results demonstrate that the Cu 3d electrons are essentially localized due to strong electron correlation at the Cu sites. We have studied sintered samples of (La~"Sr")2Cu04-~and YBa2Cu30s7 which were prepared as described in Refs. 2 and 4 and whose properties are listed in Table I. Room-temperature x-ray and ultraviolet photoemission (XPS and UPS) experiments were performed using a spectrometer fitted with a Mg Ka x-ray source (hv=1253. 6 eV) and a helium resonance lamp (h v =21.2 and 40.8 eV). The energy resolution was -0. 1 eV for UPS and -0.8 eV for XPS. The base pressure in the spectrometer was -1 x 10 ' Torr. The spectra have been corrected for the Mg Ea3 4 satellite and the analyzer transmission efficiency. Clean surfaces were obtained by in situ scraping with a diamond file. Scraping was repeated frequently so that no change in the spectra was observed during the measurements. Figure 1 shows XPS spectra in the Cu 2p core-level region, where one can see that each of the j = 2 and components exhibits a two-peak structure as in previous reports. " The higher and lower binding-energy (satellite and main) peaks correspond, respectively, to 2p3d9 and 2p3d' final-state configurations (2p denotes a Cu 2p core hole), so that the separation between the two peaks is approximately the intra-atomic Coulomb energy between the core hole and the 3d electron U,d. The satellite structure arises not only from the coexistence of the d (Cu +) and d ' (Cu '+ ) configurations in the ground state but also from a ligand-to-d charge transfer in the...
At a high oxygen pressure of 7 GPa and high temperature of 1380°C, YSr2Cu3O
y
was synthesized as a single phase. The crystal structure is a tetragonal “1-2-3” type, where a and c are 3.7949 Å and 11.4102 Å, respectively. This material is a superconductor with an onset temperature of 60 K.
We extend the spinorial geometry techniques developed for the solution of supergravity Killing spinor equations to the kappa symmetry condition for supersymmetric brane probe configurations in any supergravity background. In particular, we construct the linear systems associated with the kappa symmetry projector of M-and type II branes acting on any Killing spinor. As an example, we show that static supersymmetric M2-brane configurations which admit a Killing spinor representing the SU (5) orbit of Spin(10, 1) are generalized almost hermitian calibrations and the embedding map is pseudo-holomorphic. We also present a bound for the Euclidean action of M-and type II branes embedded in a supersymmetric background with non-vanishing fluxes. This leads to an extension of the definition of generalized calibrations which allows for the presence of non-trivial Born-Infeld type of fields in the brane actions.
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