1,354,523 research outputs found
Electron-spectroscopy study of correlation mechanisms in CuGeO3 single crystals
No full textX-ray (Al-K alpha) and resonant Cu 2p-->3d and Cu 3p-->3d photoemission valence-band spectra of high-quality CuGeO3 single crystals are reported and interpreted. In addition, an attempt is given to evaluate the charge transfer (Delta), the d-d Coulomb interaction (U-dd) energies, and the superexchange term (J) on the basis of L(2,3)M(4,5)M(4,5)-L(2,3)M(2,3)M(4,5) Auger transitions, and core-level spectra analyzed within the frame of the Anderson Hamiltonian in the impurity limit. The results clearly show that one-electron band-structure calculations do not account for the band gap approximate to 3.7 eV [M. Bassi, P. Camagni, R. Rolli, G. Samoggia, F. Parmigiani, and A. Revcolevschi (unpublished)] and the emission arising from many-body effects (correlated d(8) and d(8)-d(9) L hybridized states), while Delta and U-dd, found to be approximate to 4.2 and approximate to 6.7 eV, respectively, allow us to classify this compound as a charge-transfer insulator with a strong ionic character. In addition, energy-dependent electron-energy-loss measurements suggest that the forbidden d-d intraband transitions are centered at approximate to 1.6 eV, which justifies the blue color of CuGeO3, and the band gap is approximate to 3.7 eV, as required by the transparency of the crystal in the visible region of the electromagnetic spectrum. Finally, in the approximation allowed by the present models, J results to be of the order of -7 me
EELS INVESTIGATION OF BI2CUO4 SINGLE-CRYSTALS
No full textThe electronic structure of a Bi2CuO4 single crystal is studied by means of Electron Energy Loss Spectroscopy (EELS), at two primary beam energies (200 eV and 2 keV). The valence and shallow core electron excitation regions are investigated and the detected spectral features assigned, also exploiting the two different probing depths
Observation of strong electron correrlations in YBA2CU3O7-delta by H-NU-dependent photoelectron-spectroscopy
No full textWe present photoemission spectra from single-phase YBa2Cu3O7-Î ́which have been measured in the photon energy range (17 ÷ 130) eV, at T=80 K, T=300 K and intermediate temperatures using synchrotron radiation. Intense satellites associated with the Cu 3 d and O 2 p bands are observed to undergo resonances at the Cu 3 p and O 2 s core level excitation thresholds, respectively, indicating the presence of holes in the Cu and O hybrid bands on both Cu and O sites. The strong correlation effects for the valence electrons suggest that one-electron calculations may not be appropriate for a detailed description of the electronic properties of this material
Role of the Zhang-Rice-like exciton in optical absorption spectra of CuGeO3 and CuGe1-xSixO3 single crystals
No full textThe optical excitations in pure and Si-substituted CuGeO3 are investigated in the 15-300 K temperature range. A structure, detected for the E field parallel to the c axis of the crystals at about 3.2 eV, is due to the formation of a Zhang-Rice-like (ZR) exciton. The weak oscillator strength is due to the low mobility of the ZR-like exciton between the edge-sharing CuO4 plaquettes of CuGeO3. The temperature dependence of the ZR-like energy is explained by a renormalization effect involving phonon-electron interactions or changes in spin ordering, while the increase of ZR-like exciton intensity with the Si content is argued to arise from a different hybridization of the Si-O bond with respect to Ge-O
Role of the Zhang-Rice-like exciton in optical absorption spectra of CuGeO3 and CuGe1-xSixO3 single crystals
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Surface gap and surface electronic states in CuGeO3 single crystal
No full textThe surface and bulk electronic excitations of CuGeO3 are investigated by means of electron energy loss and polarized X-ray absorption spectroscopy. CuGeO3 shows a surface charge transfer gap of about 3.0+/-0.3 eV. The unoccupied oxygen derived density of states, as probed by X-ray absorption at the O 1s edge, is in good agreement with recent many-body calculations. (C) 1999 Elsevier Science B.V. All rights reserved
Static magnetic susceptibility, crystal field and exchange interactions in rare earth titanate pyrochlores
No full textThe experimental temperature dependence (T = 2–300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R2Ti2O7 (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole–dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed.
Amorphous ferromagnetism and re-entrant magnetic glassiness in single-crystalline Sm2Mo2O7
No full textWe discuss the magnetic properties of a Sm2Mo2O7 single crystal as investigated by means of different experimental techniques. In the literature, a conventional itinerant ferromagnetic state is reported for the Mo4+ sublattice below ∼78 K. However, our results of dc magnetometry, muon spin spectroscopy (μ+SR), and high-harmonics magnetic ac susceptibility unambiguously evidence highly disordered conditions in this phase, in spite of the crystalline and chemical order. This disordered magnetic state shares several common features with amorphous ferromagnetic alloys. This scenario for Sm2Mo2O7 is supported by the anomalously high values of the critical exponents, as mainly deduced by a scaling analysis of our dc magnetization data and confirmed by the other techniques. Moreover, μ+SR detects a significant static magnetic disorder at the microscopic scale. At the same time, the critical divergence of the third-harmonic component of the ac magnetic susceptibility around ∼78 K leads to additional evidence towards the glassy nature of this magnetic phase. Finally, the longitudinal relaxation of the μ+ spin polarization (also supported by results of ac susceptibility) evidences re-entrant glassy features similar to amorphous ferromagnets
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