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High-energy electron energy-loss spectroscopy in transmission has been used to obtain information on the symmetry of unoccupied electronic states in the new high-T, superconductors.In particular, for YBa2Cu307 and Bi2Sr2CaCu208, the 0 1s and Cu 2p absorption edges have been measured for momentum transfer parallel and perpendicular to the a, b plane. Both a. and inplane n holes on 0, formed upon doping, , are compatible with our experimental results. For YBa2Cu30q, in both cases, holes must be in the planes and in the ribbons. Out-of-plane z holes can be excluded for YBa2Cu307 and Bi2Sr2CaCu208. In the undoped and doped compounds, the unoccupied states on the Cu sites have predominantly in-plane 3d"2 y2 symmetry. There is an admixture of about 10% of lower-lying states probably with 3d3, 2 "2 symmetry.

The electronic structure of La2-Sr Cu04+~has been studied by measuring 0 1s absorption edges using high-energy electron-energy-loss spectroscopy in transmission.Upon doping the insulating compound the conduction-band states are reduced and states in the gap are formed. When going from insulating to conducting compounds there is a continuous increase of states at the Fermi level, which is at the bottom of the gap. The insulator-metal transition is probably driven by delocalization of these states.Among the cuprates showing high-T, superconductivity, the system La2 "M"Cu04 (M Ba,Sr), first discovered by Bednorz and Miiller, ' is one of the most suitable to study the electronic structure of high-T, superconductors. The undoped parent compound La2Cu04 is an antiferromagnetic insulator. This is due to strong electronic correlation efl'ects on the Cu sites which force the otherwise metallic compound with a half-filled band to become a charge-transfer insulator.A charge-transfer gap appears between the valence band (ligand band) with mainly 0 2p character and the conduction band (upper Hubbard band) with mainly Cu 3d character. Upon p-type doping with Sr, antiferromagnetism disappears and an insulatormetal transition occurs near x=0.06. For 0.06~x (0.30 superconductivity is observed with a maximum T, near x =0.15. Though numerous experimental and theoretical studies have been carried out on this system, the electronic structure as a function of dopant concentration, the mechanism of high-T, superconductivity, and even the nature of the normal state for x-0.15 still remain unclear.In this Rapid Communication we report on changes of the electronic structure of La2Cu04 upon doping with Sr, which are probably essential to understand the normalstate properties of high-T, superconductors.With increasing dopant concentration there is a continuous increase of the density of states at the Fermi level, which is located at the bottom of the gap between the ligand band and the upper Hubbard band. No discontinuity is observed close to the insulator-metal transition. This points out that this transition is driven by delocalization. The present results probably can be generalized to other cuprate systems showing high-T, superconductivity. 0 ls absorption edges of La2 "Sr"Cu04+s have been measured in the concentration range 0(x (0.30 by electron-energy-loss spectroscopy (EELS) in transmission. These edges probe the local unoccupied density of states with p symmetry at the 0 sites provided the interaction of the excited electron with the core hole is small. The latter assumption is supported, at least for metallic cuprates, by the strong similarity of 0 ls absorption edges and resonant bremsstrahlung isochromat spectroscopy (BIS) spectra on Bi2SrqCaCuqQs, and by the rather good agreement between experimental C and N ls core-level absorption spectra with the calculated total density of states in transition-metal carbides and nitrides. Ceramic samples were prepared as described previously.From these samples about 1000-A-thick films were c...

The electronic structure of fullerenes and fullerene compounds from high-energy spectroscopy Golden, M.S.; Knupfer, M.; Fink, J.; Armbruster, J.F.; Cummins, T.R.; Romberg, H.A.; Roth, M.; Sing, M.; Schmidt, M. Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Abstract The high-energy spectroscopic study of the elecuonic smcture of fullerenes and their compounds in the solid staie is reviewed. it is shown that photoemission and electron enera/-loss spectroscopy in transmission provide powerful tools for the~experimental study of the electronic structure of solid Cao, C7a and higher fullerenes. as well as the compounds formed by their intercalation wilh alkali and alkaline-earth metals. Topical issues in the field are addressed. such 3s the role played by elecmn conelmion, disorder and eleciron-phonon coupling.