摘要:

This paper gives a brief survey of several plausible mechanisms proposed for HighTcsuperconductivity based upon attractive interactions mediated either by charge or magnetic fluctuations. Close attention is paid to the testability of the models as well as how closely they conform to existing experimental and theoretical constraints.

ISSN:0094-243X    

DOI:10.1063/1.39055

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We consider an excitonic model in which the electron is localized in adorbital on a metal site, and the hole is in a linear combination ofporbitals on the surrounding oxygen sites, with both electron and hole in an insulating LaO, Bao, etc. layer adjacent to a metallic CuO2or BiO2plane. This mechanism appears to provide a natural and consistent interpretation of high‐temperature superconductivity in both the cuprates and bismuthates.

ISSN:0094-243X    

DOI:10.1063/1.39039

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This paper summarizes results of photoelectron spectroscopy and X‐ray absorption spectroscopy applied to some high Tcsuperconductors. Problems due to surface contamination of samples are pointed out. Among the questions discussed are electron‐electron interactions, charge distribution and assignment of valence to particular Cu or O atoms and the nature of O states responsible for superconductivity. (AIP)

ISSN:0094-243X    

DOI:10.1063/1.39061

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We present the results of photoemission studies of ‘‘valence disordered’’ monoclinic BaBiO3in the photon energy range 15–120 eV. The line‐shapes of the valence band photoemission spectra and the Ba contributions to the valence band are very similar to the line shapes of the total density of states and Ba partial density of states, respectively. Oxygen resonance is observed, demonstrating the existence of empty O 2p states. These results support a more covalent rather than a simple ionic picture for the electronic states of BaBiO3.

ISSN:0094-243X    

DOI:10.1063/1.39049

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have used photoemission spectroscopy to examine the symmetry of the occupied states of the valence band for the La doped superconductor Bi2(Ca, Sr, La)3Cu2O8. While the oxygen states near the bottom of the 7 eV wide valence band exhibit predominantly O 2pzsymmetry, the states at the top of the valence band extending to the Fermi level are found to have primarily O 2pxand O 2pycharacter. We have also examined anomalous intensity enhancements in the valence band feature for photon energies near 18 eV. These enhancements, which occur at photon energies ranging from 15.8 to 18.0 eV for the different valence band features, are not consistent with either simple final state effects or direct O2s transitions to unoccupied O2p states.

ISSN:0094-243X    

DOI:10.1063/1.39050

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We used the resonant photoemission technique to search for Cu1+resonances in the spectra of superconducting and non‐superconducting compounds in the Nd2−xCexCuO4family. No evidence for such resonances was found. The possible implications of this somewhat suprising result are discussed.

ISSN:0094-243X    

DOI:10.1063/1.39056

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We used photoemission spectroscopy with synchrotron radiation to search for evidence of a resonating valence bond (RVB) state in a series of thallium‐containing high‐temperature superconductors. As in the case of Bi compounds, no positive evidence was found, and the data imply a low upper limit for the energy of hypothetical spinons.

ISSN:0094-243X    

DOI:10.1063/1.39057

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

O K‐edge absorption studies show that the density of O 2p empty states near the Fermi level increases with increasing oxygen content for YBa2Cu3O6+x, and Sr concentration in La2−xSrxCuO4. With Zn substitution for Cu in YBa2Cu3O7, Tcdoes not correlate with the density of O 2p empty states. In this case Tcdramatically decreases with Zn substitution while the pre‐edge feature remains unchanged. The O K‐edge spectrum of Nd2−xCexCuO4barely changes from the undoped, insulating, to the Ce doped superconducting sample. The lack of unoccupied O 2p states near the Fermi level in this case is consistent with naive expectations for electron‐doping.

ISSN:0094-243X    

DOI:10.1063/1.39058

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Composite target rf magnetron sputtering has previously been successfully employed to make superconducting films of YBa2Cu3O7−xin‐situat substrate temperature Ts<700 °C. We report the successful growth of superconducting films of Bi2Sr2CaCu2Oxon single crystal MgO substrates by a low‐temperature process using dc magnetron sputtering from a Bi‐enriched composite target. Using a substrate temperature Ts≊645 °C, metallic films with a superconducting onset of 90–100 K and an extrapolated Tc0=56 K have been obtained. X‐ray diffraction shows the films to be c‐axis oriented. Electron microscopy reveals that the films are not significantly smoother nonsuperconducting phases had occurred. The exact mechanism by which crystallization and superconductivity occurs at such low temperatures is not yet known, but it can be speculated that the surface atoms are less constrained and thus have a smaller energy barrier to overcome in forming a crystal structure.

ISSN:0094-243X    

DOI:10.1063/1.39059

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The preparation of superconducting TlmCanBa2Cun+1Ox(m=1, 2; n=1, 2) thin films sputter‐deposited from two oxide targets using a symmetric RF diode sputtering technique are reported. Spatially resolved optical emission data suggest no significant bombardments of the growing film by energetic particles, resulting in a film composition close to that of the sputtering targets used. The superconducting properties of these films were found to depend strongly on the target composition, substrate and substrate temperature, as well as the the post‐annealing conditions used due to a substantial loss of thallium during annealing. For the films deposited on (100) MgO from Tl2Ca2Ba2Cu3Oxtargets, post annealing in a sealed quartz tube of the films wrapped in gold foils with sintered Tl2Ca2Ba2Cu3Oxpellets or powders at 895 and 870 °C routinely gave rise to single phase Tl2Ca2Ba2Cu3Oxand Tl2Ca1Ba2Cu2Oxfilms, respectively. Annealing at 895 °C with a used Tl2Ca2Ba2Cu3Oxfilms. These films are highly c‐axis textured with corresponding superconducting transition temperature of 123, 104, and 115 K, respectively.

ISSN:0094-243X    

DOI:10.1063/1.39060

出版商:AIP    

年代:1990

数据来源: AIP