Photoemission studies of high-temperature superconductors

Abstract

Photoemission studies have been performed on all classes of high temperature superconductors except the Tl-related compounds. Particular attention was paid to the surface cleanliness. Comparison with band calculation shows that the one-electron picture cannot adequately explain the electronic structure of this type of materials. Most important, Cu satellites were observed both in the valence band and the Cu 2p core level for all the samples studied, signaling the importance of the d-d correlation effects. The Cu 3d character of these satellites in the valence band was verified using resonance photoemission. The results have been interpreted in terms of a cluster model derived from the two band Anderson Hamiltonian, which in the past has been used successfully to describe the electronic structure of highly correlated systems. No clear satellite structure was observed in the O 1 s core spectrum, which is consistent with the bandlike nature of the oxygen states. Examples of changes in the electronic structure, which could be related to Tc, (such as substituting Y by Pr in the Y-Ba-Cu-O system and altering the number of Cu-O layers in the Bi-Ca-Sr-Cu-O system), are also discussed

Polarized x-ray absorption spectroscopy study of the symmetry of unoccupied electronic states near the Fermi level in the system

“…High-energy spectroscopy techniques, especially polarized x-ray absorption measurements, have been widely used in probing the electronic states near the Fermi level in these rather complex materials [1,2]. In particular, a number of polarized x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) studies have been made to obtain valuable information on the symmetry of unoccupied states in the normal state [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], and to understand the behaviour of holes in the superconducting state [19,20].…”

In situ angle-resolved photoemission study of MBE-grown (La,Ce)2CuO4 thin films

“…1 shows partially angle-integrated energy distribution curve (EDC) of the as-transferred specimens. Two features are noteworthy: the absence of a "dirt" peak around 9 eV [20], and the presence of a clear Fermi edge, both indicating that the sample surface prepared in our method is free from contamination and suitable for ARPES investigation of the low-energy electronic structure. Our preliminary experiments on hν dependence (25 -50 eV) indicated that the structure near Fermi energy (E F ) is prominent for ~50 eV photons, so the following ARPES data were obtained using 50 eV photons.…”

Photoemission study of electronic states inHgBa2CuO4+δwith various doping levels

“…In YBa 2 Cu 3 O 7Ϫ␦ ͑Y123͒ and La214, this peak has been attributed to an oxygen vacancy at the surface. 14 Figure 2 shows the spectra around the valence band in sample C, which is almost optimally doped, with different light sources. Although there is a peak at 9 eV in all spectra, we believe that this peak originates from the Hg 5d state for the following reasons.…”

“…Instead, we turned our attention to the peaks below 5 eV. In La214 and Y123, the spectra for the surface deteriorated samples show a larger intensity at 4-5 eV than the intensity near 3 eV in lower light source energy such as UPS, 14 because the vacant oxygen creates peaks at 4-5 eV. As shown in Fig.…”