Photoemission and x-ray absorption study of superconducting and semiconducting Ba1 xKxBiO3 single crystals. M. Qvarford. Department of Synchrotron ...
PHYSICAL REVIEW B
VOLUME 54, NUMBER 9
1 SEPTEMBER 1996-I
Photoemission and x-ray absorption study of superconducting and semiconducting Ba 12x K x BiO 3 single crystals M. Qvarford Department of Synchrotron Radiation Research, Institute of Physics, Lund University, Box 118, S-221 00 Lund, Sweden
V. G. Nazin, A. A. Zakharov, and M. N. Mikheeva Russian Research Center ‘‘Kurchatov Institute,’’ Kurchatov Square 1, 123182 Moscow, Russia
J. N. Andersen, M. K.-J. Johansson, and G. Chiaia* Department of Synchrotron Radiation Research, Institute of Physics, Lund University, Box 118, S-221 00 Lund, Sweden
T. Rogelet, S. So¨derholm, and O. Tjernberg Materials Physics, Department of Physics, Royal Institute of Technology, S-100 44 Stockholm, Sweden
H. Nyle´n, I. Lindau, and R. Nyholm Department of Synchrotron Radiation Research, Institute of Physics, Lund University, Box 118, S-221 00 Lund, Sweden
U. O. Karlsson Materials Physics, Department of Physics, Royal Institute of Technology, S-100 44 Stockholm, Sweden
S. N. Barilo and S. V. Shiryaev Institute of Solid State Physics and Semiconductors, 220072 Minsk, Belorussia ~Received 8 March 1996! Semiconducting Ba 0.9K 0.1BiO 3 and superconducting Ba 0.6K 0.4BiO 3 single crystals cleaved in situ have been studied by core level and valence band photoelectron spectroscopy and O K edge x-ray absorption spectroscopy. It was found that the general shape of the valence band spectrum agrees with the shape predicted by band structure calculations, but the intensity near the Fermi level was lower in the experimental spectrum as compared to the calculated. The O K edge spectra showed that the metallic phase is not related to the presence of doping inducted O 2 p holes. This property of Ba 12x K x BiO 3 shows that the semiconductor-metal transition of this system is of a different nature than that of the hole doped cuprate high-T c superconductors. The core level photoemission spectra of the cations showed a small asymmetry for Ba 0.9K 0.1BiO 3 . Corresponding spectra for Ba 0.6K 0.4BiO 3 showed a larger asymmetry resulting in a resolved high binding energy shoulder in the Bi 4 f spectrum. The origin of this feature is discussed. @S0163-1829~96!01333-1#
I. INTRODUCTION
A common property of most high-T c superconductors ~HTSC! is the presence of Cu-O 2 layers in their lattice. Band-structure calculations predict these materials to have dispersive Cu-O bands at the Fermi level (E F ).1 Ba 12x K x BiO 3 is a high-T c oxide superconductor system which has no Cu-O 2 layers in the lattice.2 The highest T c , about 30 K, is obtained for x50.4. 3 This composition gives a cubic perovskite crystal structure,3 a structure clearly different from the layered two-dimensional structures typical for the cuprate HTSC. Band-structure calculations4–6 predict that a dispersive Bi-O band crosses the Fermi level, i.e., in this material the Bi atoms are regarded to play the role of the Cu atoms in the cuprate HTSC. Strong electron correlation which is known to be crucial for cuprate HTSC is not expected in the Ba 12x K x BiO 3 system since the valence band is predicted to contain only s and p states. These differences between Ba 12x K x BiO 3 and the cuprate HTSC may imply that also the mechanism for superconductivity is different. In order to experimentally investigate possible differences 0163-1829/96/54~9!/6700~8!/$10.00
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and similarities in the electronic structure of Ba 12x K x BiO 3 and the cuprate HTSC, electron spectroscopy studies are of great importance. One main goal for such studies is to elucidate the physics behind the potassium-induced transition from a semiconductor (x,0.3) to a metal (x.0.3).7 This transition coincides with a crystallographic phase transition between cubic and distorted structures, the metallic phase being cubic and the semiconducting phase monoclinic (0
