Electronic structure ofBi2CaSr2

A. Bianconi

¹

,

Chenxi Li

²

,

Stefano Della Longa

³

et al.

Abstract

The polarized Elc oxygen EC-edge x-ray-absorption near-edge structure (XANES) spectrum of Bi2SrzCaCu208+z (Bi 2:2:1:2)has been calculated by a multiple-scattering approach and compared with experimental data. The symmetry of the conduction bands over a range of 15 eV above the Fermi level has been studied by a combined analysis of five different polarized x-ray-absorption spectra at the following edges: 0 E edge, Cu E edge, and Cu L3 edge. Information on the partial density of states of the conduction bands, 2)z, & ), projected on the atomic site X (Cu and 0) with selected orbital angular momenturn l, and its projection mi along the c axis (Sc"(& o), 2)c"(2+]), and 2)c"(2+2) for the Cu site; 2)Q(] p) RIll «5Q( ] + ] ) for the oxygen site) have been obtained. The symmetry of the itinerant states induced by doping at the Fermi level is deduced to be a mixture of 3d holes with mi =+2 and mi =0 orbital angular moments and holes on planar oxygen in the molecular-orbital combination of local b & and a~symmetry [the ligand holes L{b,) and L(a& )].

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

“…This means that there are no unoccupied 3d z 2 orbitals in the system and all 3d unoccupied states must therefore have 3d x 2 −y 2 symmetry alone. This is in agreement with the case of the La-Sr-Cu-O system reported earlier by Chen et al [18] but is in disagreement with the spectra measured earlier in TY mode [6,7,10,11,14,15] which all not only show a 10-20% intensity for the E c case but also report a shift of 300-500 meV for the WL peak towards the lower energy as one goes from E ⊥ c to E c polarization. Suzuki et al [9], on the other hand, do not report observing such a shift in their Cu L 3 spectra measured in TY mode but do report a 5% intensity in the E c polarization.…”

“…2.9 eV above peak A. This peak has been reported to be due to oxygen in the BiO layers [11][12][13]. A Bi 6p band about 3 eV above the Fermi level is also shown by band-structure calculations [32] and ARIPES studies [27] crossing the O 2p band originating from BiO and SrO layers.…”

“…It has been ascribed to transitions from 2p to a hybridized 4s, 4p and 3d z 2 state [14]. Bianconi et al [10,11] have ascribed it to a multiple-scattering resonance from the Bi-O band hybridized with the CuO 2 band with d-like symmetry. Flipse et al [16] have ascribed it to a composite state of Cu 3d z 2 , Cu 4s and O 2p z .…”

“…More investigations of the Cu L 3 edge [6,7,10,11,14,15] have reported a positive energy shift of about 300-500 meV between the E a, b and E c polarized spectra, but other studies [9,12,16] detect either no shift or a very small relative weight of d z 2 holes with no shift at all. The surface-sensitive total electron yield (TY) mode of detection was used in all the above cases.…”

“…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]. The Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212) compound has attracted more attention because interpretation in the 1-2-3 system is more complicated due to presence of the two non-equivalent Cu sites and because the surfaces of 1-2-3 compounds are unstable at room temperature under UHV conditions.…”