Symmetry of holes in high-Tcsuperconductors

Abstract

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.

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

“…A significant density of p z holes has also been established in YBa 2 Cu 3 O ≈7 [3,12] and in the La-Sr-Cu-O [18] system by such studies. There is variation in data from one system collected using the TY and FY techniques and from two different systems using the same technique.…”

“…At the same time, Qvarford et al [15] report a smaller shift (about 100 meV) for the Bi-2201 crystal also along with a significant intensity in the E c polarization. Much earlier, EELS data [12] on Bi-2212 have been reported, showing again a significant intensity in the E c polarization.…”

“…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.…”

“…The surface-sensitive total electron yield (TY) mode of detection was used in all the above cases. Later reports using the bulk-sensitive 0953-8984/96/142467+11$19.50 c 1996 IOP Publishing Ltd 2467 fluorescence yield (FY) mode [12] showed an almost insignificant intensity for the out-ofplane Cu 3d z 2 states. A recent report by Venkatesh et al [21] lists the probable reasons for the observed discrepancies in the TY and FY data.…”

“…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.…”