Orbital character of states at the Fermi level inLa2−xSr

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

“…Another question related to these is whether these predominantly arise from the apical oxygen atom and whether any increase in the number of holes at the apical O sites are detrimental to superconductivity. Pellegrin et al [17] have supported such a conclusion on the basis of the lower value for O 2p z that they observe in the Bi-2212 system as compared to that in the La-Sr-Cu-O system which has a lower T c . However, there is evidence from ARUPS [26] measurements that the Bi-O plane may also be contributing to the density of states at E F and hence to superconductivity.…”

“…Flipse et al [16] have ascribed it to a composite state of Cu 3d z 2 , Cu 4s and O 2p z . Pellegrin et al [17] have also ascribed it to the admixing of the Cu 3d z 2 with Cu 4s and Ca 4p states. The presence of this feature in the E c spectra may constitute an evidence for the non-zero intensity of the 3d z 2 orbitals.…”

“…Moreover, as borne out by ARPES and ARIPES [26,27] experiments the Bi-2212 crystal surface is known to be very stable in UHV conditions. Pellegrin et al [17] have recently indicated how the alignment error of up to ±5% may also contribute to the overall errors in the measured intensities. They report that a correction may also have to be applied due to lack of proportionality of the measured fluorescence signal to the true absorption coefficient, particularly in the case of the Cu L 3 absorption edge where the true absorption coefficient µ tr becomes of the same order as the total absorption coefficient µ tot and consequently much greater than the absorption coefficient µ rem due to the other atoms in the sample.…”

“…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.…”

Failure oft−Jmodels in describing doping evolution of spectral weight in x-ray scattering, optical, and photoemission spectra of cuprates

“…It is interesting to note that the ω c which separates the high-energy Hubbard bands and the low-energy in-gap states coincides with the isosbetic or equal absorption point in the optical spectra i.e., the residual charge-transfer gap. For hole doping, W UHB was determined by x-ray absorption spectroscopy (XAS) 10,15 , which detects the empty states above the E F . In this spirit, we compare the measured XAS spectra with the calculated emptystate DOS in Figs.…”