Electronic states near fermi level in superconductors La2 −xSrxCuO4by means of x-ray absorption spectra near Cu-Kand La-Ledges

Abstract The electronic states of La2− x Sr x CuO4 with 0.00 ≤ x ≤ 0.20 are studied by means of X-ray absorption spectroscopy (XANES, EXAFS) near the K-edge of Cu2+ ion and the L-edges of La3+ ion. It is found that characteristic white lines occurring near L II and L III edges of La3+ ion show a slight energy shift depending on substituted Sr2+ ions, x and temperature. The white lines suggest that unoccupied high-density 5dπ and 5dδ bands of La3+ ion just above a Fermi level transform to a hybridized single band of 5dπδ at 78 K in the superconductors with x = 0.10, 0.16 and 0.20. On the other hand, the XANES spectra near the Cu-K edge including a pre-edge region do not depend on x and temperature in the region of 0.00 ≤ x ≤ 0.20. It is considered that there is no reconstruction of electronic states at the Fermi level in a Mott–Hubbard band gap between an O 2p valence band and a Cu 3d conduction band. The electronic states at the Fermi level are probably consisted of the unoccupied 5dπδ band and an empty charge-transfer 3d 9 L band at low temperature, bands of which occur in a band gap between a filled O 2p valence band and an unoccupied O 2p conduction band. The insulator–superconductor–metal transitions in La2− x Sr x CuO4 are related to the 5dπδ and 3d 9 L bands and holes, which site at a top region of the O 2p valence band near the Fermi level produced by a substitution of La3+ with Sr2+ ions. Keywords: Electronic stateLa2−xSrxCuO4Superconductor