Electronic Excitations in Lanthanum Cuprates Measured by Resonant Inelastic X-ray Scattering

• Main Author: Ellis, David
• Other Authors: Kim, Young-June
• Language: en_ca
• Published: 2010
• Subjects: x-ray; cuprate; RIXS; 0611
• Online Access: http://hdl.handle.net/1807/24344

Excitations of the valence electrons in the high-temperature superconducting cuprate La2−xSrxCuO4 were measured by Resonant Inelastic X-ray Scattering (RIXS). Several types of electronic excitations resonant at the Cu 1s →4p transition were studied over a wide range of dopings 0<x<0.35. A 500 meV excitation was observed at a reduced momentum transfer q=(π 0) corresponding to the zone boundary, whose temperature and doping dependence was the same as the two-magnon Raman scattering mode. The momentum dependence of this 2-magnon excitation agrees with recent theoretical calculations. Momentum resolved measurements of the x=0 sample revealed a broad range of excitations above and below the main charge transfer peak, and their dispersions were measured across the Brillouin zone. These include a dispersionless ∼1.8 eV peak, which is either a local crystal field (d-d excitation) or dipole-forbidden charge transfer excitation, and a dispersive 2.2 eV peak identified as a Zhang-Ng type charge-transfer exciton. The 2.2 eV peak was less dispersive than predicted from the theoretical models, due to electron-phonon coupling, as illustrated by the temperature dependent shift in the peak position. With increased hole doping, the RIXS spectral weight transfers from higher to lower energies, analogous to earlier optical conductivity studies. At the finite momentum of q=(π 0), however, the changes are most systematic: an isosbestic point was observed at 2.2 eV where the spectra of all dopings cross, and spectral weight is transferred from high to low energies, with near-linear dependence on x, in a manner suggesting that the integrated RIXS intensity is preserved. The intensity and energy variations of the spectral peaks, as well as the isosbestic point and possible sum rule, could be explained qualitatively by a rigid three-band model which includes the non-bonding oxygen, upper Hubbard, and Zhang-Rice singlet bands. The estimated properties of the bands, such as width and energy separation, are in reasonably quantitative agreement with current theoretical models and angle-resolved photoemission measurements. Moreover, anomalies in the doping dependence are similar to those observed in other types of spectroscopies. These results underscore the relevance of the RIXS method in understanding the electronic behavior of the Lanthanum cuprates.