We have performed a systematic photoemission study of the chemical potential shift as a function of carrier doping in a pnictide system based on BaFe2As2. The experimentally determined chemical potential shift is consistent with the prediction of a rigid band shift picture by renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) in the Fermi surface (FS) nesting condition due to different effective masses for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA, which matches well the observed asymmetric superconducting domes in the phase diagram, strongly supports FS quasi-nesting driven superconductivity in iron pnictides.
Optical conductivity spectra sigma(omega) were used to investigate the effect of orbital ordering on the electronic structure of Ca2RuO4. Our LDA+U calculation predicts Ru 4d(xy) ferro-orbital ordering at the ground state, and well explains the present sigma(omega) as well as the reported O 1s x-ray absorption spectra. Variation of temperature (T) causes a large change of spectral weight over several eV as well as collapse of a charge gap accompanied by elongation of the c-axis Ru-O bond length. These results clearly indicate that the d(xy) orbital ordering plays a crucial role in the metal-insulator transition and the T-dependent electronic structure on a large energy scale.
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