We have investigated charge dynamics and electronic structures for single crystals of metallic layered nickelates, R2-xSrxNiO4 (R=Nd, Eu), isostructural to La2-xSrxCuO4. Angle-resolved photoemission spectroscopy on the barely-metallic Eu0.9Sr1.1NiO4
(R=Eu, x=1.1) has revealed a large hole surface of x2-y2 character with a high-energy pseudogap of the same symmetry and comparable magnitude with those of underdoped (x<0.1) cuprates, although the antiferromagnetic interactions are one order of magnitude smaller. This finding strongly indicates that the momentum-dependent pseudogap feature in the layered nickelate arises from the real-space charge correlation.
Charge dynamics of (Ti1-xVx)2O3 with x=0-0.06 has been investigated by measurements of charge transport and optical conductivity spectra in a wide temperature range of 2-600K with the focus on the thermally and doping induced insulator-metal transiti
ons (IMTs). The optical conductivity peaks for the interband transitions in the 3d t2g manifold are observed in the both insulating and metallic states, while their large variation (by ~0.4 eV) with change of temperature and doping level scales with that of the Ti-Ti dimer bond length, indicating the weakened singlet bond in the course of IMTs. The thermally and V-doping induced IMTs are driven with the increase in carrier density by band-crossing and hole-doping, respectively, in contrast to the canonical IMT of correlated oxides accompanied by the whole collapse of the Mott gap.
