Emergence Of Low-Energy Electronic States In Oxygen-Controlled Mott Insulator Ca2ruo4+Delta

Insulator-to-metal transition in Ca2RuO4 has drawn keen attention because of its sensitivity to various stimulation and its potential controllability. Here, we report a direct observation of Fermi surface, which emerges upon introducing excess oxygen into an insulating Ca2RuO4, by using angle-resolved photoemission spectroscopy. Comparison between energy distribution curves shows that the Mott insulating gap is closed by eV-scale spectral-weight transfer with excess oxygen. Momentum-space mapping exhibits two square-shaped sheets of the Fermi surface. One is a hole-like alpha sheet around the corner of a tetragonal Brillouin zone, and the other is an electron-like beta sheet around the G point. The electron occupancies of the alpha and beta bands are determined to be n(alpha) = 1.6 and n(beta) = 0.6, respectively. Our result indicates that the insulator-to-metal transition occurs selectively in d(xz) and d(yz) bands and not yet in d(xy) band. This orbital selectivity is most likely explained in terms of the energy level of d(xy), which is deeper for Ca2RuO4+delta. than for Ca1.8Sr0.2RuO4. Consequently, we found substantial differences from the Fermi surface of other ruthenates, shedding light on a unique role of excess oxygen among the metallization methods of Ca2RuO4.