Insulator-to-metal transition in Ca$_{ 2}$RuO$_{ 4}$ 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 Ca$_{ 2}$RuO$_{ 4}$, 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 $\Gamma$ 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 Ca$_{ 2}$RuO$_{ 4+\delta}$ than for Ca$_{ 1.8}$Sr$_{ 0.2}$RuO$_{ 4}$. 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 Ca$_{ 2}$RuO$_{ 4}$.
Categories: cond-mat.str-el