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 $\mathrm{\Gamma }$ 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 Ca₂RuO${}_{4+\delta }$ than for Ca_1.8Sr_0.2RuO₄. 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.

Ca ₂ RuO _4中的绝缘体到金属的转变因其对各种刺激的敏感性及其潜在的可控性而备受关注。在这里，我们报告了费米表面的直接观察，该费米表面是通过使用角度分辨光发射光谱法将过量的氧气引入绝缘的Ca ₂ RuO _4中而出现的。能量分布曲线之间的比较表明，Mott绝缘间隙是通过eV标度的光谱重量转移和过量的氧气来封闭的。动量空间映射显示了费米表面的两个正方形薄片。一个像洞一样$\alpha$ 片围绕四方布里渊区的角，另一个是类似电子的 $\beta$ 围绕 $\mathrm{\Gamma }$点。电子的电子占有率$\alpha$ 和 $\beta$ 乐队被确定为 ${ñ}_{\alpha }=\mathrm{1个}。6$ 和 ${ñ}_{\beta }=0。6$， 分别。我们的结果表明，绝缘子到金属的过渡选择性地发生在$d_{Xž}$ 和 $d_{ÿž}$ 乐队和尚未 $d_{Xÿ}$带。这种轨道选择性最有可能用能级的能级来解释。$d_{Xÿ}$，对于Ca ₂ RuO来说更深${}_{4+\delta }$比Ca _1.8 Sr _0.2 RuO ₄。因此，我们发现与其他钌酸盐的费米表面存在显着差异，从而揭示了Ca ₂ RuO ₄的金属化方法中过量氧气的独特作用。