Pseudogap of Metallic Layered NickelateR2−xSrxNiO4(R=Nd,Eu) Crystals Measured Using Angle-Resolved Photoemission Spectroscopy

We have investigated charge dynamics and electronic structures for single crystals of metallic layered nickelates, $R_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_4$ ($R=\mathrm{Nd},\mathrm{Eu}$), isostructural to ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. Angle-resolved photoemission spectroscopy on the barely metallic ${\mathrm{Eu}}_{0.9}{\mathrm{Sr}}_{1.1}{\mathrm{NiO}}_4$ ($R=\mathrm{Eu}$, $x=1.1$) has revealed a large hole surface of $x^2-y^2$ 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 1 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.