Strong Mass Renormalization at a Local Momentum Space in MultiorbitalCa1.8Sr0.2RuO4

We have studied the mass renormalization in ${\mathrm{Ca}}_{2-x}{\mathrm{Sr}}_x{\mathrm{RuO}}_4$ ($x=0.2$) using high-resolution angle-resolved photoemission spectroscopy. We observed precise band dispersions near the Fermi level ($E_F$) and the corresponding Fermi surfaces. A characteristic flat band with $\sim 4\mathrm{meV}$ dispersion accompanying sharp quasiparticle (QP) peaks shows up in a limited momentum region around ($\pi$, 0). The QP peak rapidly evolves below the crossover temperature $T^{*}\simeq 20\mathrm{K}$, which agrees well with the mass enhancement behavior indicated by thermal, magnetic, and transport properties. We discuss the origin of the mass renormalization in relation to the local flat band at ($\pi$, 0) possibly derived from the $\gamma$ ($d_{xy}$) band.