Electronic correlations and partial gap in the bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$

Abstract: The discovery of superconductivity with a critical temperature of about 80~K in La${3}$Ni${2}$O${7}$ single crystals under pressure has received enormous attention. La${3}$Ni${2}$O${7}$ is not superconducting under ambient pressure but exhibits a transition at $T^{\ast} \simeq 115$~K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La${3}$Ni${2}$O${7}$ features strong electronic correlations which significantly reduce the electron's kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at $T^{\ast}$ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni-$d{3z^{2}-r^{2}}$ orbital is removed due to the transition at $T^{\ast}$. Our experimental results provide pivotal information for understanding the transition at $T^{\ast}$ and superconductivity in La${3}$Ni${2}$O$_{7}$.