Electronic correlations and energy gap in the bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$
The discovery of superconductivity with a critical temperature of 80~K in La$_{3}$Ni$_{2}$O$_{7}$ under pressure has received enormous attention. La$_{3}$Ni$_{2}$O$_{7}$ is not superconducting under ambient pressure but exhibits a density-wave-like transition at $T^{\ast} \simeq 115$~K. Understanding the electronic correlations, charge dynamics and dominant orbitals are important steps towards the mechanism 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 it in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands dominated by the Ni-$d_{x^2 - y^2}$ and Ni-$d_{3z^2 - r^2}$ orbitals at the Fermi level. Above $T^{\ast}$, the scattering rates for both Drude components vary linearly with temperature, indicating non-Fermi-liquid behavior which may be associated with spin-fluctuation scattering. Below $T^{\ast}$, a gap opens in the Ni-$d_{3z^2 - r^2}$ orbital, suggesting the importance of the Ni-$d_{3z^2 - r^2}$ orbital in the density-wave-like instability. Our experimental results provide key insights into the mechanism of the density-wave-like order and superconductivity in La$_{3}$Ni$_{2}$O$_{7}$.