Electronic correlations and partial gap in the bilayer nickelate La_3Ni_2O_7

The discovery of superconductivity with a critical temperature of about 80 K in La_3Ni_2O_7 single crystals under pressure has received enormous attention. La_3Ni_2O_7 is not superconducting under ambient pressure but exhibits a transition at T^∗≃ 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_3Ni_2O_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^∗ 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^∗. Our experimental results provide pivotal information for understanding the transition at T^∗ and superconductivity in La_3Ni_2O_7.