Electronic correlations and partial gap in the bilayer nickelate La 3 Ni 2 O 7 .

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  ∗  ≃ 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  ∗ 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 3 z 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 3 Ni 2 O 7 .