Highly Anisotropic Charge Dynamics and Spectral Weight Redistribution in the Trilayer Nickelate La_{4}Ni_{3}O_{10}.

We study the ab-plane and c-axis charge dynamics of La_{4}Ni_{3}O_{10} using optical spectroscopy. While a pronounced Drude profile, i.e., metallic response, is observed in the ab-plane optical conductivity σ_{1}^{ab}(ω), the c-axis optical spectra σ_{1}^{c}(ω) exhibit semiconducting behavior. The zero-frequency extrapolation of the optical conductivity σ_{1}(ω→0)≡1/ρ_{dc} leads to a resistivity anisotropy ρ_{c}/ρ_{ab} exceeding 1000 at low temperatures, which is much larger than the values in iron-based superconductors but comparable to those in high-T_{c} cuprates. The interband response is also highly anisotropic, showing salient orbital selectivity for light polarized in the ab plane and along the c axis. The interband-transition peaks in both σ_{1}^{ab}(ω) and σ_{1}^{c}(ω) are located at lower energies compared to density-functional-theory predictions, signifying considerable electronic correlations. By investigating the spectral weight transfer, we find that above the density-wave transition, Coulomb correlations have a marked impact on the charge dynamics of La_{4}Ni_{3}O_{10}, whereas in the density-wave state, a gap opens with the Ni-d_{z^{2}} orbital being involved.