Far infrared (IR) measurements in the photon energy range from 10 to 85 meV were performed using synchrotron edge radiation from the 2.5 GeV electron storage ring at the IR beamlines of the Karlsruhe Research Accelerator (KARA) at the Karlsruhe Institute of Technology, Germany. Synchrotron-based IR ellipsometry enabled precise determination of the complex optical conductivity of superconducting La₃Ni₂O₇ [1] at ambient pressure over temperatures from 300 K to 10 K. Complementary reflectance spectra were acquired with a Bruker HYPERION II IR microscope, with the sample positioned in a high-pressure cell. The setup provided a spatial resolution of (20x20 μm²) and allowed measurements at pressures up to 20 GPa and temperatures between 35 K and 300 K.

Optical conductivity exhibits uniaxial charge anisotropy, with metallic behavior in the plane and no electronic background in the out-of-plane phonon spectra. The in-plane response reveals a density wave transition with the formation of a pseudogap below 115 K. Phonon bond-bending modes show spot-dependent variations. Applying pressure induces a linear mode hardening. High-resolution reflectance indicates metallic inhomogeneity and remains essentially unchanged across T_c. These observations confirm the coexistence of multiple crystallographic phases with alternating single and triple NiO₆ layers and reveal no superconducting gap, supporting filamentary superconductivity at phase boundaries [1].

[1] Phys. Rev. Lett. 133, 146002 (2024)