Interaction of in-plane Drude carrier with c-axis phonon in PdCoO₂

We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO2 thin films. For the ab-plane, an optical peak near Omega approximate to 750 cm(-1) drives the scattering rate 1/tau(omega) and effective mass m*(omega) of the Drude carrier to increase and decrease respectively for omega >= Omega. For the c-axis, a longitudinal optical phonon (LO) is present at Omega as evidenced by a peak in the loss function Im[-1/epsilon(c)(omega)]. Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that the Peak at Omega results from an electron-phonon coupling of the ab-plane carrier with the c-LO phonon, which leads to the frequency-dependent 1/tau(omega) and m*(omega). This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared (IR) band and a c-LO. On the contrary, it is the Drude carrier that couples in PdCoO2. The coupling between the ab-plane Drude carrier and c-LO suggests that the c-LO phonon may play a significant role in the characteristic ab-plane electronic properties of PdCoO2, including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.