We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO2thin films. For the
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Abstract a b -plane, an optical peak near Ω ≈ 750 cm−1drives the scattering rate 1/τ (ω ) and effective massm *(ω ) of the Drude carrier to increase and decrease respectively forω ≧ Ω. For thec -axis, a longitudinal optical phonon (LO) is present at Ω as evidenced by a peak in the loss function Im[−1/ε c(ω )]. Further polarized measurements in different light propagation (q ) and electric field (E ) configurations indicate that the Peak at Ω results from an electron-phonon coupling of thea b -plane carrier with thec -LO phonon, which leads to the frequency-dependent 1/τ (ω ) andm *(ω ). This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared (IR) band and ac -LO. On the contrary, it is the Drude carrier that couples in PdCoO2. The coupling between thea b -plane Drude carrier andc -LO suggests that thec -LO phonon may play a significant role in the characteristica b -plane electronic properties of PdCoO2, including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.