Interaction of in-plane Drude carrier with c -axis phonon in PdCoO 2

We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO 2 thin films. For the a b -plane, an optical peak near Ω ≈ 750 cm −1 drives the scattering rate 1/ τ ( ω ) and effective mass m * ( ω ) of the Drude carrier to increase and decrease respectively for ω ≧ Ω. For the c -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 the a b -plane carrier with the c -LO phonon, which leads to the frequency-dependent 1/ τ ( ω ) and m * ( ω ). 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 PdCoO 2 . The coupling between the a b -plane Drude carrier and c -LO suggests that the c -LO phonon may play a significant role in the characteristic a b -plane electronic properties of PdCoO 2 , including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.