  We report on the successful synthesis of highly conductive PdCoO2 ultrathin
films on Al2O3 (0001) by pulsed laser deposition. The thin films grow along the
c-axis of the layered delafossite structure of PdCoO2, corresponding to the
alternating stacking of conductive Pd layers and CoO2 octahedra. The
thickness-dependent transport measurement reveals that each Pd layer has a
homogeneous sheet conductance as high as 5.5 mS in the samples thicker than the
critical thickness of 2.1 nm. Even at the critical thickness, high conductivity
exceeding 104 Scm-1 is achieved. Optical transmittance spectra exhibit high
optical transparency of PdCoO2 thin films particularly in the near-infrared
region. The concomitant high values of electrical conductivity and optical
transmittance make PdCoO2 ultrathin films as promising transparent electrodes
for triangular-lattice-based materials.
