Infrared ellipsometry study of the charge dynamics in K3p -terphenyl

We report an infrared ellipsometry study of the charge carrier dynamics in polycrystalline ${\mathrm{K}}_xp$-terphenyl samples with nominal $x=3$, for which signatures of high-temperature superconductivity were previously reported. The infrared spectra are dominated by two Lorentzian bands with maxima around 4000 and $12 000 {\mathrm{cm}}^{\ensuremath{-}1}$ which, from a comparison with calculations based on a H\"uckel model, are assigned to intramolecular excitations of $\ensuremath{\pi}$ electrons of the anionic $p$-terphenyl molecules. The intermolecular electronic excitations are much weaker and give rise to a Drude peak and a similarly weak Lorentzian band around $220 {\mathrm{cm}}^{\ensuremath{-}1}$. A dc resistivity of about $0.3 \mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ at $300\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ is deduced from the IR data, comparable to values measured by electrical resistivity on a twin sample. The analysis of the temperature dependence of the low-frequency response reveals a gradual decrease of the plasma frequency and the scattering rate of the Drude peak below $300\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ that gets anomalously enhanced below $90\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The corresponding missing spectral weight of the Drude peak appears blueshifted towards the Lorentz band at $220 {\mathrm{cm}}^{\ensuremath{-}1}$. This characteristic blueshift signifies an enhanced localization of the charge carriers at low temperatures and contrasts the behavior expected for a bulk superconducting state for which the missing spectral weight would be redshifted to a delta function at zero frequency that accounts for the loss-free response of the superconducting condensate. Our data might still be compatible with a filamentary superconducting state with a volume fraction well below the percolation limit for which the spatial confinement of the condensate can result in a plasmonic resonance at finite frequency.