Infrared ellipsometry study of the charge dynamics in K$_3$\,\!p-terphenyl

Abstract

We report an infrared ellipsometry study of the charge carrier dynamics in polycrystalline Kxpterphenyl 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 4 000 cm −1 and 12 000 cm −1 which, from a comparison with calculations based on a Hückel model are assigned to intra-molecular excitations of π electrons of the anionic p-terphenyl molecules. The inter-molecular electronic excitations are much weaker and give rise to a Drude peak and a similarly weak Lorentzian band around 220 cm −1 . A dc resistivity of about 0.3 Ωcm at 300K 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 300K that gets anomalously enhanced below 90K. The corresponding missing spectral weight of the Drude peak appears blueshifted towards the Lorentz-band at 220 cm −1 . This characteristic blue-shift 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.