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Experimental and theoretical study of infrared active phonons in heavily doped Kxp-terphenyl

Qi He1,*, A. Guijarro2, J. L. McDonald2, Yu. G. Pashkevich1,3, N. Pinto4, C. Di Nicola5, P. Marsik1, F. Le Mardeĺe6, B. Xu7,8 et al.

C. Bernhard1,†

  • *Contact author: qi.he@unifr.ch
  • †Contact author: christian.bernhard@unifr.ch
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Phys. Rev. Research 8, 013238 – Published 5 March, 2026

DOI: https://doi.org/10.1103/gk5y-g1np

Abstract

We present an experimental and theoretical study of the infrared-active phonon modes of heavily potassium (K) doped p-terphenyl, a material of great current interest due to reports of high-temperature superconductivity. First, we show that the phonon features in the fingerprint region of the midinfrared domain can be well reproduced through the density functional theory (DFT) quantum-chemical simulations. Compelling evidence for a distorted organic molecular framework is obtained here from the emergence of up to three absorption bands that are predicted to be infrared (IR)-inactive under idealized D2h planar symmetry and a systematic softening of specific vibrational modes. Computational models of twisted conformers exhibiting reduced D2 or C2h symmetry are shown to successfully reproduce the activation of these silent modes and the redshifting shown by others. Second, we present an analysis of the lineshape of the experimental IR-phonon modes in a wide temperature range from 6 to 300 K. We show that several of the IR phonons have a strongly asymmetric shape that is indicative of a Fano-like interaction with the doped charge carriers. Notably, we find that the phonon asymmetries, and the corresponding Fano parameters, undergo anomalous changes around 100 K that are reminiscent of a charge localization transition around 90 K that has been previously reported from a corresponding study of the low-energy electronic response.

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Phys. Rev. Research 8, 013238– Published 5 March, 2026
  • Received 1 October 2025
  • Accepted 9 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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