This thesis presents an ellipsometric study of the infrared response of
bulk samples [1] and thin films of K doped p-terphenyl. In both cases,
a similarly weak Drude-response has been observed that undergoes
anomalous changes around 90 K which can be associated with an
enhanced charge localization. It seems likely that this charge
localization is caused by a structural transition due to a twisting of
the p-terphenyl molecules which reduces the hopping probability
of the doped electrons in the pi-orbitals. Such a structural transition
of the molecules occurs in the pristine p-terphenyl around 195 K. The
K doping is known to suppress this twisting, the corresponding
transition thus may well be reduced to about 90 K.

Notably, an activation barrier due to such a twisting of the
molecules could also explain the energy gap feature that has been
reported from an ARPES study [2] on the surface of p-terphenyl
crystals that have been covered with K ions.

While the details of this charge localization remain to be fully
understood, the presented infrared spectra certainly exclude the
possibility that a bulk-like superconducting state develops in these
K3 p-terphenyl samples. Further studies with microscopic probes will
be needed to settle the question of whether the charge localization
might be accompanied by the formation of a high temperature
superconducting phase in a small fraction of the sample volume.

[1] He Q, Marsik P, Le Mardelé F, et al. Infrared ellipsometry study of
the charge dynamics in K 3 p-terpheny. Physical Review B, 2023,
107(9): 094520.
[2] H. Li, X. Zhou, S. Parham, T. Nummy, J. Griffith, K. N. Gordon, E. L.
Chronister, and D. S. Dessau, Spectroscopic evidence of low-energy
gaps persisting up to 120 K in surface-doped p-terphenyl crystals,
Phys. Rev. B 100, 64511 (2019)