Systems with reduced dimensionality

Infrared Plasmarons in Elemental Bismuth

Elemental semi-metals, such as graphite and bismuth, are materials of much long-term interest due to their exceptional properties. Though some of the basic aspects of these (only apparently) simple materials are already known, many effects observed in experiments remain unexplained. Despite the strong similarities with graphite, which is now a day very popular because of its monoatomic layered derivate, graphene, the optical and infrared properties of bismuth have been under-investigated.
Our study of the frequency and temperature dependence of the optical properties of single-crystal bismuth was carried out using FT spectroscopy in the IR and visible spectral range. The analysis of the optical data revealed an absorption anomaly in an energy range not compatible with any interband transitions. Such effect appears as a pronounced absorption peak in the MIR region preceded by a tiny prepeak structure.

The temperature dependence of this anomaly, as shown in figure, seems to follow the progressive reduction of the screened plasma frequency with decreasing T. In particular the onset of the prepeak absorption it is found to be always above the screened plasma frequency indicating a clear relation between the two effects. The extended Drude analysis revealed that the position of the absorption feature corresponds to an abrupt increase of the scattering rate much similar to what is predicted in the case of electron-boson interaction. The electron-energy-loss function extracted from the experimental data showed together with the expected plasmon peak a second feature centered at the same frequency of the prepeak.

This observation proves unambiguously a coupling between the electron system and a longitudinal bosonic collective mode. Such an interaction has been called plasmaron. To the best of our knowledge this represents the first optical observation of such interaction.