Anomalous Behavior of the $$E_{u}^{1}$$ Infrared-active Phonon Mode in Bi2 – xSrxSe3 Crystal

We have studied spectral evolution of the $$E_{u}^{1}$$ phonon line of a topological insulator Bi2 – xSrxSe3 with temperature. Unlike the Raman-active phonons, the $$E_{u}^{1}$$ mode demonstrates softening upon cooling the crystal, and the corresponding spectral line acquires a pronounced Fano-like shape at temperatures T ≲ 100 K. We interpret the latter effect as a signature of specific coupling of the bulk infrared active phonons to surface Dirac electrons. Using coherent resonant excitation of the $$E_{u}^{1}$$ mode as a surface sensitive tool, we have detected softening of the surface counterpart of the bulk $$E_{u}^{1}$$ phonon mode upon strontium doping. This observation can be an evidence of enhanced electron–phonon interaction at the surface of the Bi2 – xSrxSe3 crystal.