IR spectroscopy in tin and indium doped PbTe

Infrared spectroscopy is a powerful technique for probing the superconducting gap and low-energy excitations in superconducting materials. In this study, we investigate lead telluride (PbTe), with a focus on the superconducting behavior when doped with tin and indium. Using a novel continuous-wave far-infrared spectrometer in conjunction with specific heat measurements, we directly compare the measurements with predictions from the Mattis-Bardeen theory of surface impedance in superconductors. The results reveal deviations from the theoretical expectations, indicating the possible presence of unconventional collective excitation modes associated with superconductivity.