THz spectroscopy of superconducting ultrathin films

Coherent-source THz spectroscopy was employed to measure the charge carrier dynamics and excitations of ultrathin superconducting films at low temperatures. While the properties of weakly disordered superconductors can be well described by the BCS theory, significant deviations are observed as disorder increases towards the superconductor-insulator transition. A comparative study including tunneling and optical spectroscopy reveals the importance of Coulomb interactions on the energy gap of disordered superconductors. We also discuss the possibility of collective low-frequency excitations due to the Higgs mode, which become long-lived and well defined in the vicinity of a quantum critical point.