Institute of Physics, Czech Academy of Sciences, Czech Republic
Title: Departure from BCS response in photoexcited superconducting films observed by terahertz spectroscopy
Biography: Christelle Kadlec
We investigate thin superconducting NbN films with various thicknesses by time-resolved terahertz spectroscopy. In agreement with previous reports, the equilibrium THz conductivity can be described by the BCS theory. Upon strong photoexcitation by femtosecond laser pulses, when the superconducting state is completely broken, the recovery dynamics occurs by a growth of initially spherical isolated superconducting islands in the normal-state environment. These islands subsequently merge towards a nearly percolated superconducting network. The recovery process is accompanied by a shift in the conductivity spectral weight, indicating a departure from the BCS character of the density of electron states in these islands. While the superconductivity recovers on the hundreds-picosecond time scale, the properties characterizing the superconducting state (such as the gap width and the density of states) recover much more slowly, at least on the nanosecond time scale.