Abstract-Transient increase of the energy gap of superconducting NbN thin-films excited by resonant narrow-band terahertz pulses

M. Beck, I. Rousseau, M. Klammer, P. Leiderer, M. Mittendorff, S. Winnerl, M. Helm, G. N. Gol’tsman, and J. Demsar

http://prl.aps.org/accepted/75075Y06Jb413d3903430035b96813be3c98ad3c4

Observations of radiation-enhanced superconductivity have thus far been limited to a few type-I superconductors (Al, Sn) excited at frequencies between the inelastic scattering rate and the superconducting gap frequency $2\Delta/h$. Utilizing intense, narrowband picosecond THz pulses, tuned to just below and above $2\Delta/h$ of a BCS superconductor NbN, we demonstrate that superconducting gap can be transiently increased also in a type-II dirty-limit superconductor. The effect is particularly pronounced at higher temperatures and is attributed to radiation induced non-thermal electron distribution persisting on a 100 ps timescale.