Berlin 2012 – scientific programme
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TT: Fachverband Tiefe Temperaturen
TT 42: Superconductivity: Fabrication, Properties, Electronic Structure
TT 42.9: Talk
Thursday, March 29, 2012, 17:15–17:30, H 3005
Energy-Gap Dynamics of a Superconductor NbN Studied by Time-Resolved Terahertz Spectroscopy — •Matthias Beck1, Paul Leiderer1, Viktor V. Kabanov2, Gregory Gol’tsman3, Manfred Helm4, and Jure Demsar1,2 — 1Dept. of Physics and Center for Appl. Photonics, Univ. of Konstanz — 2Zukunftskolleg, Univ. of Konstanz, — 3Moscow State Ped. Univ., Moscow — 4Helmholtz-Zentr., Dresden-Rossendorf
Using time-resolved terahertz (THz) spectroscopy we performed direct studies of the photoinduced suppression and recovery of the SC gap in a conventional SC NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the important microscopic constants: the Cooper pair-breaking rate via phonon absorption and the bare quasiparticle recombination rate. From the latter we were able to extract the dimensionless electron-phonon coupling constant, λ=1.1 ± 0.1, in excellent agreement with theoretical estimates. The technique also allowed us to determine the absorbed energy required to suppress SC, which in NbN equals the thermodynamic condensation energy (in cuprates the two differ by an order of magnitude). Finally, we present the first studies of dynamics following resonant excitation with intense narrow band THz pulses tuned to above and below the superconducting gap. These suggest an additional process, particularly pronounced near Tc, that could be attributed to amplification of SC via effective quasiparticle cooling.