Energy-Gap Dynamics of Superconducting NbN Thin Films Studied by Time-Resolved Terahertz Spectroscopy

Abstract

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor 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 bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.

Energy dependence of the electron-boson coupling strength in the electron-doped cuprate superconductor Pr1.85Ce0.15CuO4−δ

“…It follows that the maximum gap frequency 2∆/ ≈ 1.7 THz (∆ = 3.5 meV), in good agreement with other studies on c-axis films [23,26]. Unlike in the BCS case [16], 2∆ displays only weak T-dependence.…”

“…In PCCO 2∆ ≈ 7 meV [23][24][25][26], well below the acoustic phonon cut-off frequency of ≈ 20 meV [27], as well as the energy of the collective electronic mode of ≈ 11 ± 2 meV [28,29]. Like in NbN [16], we expected the absorbed energy density required to deplete superconductivity, A dep , to be in the low temperature limit the same in the two configurations, with A dep ≈ E c . We demonstrate that this is not the case, suggesting that the Eliashberg electron-boson coupling arXiv:1603.04155v1 [cond-mat.supr-con] 14 Mar 2016 function in cuprates depends strongly on the electron energy, unlike to what is commonly assumed [10,30].…”

“…the electron-phonon (e-ph) coupling strengths, numerous inconsistencies have been noted (even for the case of simple metals) [11][12][13][14]. An alternative timedomain approach, based on the dynamics in the superconducting state, has been put forward [15,16]. Under the assumption that the absorbed optical energy is distributed between quasiparticles and high frequency ( ω > 2∆) bosons on the sub-picosecond timescale, and taking into account the nonlinearity of relaxation processes (pairwise recombination of quasiparticles), the electron-boson coupling strength is determined by studying the excitation density dependence of the Cooper pair-breaking process [15,16].…”

“…Under the assumption that the absorbed optical energy is distributed between quasiparticles and high frequency ( ω > 2∆) bosons on the sub-picosecond timescale, and taking into account the nonlinearity of relaxation processes (pairwise recombination of quasiparticles), the electron-boson coupling strength is determined by studying the excitation density dependence of the Cooper pair-breaking process [15,16]. While this approach has been successfully applied to conventional superconductors [16,17], the results on cuprates show that the energy density required to suppress superconductivity exceeds the thermodynamic condensation energy, E c , by an order of magnitude [2,[18][19][20]. Therefore, the assumption that the absorbed energy is distributed between QPs and the coupled high frequency bosons fails.…”

Multiple amplitude modes in strongly coupled phonon-mediated superconductors

“…The material used in the recent experiments [15,16], NbN, has a relatively large dimensionless electron-phonon coupling λ eff 1, corresponding to the strong-coupling regime [28][29][30][31]. Hence it is necessary to understand how strong electronphonon (el-ph) couplings can affect collective excitations in conventional superconductors.…”