Quasiparticle relaxation across the multiple superconducting gaps in the electron-doped BaFe1.85Co0.15As2

Abstract

We study the quasiparticle relaxation dynamics in the electron-doped superconductor, BaFe1.85Co0.15As2 (BFCA) by using optical spectroscopy in both time and frequency domains. The estimate of the electron−phonon (el–ph) coupling constant from quasiparticle relaxation rate in time domain gives a value of ∼0.16 that is much too small to give a TC of 25 K. We obtain a negative value of the Coulomb pseudopotential that implies that the el−ph interaction alone is not strong enough to induce a superconducting transition in BFCA. More importantly, the electron-boson spectral function determined from optical scattering rate in frequency domain does not resemble the measured phonon density of states and has a dominant peak (at 53 meV) beyond the phonon frequencies. Our results support that the BFCA is a multi-gap s-wave superconductor and that its electron pairing is based on a spin-mediated pairing mechanism.

Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li0.84Fe0.16)OHFe0.98Se

“…To avoid systematic discrepancies due to different experimental methods, we mainly summarize experimental 𝜆 𝐴1g values obtained by using time-resolved ultrafast optical spectroscopy (including time-resolved angle-resolved photoemission spectroscopy, red spheres and squares), [5,6,8,9,53] as well as theoretical results (blue spheres and squares). [35,37,39,42,43,46,48] Note that some of the data are retrieved by the authors (see the Supplementary Material).…”

“…Furthermore, we perform ultrafast optical spectroscopy experiments on Fe 1.05 Se 0.2 Te 0.8 (𝑇 c = 10 K) and Fe 1.01 Se 0.2 Te 0.8 (𝑇 c = 13.5 K) single crystals, and obtain the 𝜆 𝐴1g values to be 0.11 and 0.13, respectively (see the Supplementary Material). [5,6,8,9,53]). Blue sphere (squares): retrieved theoretical calculation results of FeSe (FeAs)-based superconductors.…”

“…[37]) CaFe 1.85 Co 0.15 As 2 20 [43] 0.115 (retrieved from Ref. [43]) BaFe 1.85 Co 0.15 As 2 23 [53] 0.125 [53] BaFe 1.84 Co 0.16 As 2 24 [9] 0.12 [9] BaFe 1.85 Co 0.15 As 2 25 [8] 0.16 [8] LaFe 1.85 AsO 0.875 F 0.125 26 [2] 0.16 [42] K𝑥Fe 2 Se 2 30 [15] 0.175 (retrieved from Ref. [46]) Ba 0.6 K 0.4 Fe 2 As 2 38 [48] 0.21 (retrieved from Ref.…”

“…The SC 𝑇c positively correlates to the EPC strength 𝜆 𝐴 1g (𝜆). Red spheres (squares): ultrafast spectroscopy results of FeSe (FeAs)-based superconductors (three data from this work and Refs [5,6,8,9,53]…”

Theory of photoexcited carrier relaxation across the energy gap of phase-ordered materials

“…Usually the anomalous T dependence of τ exp is observed in many gapped systems, for example, phaseordered systems showing an energy gap opening in the electron band below the transition temperature T c . It has been found that a wide variety of superconductors [1][2][3][4][5][6][7][8][9][10][11][12][13] and density-wave compounds [14][15][16][17][18][19] show diverging behavior of τ exp near T c , as confirmed by femtosecond time-resolved optical spectroscopy. 1,4,5,[7][8][9][10][11][12][13][14][15][16][17]19 The divergence of τ exp in these gapped systems is believed to result from a recursive energy transfer between electrons and phonons.…”