Two-band conductivity of a FeSe$_{0.5}$Te$_{0.5}$ film by reflectance measurements in the terahertz and infrared range

Abstract

Abstract. We report an infrared spectroscopy study of a 200 nm thick FeSe 0.5 Te 0.5 film grown on LaAlO 3 with T c =13.7 K. We analyze the 20 K normal state absolute reflectance R N measured over a broad infrared range and the reflectance ratio R S /R N , R S being the superconducting state reflectance, measured at 6 K in the terahertz range down to 12 cm −1 . We show that the normal state model conductivity is given by two Drude components, one of which much broader and intense than the other. In the superconducting state, we find that a gap ∆=37±3 cm −1 opens up in the narrow Drude band only, while the broad Drude band results to be ungapped, at least in the explored spectral range. Our results show that only a two-band model can coherently describe both normal and superconducting state data. ‡ Present address: Elettra Sincrotrone Trieste,

FeTe0.55Se0.45: A multiband superconductor in the clean and dirty limit

“…2, revealing two distinct Drude components; a narrow response with ω p,D;1 2630 cm −1 and 1/τ D,1 32 cm −1 , and a much broader and stronger component with ω p,D;2 14 110 cm −1 and 1/τ D,2 1770 cm −1 . These values are consistent with the results from the two-Drude analysis performed on FeTe 0.5 Se 0.5 thin films [32]. The structure in the mid-infrared region is described by two oscillators centered at ω 1 1720 cm −1 and ω 2 4010 cm −1 ; other high-frequency oscillators have been included to describe the optical conductivity in the near-infrared and visible regions, but they are not shown in this plot.…”

“…Perpendicular to the planes (c axis) the transport appears incoherent and displays little temperature dependence; below T c no evidence of a gap or a condensate is observed [33]. In this work the detailed optical properties of FeTe 0.55 Se 0.45 in the a-b planes are examined at a large number of temperatures in the normal state and analyzed using the two-Drude model [34,35], which considers two electronic subsystems rather than a single electronic band; this approach has been successfully applied to thin films of this material [32]. The single-band approach was used in a previous study of this material and was the basis for the application of the generalized Drude model [30]; however, we demonstrate that the multiband nature of this material precludes the use of the generalized Drude model.…”

“…The optical properties in the Fe-Te/Se (a-b) planes of FeTe 0.55 Se 0.45 reveal a material that appears to be almost incoherent at room temperature but that develops a metallic character just above T c . Below T c the emergence of a superconducting state is seen clearly in the in-plane optical properties [30][31][32]. Perpendicular to the planes (c axis) the transport appears incoherent and displays little temperature dependence; below T c no evidence of a gap or a condensate is observed [33].…”

Phonon anomalies in some iron telluride materials

“…The substitution of Se for Te in FeTe 1−x Se x suppresses the structural and magnetic transition and results in a superconducting phase for a broad range of compositions [12,13], with a maximum critical temperature of T c ≃ 14 K [14][15][16][17]. The optical and transport properties of Fe 1+δ Te and FeTe 1−x Se x have been investigated [17][18][19][20][21][22][23] and in addition there have been a number of reports on the vibrational properties of these materials; however, only the Raman-active modes have been studied [24][25][26][27][28], leaving the infrared-active modes largely unexplored. * homes@bnl.gov † ymdai@lanl.gov; Los Alamos National Laboratory, Center for Integrated Nanotechnologies, MPA-CINT, MS K771, Los Alamos, New Mexico 87545, USA…”