Optical properties of optimally doped single-crystal Ca8.5La1.5(Pt3As8)(

Abstract

We have measured the reflectivity of the optimally doped Ca8.5La1.5(Pt3As8)(Fe10As10) single crystal (Tc = 32.8K) over the broad frequency range from 40 cm −1 to 12000 cm −1 and for temperatures from 8K to 300 K. The optical conductivity spectra of the low frequency region (< 1, 000 cm −1 ) in the normal state (80 K < T ≤ 300 K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low frequency Drude spectra develops pseudogap (PG) hump structure at around ≈ 100 cm −1 and continuously evolves into the fully opened superconducting (SC) gap structure below Tc. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above Tc into the SC gap structure below Tc. The extracted two SC gap sizes are ∆S = 4.9 meV and ∆L = 14.2 meV , suggesting Ca8.5La1.5(Pt3As8)(Fe10As10) as a multiple gap superconductor with a mixed character of the weak coupling and strong coupling superconductivity.

Temperature dependence of the superconducting energy gaps in Ca9.35La0.65(Pt3As8)(Fe2As2)5 single crystal

“…2) using a larger plasma frequency at 30 K. The penetration depth due to the smaller gap Δ 1 exhibits a smaller penetration depth and dominates the value of the penetration depth within the error bars. Moreover, the variation of the penetration depth with temperature is found to fit very closely to the relation with and The penetration depth at 0 K and are consistent with the result obtained for optimally La-doped Ca10-3-8 samples 12 and the result obtained from the electrical resistivity mentioned above, respectively. The inset of Fig.…”

“…The plasma frequencies (corresponding scattering rates) were evaluated to be  = 1446 cm −1 ( = 20 cm −1 ) for the narrow Drude band and  = 6322 cm −1 ( = 125 cm −1 ) for the broad Drude band, respectively. The high-frequency interband transitions are explained by two Lorentz oscillator components, similar to the results for Ca 8.5 La 1.5 (Pt 3 As 8 )(Fe 2 As 2 ) 5 reported previously 12 , with the far-infrared interband transitions well fitted by two Lorentz oscillator components at 150 and 230 cm −1 . In addition, the transition from the Pt 5 d band mentioned above was included in this fitting using a broad Lorentz oscillator at approximately 2000 cm −1 , with the transitions due to the broad band included using two relatively broad Lorentz oscillators at approximately 300 and 600 cm −1 .…”

“…This inconsistency comes from the uncertainty in the low-frequency extrapolations; thus, to solve this problem of inconsistency, lower-frequency data are required. The gap magnitudes at 8 K are smaller than those for the optimally La-doped 10-3-8 compound 12 ; the magnitude of the smaller gap is reduced by 25%, while that of the larger gap is reduced by 60%. The scattering rates corresponding to the smaller (larger) gaps are similar to those for the broad (narrow) Drude bands shown in Fig.…”

“…According to a former suggestion, such differences arise from the difference in the metallicity of the Pt n As 8 layers 14 , with the latter suggestion that the differences can develop from the number of band-edge singularities 15 . To investigate the effect of such subtle differences on the electronic structure and superconductivity, various experimental studies based on transport 11 , pressure effects 16 , NMR 17 , ARPES 18 , neutron 19 , and IR spectroscopy 12 have been carried out; thus, much progress has been made. However, detailed studies on the characteristic changes of superconducting gaps due to such differences in the electronic structure have rarely been carried out, with the in-depth understanding of the superconductivity mechanism, as well as the systematic development of superconductivity, having hardly progressed.…”

Evidence for a preformed Cooper pair model in the pseudogap spectra of a Ca10(Pt4As8)(Fe2As2)5 single crystal with a nodal superconducting gap

“…In particular, many attempts have been made to solve the mystery of the role and origin of the pseudogap seen in the temperature region higher than T c . Pseudogap phenomena have also been found in heavy Fermion superconductors 8 and more recently in iron-based superconductors 9,10 . This implies that the pseudogap phenomenon is a common phenomenon in unconventional superconductors.…”