Complex conductivity of FeSe_1–x Te_x (x = 0 – 0.5) films

We measured the complex conductivity, σ, of the FeSe_1−xTe_x (x = 0 – 0.5) films below T _c which show a drastic increase of the superconducting transition temperature, T _c, when the nematic order disappears. Since the magnetic penetration depth, λ (> 400 nm) of Fe(Se, Te) is longer than the typical thickness of the film (∼100 nm), we combined the coplanar-waveguide-resonator- and cavity-perturbation techniques to evaluate both the real and imaginary parts of σ. Films with the nematic order showed a qualitatively different behavior of the quasiparticle scattering time compared with those without the nematic order, suggesting that the nematic order influences the superconducting gap structure. On the other hand, the proportionality between the superfluid density, n _s/m* (∝ λ⁻²), and T _c was observed irrespective of the presence or absence of the nematic order. This result indicates that the amount of the superfluid has a stronger impact on T _c of Fe(Se, Te) than the presence or absence of the nematic order itself. (paper)