Relationship between superconductivity and nematicity in FeSe1-xTex (x=0-0.5) films studied by complex conductivity measurements

We measured the complex conductivity, sigma, of FeSe1-xTex (x = 0 - 0.5) films in the superconducting state which show a drastic increase of the superconducting transition temperature T-c when the nematic order disappears. Since the magnetic penetration depth lambda (>400 nm) of Fe(Se,Te) is longer than the typical thickness of the film (similar to 100 nm), we combined the coplanar waveguide resonator and cavity perturbation techniques to evaluate both the real and imaginary parts of sigma. Films with a nematic order showed a qualitatively different temperature dependence in penetration depth and quasiparticle scattering time when compared with those without nematic order, suggesting that nematic order influences the superconducting gap structure. Conversely, the proportionality between superfluid density n(s) (proportional to lambda(-2) ) and T-c was observed irrespective of the presence or absence of nematic order. This result indicates that the amount of superfluid has a stronger impact on the T-c of Fe(Se,Te) than the presence or absence of nematic order. Combining these results with band dispersions calculated using density functional theory, we propose that the change of the Fermi surface associated with nematicity is the primary factor influencing the change of T-c and the superconducting gap structure in Fe(Se,Te).

Automated summary

The study measured the complex conductivity of FeSe1-xTex films in different superconducting states, finding that the disappearance of nematic order significantly increases the superconducting transition temperature Tc, impacting the superconducting gap structure. The proportionality between superfluid density and Tc was observed, with changes in the Fermi surface identified as the primary factor affecting Tc and the superconducting gap structure.