Relationships between Superconductivity and Nematicity in FeSe$_{1-x}$Te$_x$ ($x=0-0.5$) Films Studied by Complex Conductivity Measurements

TL;DR

This study investigates how nematic order affects superconductivity in FeSe$_{1-x}$Te$_x$ films by measuring complex conductivity, revealing that nematicity influences the superconducting gap structure but superfluid density correlates more strongly with $T_c$.

Contribution

It provides new insights into the relationship between nematic order and superconducting properties in FeSe$_{1-x}$Te$_x$ films, highlighting the role of Fermi surface changes.

Findings

Nematic order affects the temperature dependence of penetration depth and quasiparticle scattering.

Superfluid density correlates with $T_c$ regardless of nematicity.

Fermi surface changes due to nematicity influence $T_c$ and gap structure.

Abstract

We measured the complex conductivity, $\sigma$, of FeSe$_{1-x}$Te$_x$ ($x=0-0.5$) films in the superconducting state which show a drastic increase of the superconducting transition temperature, $T_\textrm{c}$, 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 ($\sim$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, $n_\textrm{s}$ ($\propto\lambda^{-2}$), and $T_\textrm{c}$ 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 $T_\textrm{c}$ 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 $T_\textrm{c}$ and the superconducting gap structure in Fe(Se,Te).