Superconducting fluctuations in ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ thin films probed via microwave spectroscopy

We investigated the microwave conductivity spectrum of ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ epitaxial films on ${\mathrm{CaF}}_2$ in the vicinity of the superconducting transition. We observed the critical behavior of the superconducting fluctuations in these films with a dimensional crossover from two dimensional (2D) to three dimensional (3D) as the film thickness increased. From the temperature dependence of the scaling parameters we conclude that the universality class of the superconducting transition in ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ is that of the 3D-XY model. The lower limit of the onset temperature of the superconducting fluctuations $T^{\mathrm{onset}}$ determined by our measurements was $1.1T_{\mathrm{c}}$, suggesting that the superconducting fluctuations of ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ are at least as large as those of optimally and overdoped cuprates.

Figure 1

Frequency dependence of the microwave complex conductivity of ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ with thickness of (a) 30 nm, (b) 60 nm, (c) 116 nm, and (d) 150 nm. Inset shows the temperature dependence of the normalized resistivity of the films.

Figure 2

Results of the dynamical scaling analysis for ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ thin films with thickness of (a) 30 nm, (b) 60 nm, (c) 116 nm, and (d) 150 nm. Broken curves are those of 2D and 3D fluctuations calculated by Schmidt [36].

Figure 3

Temperature dependence of the scaling parameters ${\omega }_0$, ${\sigma }_0$, and their product ${\omega }_0{\sigma }_0$, for ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ thin films. The left panel shows those for 30- and 60-nm-thick films, and the right panel shows those for 116- and 150-nm-thick films. Solid and broken lines are theoretically expected behaviors with the corresponding notes.

Figure 4

Temperature dependence of the imaginary part of the complex conductivity ${\sigma }_2$ at 1 GHz in an ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ thin film with thickness of 60 nm. Inset shows the relative temperature variation of the phase of the complex reflection coefficient $S_{11}$ for the ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ film. Those of normal metals are also plotted.