Microwave Study of ${\rm FeSe}_{0.3}{\rm Te}_{0.7}$ Thin Film by ${\rm TE}_{011}$ -Mode Sapph

High quality epitaxial thin films of FeSe_1-xTe_x (x=0-1) have been successfully fabricated. Their superconducting transition temperatures are around 8-13 K. Microwave properties of a film (x=0.7) was studied by a sapphire dielectric cavity at 9.315 GHz. The cavity, which has a quality factor of 45000 in room temperature with TE₀₁₁-mode, is specially designed for the measurement of small samples with the sapphire cylinder having a small hole in the center. Thin film samples with dimension of 1-2 mm can be put in the middle of the hole, supported by a very thin sapphire rod. The cavity is sealed in a vacuum chamber soaked in the liquid ⁴He and the temperature of the thin sapphire rod (hence the sample) can be controlled from 1.6 K to 60 K with a stability about ±1 mK. Temperature dependence of transmission response and Q-factors were measured by a network analyser (Agilent N5230C). The results showed a clear signature of multi-gap superconductivity. No evidences of existence of node in the energy gap were found as the normalized change in the surface reactance and the corresponding normalized change in the in-plane penetration depth have flat dependence at low temperatures.