Terahertz Time-Domain Spectroscopy in FeSe thin films

FeSe is one of the simplest crystal structures of iron-based superconductors, and exhibits remarkable control of its critical temperature, as well as displays competition of magnetic and superconducting order in its ground state. In addition to possible applications, the system offers great potential for experiments in the area of strongly correlated systems and understanding of unconventional superconductivity.

Previous studies have revealed a dependence of Tc on pressure, with compressive strain increasing Tc up to 37 K from the bulk value of 8 K. In addition, tensile strain has been shown to decrease Tc in this compound, and completely suppress it down to Tc = 200 mK.

In this study, we use broadband terahertz time-domain spectroscopy to probe the superconductivity of FeSe thin films grown by pulsed laser deposition on MgO, and characterized by X-ray diffraction and photoelectron spectroscopy with depth profiling. In our measurements, we evaluate the electrical conductivity down to T = 200 mK. Our work may provide insight into the viability of this and similar compounds for optical study in low temperature environments.