Gapless superconductivity observed in Fe(Te,Se) thin films and heterostructures

Using time-domain terahertz spectroscopy, we observe a gapless superconducting state in Fe(Te,Se) epitaxial thin films and in Fe(Te,Se) heterostructures with Bi₂Te₃ and MnTe grown by hybrid symmetry epitaxy. Clear 1/ω behavior arises in the imaginary conductance below the superconducting transition; however, there is no concomitant suppression of the real conductance, indicating a gapless superconducting state. Furthermore, a sharp, low-frequency peak in the real conductance emerges alongside the superconductivity. We model these anomalous features of the superconducting state to determine the origin of this novel behavior.