Terahertz spectroscopy and polarimetry of unconventional superconductors

Superconductors are naturally well suited for spectroscopic studies in the terahertz (THz) range, as their gap energies are of comparable order to THz photon energies, meV. In this talk I will present work on two unconventional superconductors, FeTe1-xSex (FTS) and a BiNi bilayer. Firstly, using time-domain terahertz spectroscopy I elucidate the low temperature conducting behavior of two FTS samples. Constrained with DC resistivity, I find two conduction channels that add in parallel, one which has a scattering rate that goes as the Planckian limited rate, ∼kT/h. Through analysis of its spectral weight, I show the superconducting condensate is mainly drawn from this channel that undergoes Planckian scattering. Secondly, implementing a novel design for high-precision THz polarimeter, I find a finite Kerr rotation, the smallest rotation measured in THz frequencies to date, in the superconducting state of BiNi at zero magnetic field. This is indicative of a possible time reversal symmetry breaking (TRSB) order parameter. Using general Kramers-Kronig relations one can relate the Kerr rotation at high frequencies, i.e. optical probes, to rotations at low frequency, i.e. THz. With this relation I propose a possible scenario in which this TRSB is able to manifest.

1. FTS, https://arxiv.org/abs/2505.00623
2. High precision polarimeter, Zhenisbek Tagay, Ralph Romero III, and N. P. Armitage, "High-precision measurements of terahertz polarization states with a fiber coupled time-domain THz spectrometer," Opt. Express 32, 15946-15954 (2024)