Terahertz conductivity of superconducting Beta-tungsten thin films*

Ultrathin films of β-tungsten host superconductivity with strong spin-orbit coupling (SOC). This non-equilibrium phase of tungsten has attracted considerable attention in recent years for spintronic applications due to its giant spin Hall effect and the potential promise of exotic superconductivity. After more than 60 years of its discovery, the superconductivity in this material is still not well understood. We investigate the superconducting order in β-Tunsgten using high precision time-domain THz transmission spectroscopy. Superconductivity is exhibited below 4.2 K for 70 nm film and the transition temperature reduces with increasing thickness. We study the frequency response of the complex conductivity of a thin film with a Tc of 4.2 K. In the superconducting state down to 1.6 K, the superfluid spectral weight developed is much less than the expected spectral weight for a BCS-type superconductor. This anomalous spectral weight is a possible indication of unconventional superconductivity in this system. The magnetic field response of the optical conductivity that can give additional information about the superconducting order parameter of this system, is currently under investigation.