Low-energy charge dynamics of infinite-layer nickelates: evidence for d-wave superconductivity

The discovery of superconductivity in infinite-layer nickelates establishes a new category of unconventional superconductors that shares structural and electronic similarities with cuprates. Despite the exciting advances, the key issues of the superconducting pairing symmetry, gap amplitude, superconducting fluctuation and collective modes remain elusive. In this talk, I will show how we utilize static and ultrafast terahertz spectroscopy to address these outstanding problems. We demonstrate that the equilibrium terahertz conductivity and nonequilibrium terahertz responses of an optimally Sr-doped nickelate film (Tc = 17 K) are in line with the electrodynamics of d-wave superconductivity in the dirty limit. The gap-to-Tc ratio is extracted to be 3.4, which indicates the superconductivity falls in the weak-coupling regime. In addition, we observed significant superconducting fluctuation near Tc. These results highlights a new d-wave system which closely resembles the electron-doped cuprates, expanding the family of unconventional superconductivity in oxides. Finally, we will present evidence for highly damped Higgs mode in infinite-layer nickelates.