Effect of radiation-induced defects on the superfluid density and optical conductivity of overdoped La2-x Srx CuO4

Using a combination of time-domain terahertz (THz) spectroscopy (TDTS) and mutual inductance measurements, we have investigated the low-energy electrodynamic response of overdoped La2-xSrxCuO4 films that have been exposed to ion irradiation. Films went through three rounds of irradiation (2, 4, and 6×1013ions/cm2), and mutual inductance and TDTS experiments were performed between each step. Together with the as-grown film, this gives four different levels of disorder. The transport scattering rate that is measured directly in the THz experiments is an approximately linear function of the radiation dose at all temperatures. This is consistent with a proportionate increase in elastic scattering. In the superconducting state we find that the relation between Tc, the superfluid density, and the scattering rates is quantitatively at odds with the predictions based on the extant theory of Abrikosov-Gorkov-like pair breaking in a dirty d-wave superconductor. Increasing disorder causes only a small change in the superconducting transition temperature for the overdoped films, but the changes in the ω∼0 superfluid density are much larger.