Overdoped high-temperature cuprate superconductors have often been understood within the standard BCS framework of superconductivity. However, measurements in a variety of overdoped cuprates indicate that the superfluid density is much smaller than expected and decreases smoothly to zero as the doping is increased. In this talk, I will discuss how we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on a number of as-grown and ion-irradiated overdoped La_2−xSr_xCuO₄ films to determine the total, superfluid, and uncondensed spectral weight as a function of doping and disorder. We find that a significant fraction of the carriers remains uncondensed in a wide Drude-like peak as the temperature goes to zero while the superfluid density remains linear-in-temperature. Increasing disorder causes only a small change in the superconducting transition temperature for the overdoped films, but the changes in the superfluid density are much larger. I will discuss our findings in the context of existing theories of pair-breaking in a dirty d-wave superconductor, intrinsic electronic inhomogeneity, and the possible presence of phase fluctuations. I will also briefly highlight how new THz techniques based on non-linear response might be able to shed light onto the anomalous nature of the superfluid density in overdoped cuprates.

References:

Fahad Mahmood, Xi He, Ivan Božovic, and N. P. Armitage. Phys. Rev. Lett. 122, 027003 (2019)

Fahad Mahmood, David Ingram, Xi He, J. A. Clayhold, Ivan Božovic, and N. P. Armitage. Phys. Rev. B 105, 174501 (2022)