Effect of radiation-induced defects on the superfluid density and optical conductivity of overdoped La$_{2-x}$Sr$_x$CuO$_4$

TL;DR

This study investigates how radiation-induced defects affect the superfluid density and optical conductivity in overdoped La$_{2-x}$Sr$_x$CuO$_4$ films, revealing that disorder significantly impacts superfluid density more than transition temperature.

Contribution

It provides experimental insights into the effects of controlled disorder on superfluid density and electrodynamic response in overdoped cuprate superconductors, challenging existing pair-breaking theories.

Findings

Transport scattering rate increases linearly with radiation dose.

Superfluid density decreases significantly with disorder, while $T_c$ remains relatively stable.

Results are inconsistent with traditional Abrikosov-Gorkov pair-breaking predictions.

Abstract

Using a combination of time-domain THz spectroscopy (TDTS) and mutual inductance measurements, we have investigated the low-energy electrodynamic response of overdoped La$_{2-x}$Sr$_x$CuO$_4$ films that have been exposed to ion irradiation. Films went through three rounds of irradiation (2, 4, and 6 $\times 10^{13}$ ions/cm$^2$) 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 $T_c$, the superfluid density, and the scattering rates are 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 to the $\omega \sim 0$ superfluid density are much larger.