Locating the missing superconducting electrons in overdoped cuprates

Understand

Overdoped high-temperature cuprate superconductors have been widely believed to be described by the physics of d-wave BCS-like superconductivity.

• However, recent measurements indicate that as the doping is increased, the superfluid density decreases smoothly to zero rather than increasing as expected by BCS theory in the absence of disorder.
• Here, we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on the same overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ films to determine both the superfluid and the uncondensed carrier density as a function of doping.
• A significant fraction of the carriers remains uncondensed in a wide Drude-like peak even as $T\rightarrow0$, which, when taken with the linear-in-temperature superfluid density, is inconsistent with existing theories for the role of disorder in suppressing the superfluid density in a d-wave superconductor.