Locating the missing superconducting electrons in overdoped cuprates

TL;DR

This study uses advanced spectroscopy and inductance measurements to reveal that in overdoped cuprates, a large fraction of carriers remain uncondensed, challenging existing theories of superfluid suppression and suggesting quantum phase fluctuations.

Contribution

It provides the first comprehensive frequency range analysis of superfluid and uncondensed carriers in overdoped cuprates, revealing discrepancies with current theories.

Findings

Significant uncondensed carrier density persists at low temperatures.

Superfluid density decreases smoothly to zero with doping, contrary to BCS expectations.

Evidence suggests quantum phase fluctuations near critical doping.

Abstract

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. Our almost eight orders of magnitude in measurement frequency range gives us a unique look at the low frequency spectral weight distribution, which may suggest the presence of quantum phase fluctuations as the critical doping is approached.