Why the superfluid density collapses in overdoped cuprates
In plain words
Adding holes beyond the optimum lowers the superconducting temperature, and the density of paired electrons falls in step even though the metal looks more ordinary; disorder and new physics are both proposed as the cause.
Precise statement
In overdoped La2-xSrxCuO4 films the zero-temperature phase stiffness rho_s0 is proportional to $T_c$ and far below the BCS value expected from the normal-state Drude weight (2016). Determine whether d-wave BCS theory with realistic (forward and point) disorder accounts quantitatively for rho_s0(p), the T-linear $\rho_s(T)$, and the residual uncondensed Drude weight seen in terahertz optics, or whether intrinsic inhomogeneity or a different pairing state is required.
What would settle it
Optical, penetration-depth and tunneling data on the same overdoped films compared with disorder-averaged Bogoliubov-de Gennes calculations using independently measured scattering rates.
Status in the literature
Dirty d-wave calculations (2017 onward) reproduce several trends, while optical data showing a large uncondensed fraction are cited against them.