  The spontaneous expulsion of applied magnetic field, the Meissner effect, is
a defining feature of superconductors; in Type-II superconductors above the
lower critical field, this screening takes the form of a lattice of magnetic
flux vortices. Using implanted spin-1/2 positive muons, one can measure the
vortex lattice field distribution through the spin precession and deduce key
parameters of the superconducting ground state, and thereby fundamental
properties of the superconducting pairing. Muon spin rotation/relaxation
($\mu$SR) experiments have indeed revealed much interesting physics in the
underdoped cuprates, where superconductivity is closely related to, or
coexistent with, disordered or fluctuating magnetic and charge excitations.
Such complications should be absent in overdoped cuprates, which are believed
to exhibit conventional Fermi liquid behaviour. These first transverse field
(TF)-$\mu^+$SR experiments on heavily-overdoped single crystals reveal a
superfluid density exhibiting a clear inflection point near 0.5$T_c$, with a
striking doping-independent scaling. This reflects hitherto unrecognized
physics intrinsic to $d$-wave vortices, evidently generic to the cuprates, and
may offer fundamentally new insights into their still-mysterious
superconductivity.
