MAGNETIC-FIELD PENETRATION DEPTH IN TL2BA2CUO6+DELTA IN THE OVERDOPED REGIME

THE magnetic field penetration depth lambda is a basic parameter of superconductivity, related to n(s)/m* (superconducting carrier density/effective mass) as lambda-2 is-proportional-to n(s)/m* in the limit where the coherence length xi is much shorter than the mean free path l (the 'clean limit'). Muon spin relaxation (muSR) measurements1,2 of lambda in high-transition-temperature (high-T(c)) copper oxide superconductors have revealed remarkable correlations between T(c) and n(s)/m*: T(c) increases linearly with n(s)/m* in the underdoped region, followed by a saturation with increasing carrier doping. Here we report muSR measurements of lambda in ceramic specimens of the superconductor Tl2Ba2CuO6+delta (Tl-2201) in the 'overdoped' region where T(c) decreases with increasing hole doping. Recent measurements of upper critical field and resistivity3 confirm that overdoped Tl-2201 lies well in the clean limit with xi << L We find that the muon spin relaxation rate sigma(T --> 0) is-proportional-to lambda-2 is-proportional-to n(s)/m* in Tl-2201 decreases with increasing hole doping, implying that either n(s) no longer scales with the normal-state carrier density n(n), and/or m* for a superconducting pair becomes much larger than the value expected from the normal-state effective mass m(n)*. This behaviour of overdoped Tl-2201 is in marked contrast to conventional metallic superconductors having a retarded interaction, in which the normal-state properties (n(n), m(n)*, l) directly represent the corresponding values in the superconducting state.