MAGNETIC-FIELD PENETRATION DEPTH IN TL2BA2CUO6+DELTA IN THE OVERDOPEDREGIME

THE magnetic field penetration depth lambda is a basic parameter of su perconductivity, related to n(s)/m (superconducting carrier density/e ffective mass) as lambda-2 is-proportional-to n(s)/m in the limit whe re the coherence length xi is much shorter than the mean free path l ( the 'clean limit'). Muon spin relaxation (muSR) measurements1,2 of lam bda in high-transition-temperature (high-T(c)) copper oxide supercondu ctors have revealed remarkable correlations between T(c) and n(s)/m: T(c) increases linearly with n(s)/m in the underdoped region, followe d by a saturation with increasing carrier doping. Here we report muSR measurements of lambda in ceramic specimens of the superconductor Tl2B a2CuO6+delta (Tl-2201) in the 'overdoped' region where T(c) decreases with increasing hole doping. Recent measurements of upper critical fie ld and resistivity3 confirm that overdoped Tl-2201 lies well in the cl ean limit with xi << L We find that the muon spin relaxation rate sigm a(T --> 0) is-proportional-to lambda-2 is-proportional-to n(s)/m in T l-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 expec ted from the normal-state effective mass m(n). This behaviour of over doped Tl-2201 is in marked contrast to conventional metallic supercond uctors having a retarded interaction, in which the normal-state proper ties (n(n), m(n), l) directly represent the corresponding values in t he superconducting state.