THE magnetic field penetration depth λ is a basic parameter of superconductivity, related to ns/m* (superconducting carrier density/effective mass) as λ-2 ∝ ns/m* in the limit where the coherence length ζ is much shorter than the mean free path l (the 'clean limit'). Muon spin relaxation (µSR) measurements1,2 of λ in high-transition-temperature (high-Tc ) copper oxide superconductors have revealed remarkable correlations between Tc and ns/m*: Tc increases linearly with ns/m* in the underdoped region, followed by a saturation with increasing carrier doping. Here we report µSR measurements of λ in ceramic specimens of of the superconductor TI-2Ba2CuO6+δ (TI-2201) in the 'overdoped' region where Tc decreases with increasing hole doping. Recent measurements of upper critical field and resistivity3confirm that overdoped TI-2201 lies well in the clean limit with a ζ<<l. We find that the muon spin relaxation rate σ(T → 0) ∝ λ −2 ∝ ns/m* in TI-2201 decreases with increasing hole doping, implying that either ns no longer scales with the normal-state carrier density nn , 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 TI-2201 is in marked contrast to conventional metallic superconductors having a retarded interaction, in which the normal-state properties (nn , m*n , l)directly represent the corresponding values in the superconducting state.
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Uemura, Y., Keren, A., Le, L. et al. Magnetic-field penetration depth in TI2Ba2CuO6+δ in the overdoped regime. Nature 364, 605–607 (1993). https://doi.org/10.1038/364605a0
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