The muon spin depolarization rate a was measured in overdoped Tl2Ba2Cu06+s. o(T-+ 0) was found to decrease proportional to the superconducting transition temperature T c as doping 8 is increased. In the framework of the clean-limit London model, o(0)~~X~2~-n s /m*, this implies that the depression of T c by overdoping is associated with a decrease of the superconducting condensate density n s in spite of the increasing normal-state carrier density. This can be largely accounted for in terms of strong pair breaking, which depresses both the condensate density and T c with increased doping.PACS numbers: 74.72. Dn, 74.72.Fq, 76.75.+i The muon-spin-rotation (//SR) technique provides a powerful tool to measure the magnetic penetration depth X in type II superconductors [1][2][3][4][5]. X is derived from the muon spin depolarization rate o~~X~2 which reflects the field distribution in the vortex state in a high external magnetic field. In the clean-limit London model, X~2 is determined essentially by the superconducting condensate density n s divided by the effective mass ra*. The remarkable result of previous juSR experiments was that, in the low-doping regime, the data appeared to trace a common line in a plot of T c versus do [a(T-+ 0)] (see dashed line in Fig. 1). Close to optimum doping, where n s /m* becomes large, T c deviates from this line, showing saturation followed by a slight decline with increasing n s /m*. This experimental finding was taken as evidence for a high-energy-scale pairing mechanism (Epairing^>Ef) and suggests a picture of real-space paired bosons, which Bose condense in a common state at T c [6]. Such a model is able to explain the linear growth of T c with increasing carrier concentration.In this Letter we report the first systematic //SR measurements for the heavily overdoped region. We show that n s /m* falls to zero as T c is depressed in overdoped specimens in which the normal-state charge-carrier concentration increases as T c decreases. The Tl2Ba2Cu06+2 planes by the excess oxygen incorporated into the TI2O2 bilayers [7,8]. Three sintered polycrystalline Tl-2201 disks (16 mm in diameter and 2 mm thick) were prepared with transition temperatures of 84, 53, and 13 K by annealing at various oxygen partial pressures and temperatures then quenching into liquid ni-1764 0031 -9007/93/71 (11 )/l 764(4)$06.00

Muon spin relaxation studies on a variety of polycrystalline YBa2Cu30», Y2Ba&Cu70» &, and YBa2Cu40, samples show that the 8-dependent depolarization rate (and hence n, /m") is greatly enhanced when the Cuo chains are free of disorder (8 0). Ca substitution allows the doping on the planes and chains to be independently controlled and the enhanced depolarization rate can be unambiguously attributed to condensation of mobile carriers on the chains as Cooper pairs, thus increasing the total condensate density n, . The in-plane London penetration depth is highly anisotropic: = 155 nm but Ab falls as low as 80 nm when 6 0.PACS numbers: 74.25.Ha, 74.62.Dh, 74.72.8k, 76.75.+i In contrast to all other high-T, superconducting cuprates (HTSC), the compound YBa2Cu307 s (1-2-3), together with its homologs 2-4-7 and 1-2-4, has linear Cu-0 chains additional to the Cu02 planes. These chains have a relatively high carrier density and a metallic character as evidenced by nuclear quadrupole resonance relaxation times [1], field-dependent heat capacity [2], infrared conductivity [3], and dc transport properties [4,5]. From muon spin relaxation (p, SR) data we show that, when these chains are free of disorder, the superconducting condensate density originating in the Cu02 planes is greatly enhanced due to the additional carriers on these chains condensing as Cooper pairs. That is, the CuQ chains become superconducting by proximity to the intrinsically superconducting planes providing significant enhancement in the condensate density, condensate energy, in-plane anisotropy, interlayer coupling, and flux pinning. Moreover, we argue that pair breaking on oxygen-deficient chains could provide the low-energy excitations deduced from several studies, without having to invoke d-wave symmetry.The HTSC are characterized by a generic phase behavior in which the transition temperature T, has an approximately parabolic dependence on hole concentration p, rising to a maximum T, " then falling on the overdoped side. In La2 Sr, Cu04 (2-1-4) the superconducting phase curve is conveniently approximated by [6] T,/T, '" = 1 -82. 6(p -0.16),and there is some evidence that this also well describes other, if not all, HTSC [5 -7]. Recently we have proposed from p, SR [8) and heat capacity [9] data that the decline in T"condensate density, and pair density in the overdoped region is due to pair-breaking interactions which reduce the lifetime of superconducting pairs. The p-dependent condensate density can be measured from the muon spin n, (0) = 0 5n/(1 + be. /Zvb),(3) and the curve in Fig. 1(a) is obtained by combining Eqs. (1) and (3).The reflex loop shown in Fig. 1(a) is the behavior to be expected for all HTSC with Cuo, planes only. It has been confirmed for several other systems [11 -13]. For 1-2-3, depolarization rate o, which is related to the London penetration depth AL as follows: tr~AL2 = poe'n, /m' Here n, is the superconducting condensate density and

We summarize our results of transverse-field muon-spin-relaxation (TF-pSR) experiments on a variety of polycrystalline cuprate high-T, superconducting systems. In this paper we present the conditions under which the @SR depolarization rate at low temperatures o. o~A , ,b (0)~n, (0)/m, & is determined in a unique way by the doping state of Cu02 planes and the consequent critical temperature T, . Disorder and pinning e6'ects due to various sorts and amounts of dopant atoms do not a6'ect era. From our results on YBa2Cu307 z, YBa2Cu40&, and Y2Ba4Cu70» z (systems having both planes and chains) we conclude that a superconducting condensate is not only formed in the Cu02 planes but is induced additionally in the CuO chains. This chain condensate is rapidly suppressed by any disorder in the chains and depends strongly on the oxygen ordering.

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