We have performed transverse-field muon spin relaxation measurements of the Zn-substituted cuprate high-T c superconductors: La 22x Sr x ͑Cu 12y Zn y ͒O 4 and YBa 2 ͑Cu 12y Zn y ͒ 3 O 6.63 . The superconducting carrier density͞effective mass n s ͞m ‫ء‬ ratio at T ! 0 decreases with increasing Zn concentration, in a manner consistent with our "swiss cheese" model in which charge carriers within an area pj 2 ab around each Zn are excluded from the superfluid. We discuss this result in the context of Bose condensation, pair localization, and pair breaking. [S0031-9007(96)02011-X]

We report elastic neutron diffraction and muon spin relaxation (µSR) measurements of the quasi one-dimensional antiferromagnets Sr2CuO3 and Ca2CuO3, which have extraordinarily reduced TN/J ratios.We observe almost resolution-limited antiferromagnetic Bragg reflections in Sr2CuO3 and obtain a reduced ordered moment size of ∼0.06µB. We find that the ratio of ordered moment size µ(Ca2CuO3)/µ(Sr2CuO3) = 1.5(1) roughly scales with their Néel temperatures, which suggests that the ordered moment size of quasi one-dimensional antiferromagnets decreases continuously in the limit of vanishing inter-chain interactions.PACS numbers: 76.75.+i, 75.25.+z, 75.10.Jm One-dimensional spin systems with antiferromagnetic interactions have received considerable attention because of their pronounced quantum mechanical effects. In the absence of inter-chain interactions, both integer and halfodd integer spin-chain systems have spin-singlet ground states, rather than an antiferromagnetically ordered Néel state [1][2][3]. Yet, for half odd-integer spin-chains, the spin-excitations are gap-less at momentum k = 0 and π [4]; this indicates that the ground state of a half-odd integer spin-chain is closer to the Néel ordered state than the integer spin systems, which have a so-called Haldane gap [3].Because of the gap-less feature of half-odd integer spinchains, one interesting question is whether the ground state is ordered or disordered when inter-chain interactions are introduced. Previously, it was proposed that there is a non-zero critical coupling ratio (J ′ /J = R c ), below which the system retains a singlet ground-state [5]. Recent renormalization group calculations however suggest that the ground state may depend on microscopic details of the model which describes the spin-spin interactions [6,7]. Numerical studies of the Heisenberg model suggested a vanishing critical coupling ratio (R c ∼ 0); namely, for infinitesimally small inter-chain couplings, half odd-integer spin-chains should exhibit Néel order [7].Experimentally, KCuF 3 is the most investigated quasione-dimensional S=1/2 antiferromagnet. Unfortunately, this material has relatively large coupling ratio R = J ′ /J ∼ 2 K/203 K = 1.0 × 10 −2 , as shown from neutron inelastic scattering measurements [8]. Probably reflecting the large coupling ratio R, the T N /J ratio (∼ 39 K/203 K = 0.2) and the ordered moment size (= 0.49(7)µ B [9]) were also found to be relatively large.To investigate the regime of the critical coupling ratio, model materials with smaller inter-chain couplings are needed; the quasi one-dimensional S=1/2 antiferromagnets Sr 2 CuO 3 and Ca 2 CuO 3 are suitable candidates. The intra-chain interaction (2J ∼ 2600 K) of these materials have been estimated from susceptibility [10,11] and infrared light absorption [12]. Néel ordering of these compounds was first observed in µSR measurements [13], with a significantly reduced T N /J ratio of ∼ 5 K/1300 K = 4 × 10 −3 for Sr 2 CuO 3 and T N /J ∼ 11 K/1300 K = 8 × 10 −3 for Ca 2 CuO 3 . Since T N /J is a measure of ...

We present zero-field muon spin relaxation ͑SR͒ measurements of La 1.6Ϫx Nd 0.4 Sr x CuO 4 with xϭ0.125,0.15,0.2; La 1.475 Nd 0.4 Ba 0.125 CuO 4 , La 1.875 Ba 0.125 CuO 4 , and La 1.875 Ba 0.125Ϫy Sr y CuO 4 with y ϭ0.025,0.065. All of the samples with dopant concentrations xϩyр0.15 show similar static magnetic order with coherent precession of the muon spins below T N Ϸ30 K, with a T→0 ordered Cu moment Ϸ0.3 B . The samples with xϭ0.20 show no coherent precession but manifest two distinct relaxation regimes, typical of quasistatic magnetism. We then present transverse-field SR hysteresis measurements of the La 1.45 Nd 0.4 Sr 0.15 CuO 4 and La 1.4 Nd 0.4 Sr 0.2 CuO 4 systems that show a large superconducting response below approximately 7 K and 12 K, respectively. We argue that superconductivity and magnetic order coexist in the xϭ0.15 system.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the AFe 2 As 2 ͑A =Ca,Ba,Sr͒ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of ͑Ba 0.5 K 0.5 ͒Fe 2 As 2 exhibits a nearly linear variation in T at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.

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