In addition to its importance for existing and potential applications, superconductivity [1] is one of the most interesting phenomena in condensed matter physics.Although most superconducting materials are well-described in the context of the Bardeen Cooper and Schrieffer (BCS) theory [2], considerable effort has been devoted to the search for exotic systems whose novel properties cannot be described by the BCS theory. Conventional superconductors break only gauge symmetry by selecting a definite phase for the Cooper pair wavefunction; a signature of an unconventional superconducting state is the breaking of additional symmetries [3].Evidence for such broken symmetries include anisotropic pairing (such as d-wave in the high-T c cuprates) and the presence of multiple superconducting phases (UPt 3 and superfluid 3 He[4]). We have performed muon spin relaxation measurements of Sr 2 RuO 4 and observe a spontaneous internal magnetic field appearing below T c . Our measurements indicate that the superconducting state in Sr 2 RuO 4 is characterized by broken time reversal symmetry which, when combined with symmetry considerations indicate that its superconductivity is of p-wave (odd-parity) type, analagous to superfluid 3 He. Despite the structural similarity with the high T c cuprates, the origin of the unconventional superconductivity in Sr 2 RuO 4 is fundamentally different in nature.Sr 2 RuO 4 , which is isostructural to the high-T c cuprate La 1.85 Sr 0.15 CuO 4 , is to date the only known layered perovskite superconductor which does not contain copper. Although first synthesized in the 50's, [5] its superconductivity was only found in 1994[6]; T c 's of early samples were roughly 0.7 K but have increased to T c = 1.5 K in recent high quality single crystals [7]. Despite its low transition temperature, Sr 2 RuO 4 is of great interest as there is growing evidence for an unconventional superconducting state. In this system, strong correlation effects enhance the effective mass seen in quantum oscillation [8] and Pauli spin susceptibility measurements, in the same way as in 3 He [9]. Combining this feature with Sr 2 RuO 4 's expected tendency to display ferromagnetic spin fluctuations, Rice and Sigrist [10], and later Baskaran [11] argued that the pairing in Sr 2 RuO 4 could be of odd parity (spin triplet) type.The strong suppression of the superconducting T c by even non-magnetic impurities suggests non-s-wave pairing [7]. Specific heat [12] and NMR 1/T 1 [13] measurements indicate the presence of a large residual density of states (RDOS) at low temperatures (well within the superconducting state); in high quality samples, this RDOS as T→ 0 seems to approach half of the normal state value. Several authors [14,15] have proposed so-called non-unitary p-wave superconducting states for Sr 2 RuO 4 to account for this RDOS as well as the absence of a Hebel-Slichter peak in NMR measurements [13]. A finite RDOS is not a unique signature of unconventional superconductivity; for example it is observed in so-called gapless sup...

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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 ...