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 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 have performed muon spin relaxation measurements of the frustrated kagome lattice spin system SrCr8Ga4019. Our results demonstrate the slowing down of the Cr spin fluctuations when cooling toward the susceptibility-cusp temperature Tg 3 5 K The saturation of the relaxation rate below Tg, together with its weak dependence on longitudinal field (LF) between 0 and 2 kG, indicates the presence of dynamic spin fluctuations persisting even at T -100 mK without static order parameter. We propose a spin-liquid type ground state to explain an undecouplable Gaussian shape of the relaxation function observed at T~T , .

We investigated the optical spectrum of Ba(Fe1−xCox)2As2 single crystals with various doping levels. It is found that the low-energy optical conductivity spectrum of this system can be decomposed into two components: a sharp Drude term and a broad "incoherent" term. For the compounds showing magnetic order, a gap appears predominantly in the "incoherent" component, while an swave like superconducting gap opens in both components for highly doped compounds. The Drude weight steadily increases as doping proceeds, consistent with electron doping in this system. On the other hand, the "incoherent" spectral weight is almost doping independent, but its spectral feature is intimately connected with the magnetism. We demonstrate that the presence of two distinct components in the optical spectrum well explains the doping and temperature dependences of the dc resistivity.