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The anisotropy and temperature dependence of the magnetic-field penetration in superconducting UPt3 have been measured by muon spin relaxation. The extrapolated zero-temperature values for the penetration depths parallel and perpendicular to the c axis are Xw =7070 ± 30 A and X± =7820 ± 30 A, respectively. The temperature dependences of Xw and X± are different and can both be accounted for by a superconducting gap function with a line of nodes in the basal plane and axial point nodes.PACS numbers: 74.70.Tx, 74.60.Ec, 76.75.+i There are many experiments which give indirect evidence for anisotropic superconductivity in heavy-fermion systems. These fall into two categories, namely, measurements of transport coefficients or specific heat at low temperatures (70, 12 and measurements showing transitions between different superconducting states. 3,4 Experiments of the first type have shown that the temperature dependence of bulk properties of heavy-fermion superconductors follow power laws at low temperatures as opposed to the exponentially activated behavior characteristic of conventional superconductors. The absence of activated T dependence implies nodes in the gap functions, which can be due to either unconventional pairing in a clean sample or conventional pairing in the presence of magnetic impurities. 5 In spite of the great activity 5 on heavy-fermion superconductivity, the only experiment which by itself resolves this ambiguity in favor of unconventional pairing is a transverse ultrasound measurement on UPt3, 2 where the attenuation was found to rise (from 7=0) by Aa~~T and Aa~~T 3 for sound propagating in the basal plane with polarization parallel and perpendicular to the basal plane, respectively. For conventional gapless superconductors, Aa~ T 2 , independent of polarization direction. Ultrasound attenuation is a nonequilibrium property, and a prerequisite for its interpretation is a detailed understanding of the mean free path for thermally excited quasiparticles. In the present paper, we describe the first demonstration that a static property, namely, the magnetic penetration depth X, of a heavyfermion superconductor has an anisotropic T dependence. Previous measurements of X for several heavyfermion systems showed that X increased from its 7=0 value in proportion to T 2 , but did not address the issue of anisotropy. 6,7 The technique used here, muon spin re-laxation (n + SR), is unique in that it both gives absolute values for X and allows anisotropy to be measured in a single experimental run for a single face of the sample, with no mechanical disturbance of the apparatus. Furthermore, the 4.2-MeV muons used in this experiment penetrate ~ 70 jim into UPt3 and thus probe bulk superconducting material. The important results of our experiment are (i) that X(T=0) is consistent with the London formula where the effective mass is the renormalized (heavy) mass, and (ii) that the T dependence of X is anisotropic in a manner which implies that the superconducting gap function has a line of nodes in the basal p...

We report on transport and magnetization studies of the critical current in single crystal ErNi2B2C for applied fields below 4 kG. Below T approximately 2.5 K superconductivity coexists with weak ferromagnetism. We find that the critical currents are strongly enhanced for all field orientations in this ferromagnetic regime, corresponding to a threefold increase of the pinning force of the flux line lattice. We speculate that this increase is due to strong pair breaking by the ferromagnetism.

We have investigated vortex motion below the quasistatic melting transition in a detwinned YBaqCu307 q single crystal by measuring the differential resistance and the voltage noise. With the magnetic field parallel to the copper-oxygen planes and the large, flat surface of the sample, the noise power spectrum permits us to deduce that the surfaces are the dominant source of the noise and that the vortices appear to be fiowing in channels. In certain regimes, the noise power spectrum exhibits unusually sharp and distinct peaks which are periodic in frequency.PACS numbers: 74.60.GeIn a type-II superconductor, in the mixed state, the sample is penetrated by an array of magnetic vortices each of which contains a quantized amount of magnetic Aux. In an applied current, there will be a Lorentz force density f = J X B on the vortices, where 1 is the current density and B is the flux density [1]. If the vortices are at rest, there will be no electric field and the resistance