We present highly sensitive Hall effect measurements of the heavy fermion compound CeCoIn5 down to temperatures of 55 mK. A pronounced dip in the differential Hall coefficient | partial differential rho(xy)/ partial differential H| at low temperature and above the upper critical field of superconductivity, H(c2), is attributed to critical spin fluctuations associated with the departure from Landau Fermi liquid behavior. This identification is strongly supported by a systematic suppression of this feature at elevated pressures. The resulting crossover line in the field-temperature phase diagram favors a field induced quantum critical point at mu(0)H(qc) approximately 4.1 T below H(c2)(T=0) suggesting related, yet separate, critical fields.

A polycrystalline sample of the MgB 2 superconductor was investigated by measurements of the electrical resistivity, the thermopower and the thermal conductivity in the temperature range between 1.8K and 300K in zero magnetic field. The electrical resistivity shows a superconducting transition at Tc = 38.7K and, similarly to borocarbides, a T 2.4 behaviour up to 200K. The electron diffusion thermopower and its bandstructure-derived value indicate the dominant hole character of the charge carriers. The total thermopower can be explained by the diffusion term renormalized by a significant electronphonon interaction and a phonon drag term. In the thermal conductivity, for decreasing temperature, a significant decrease below Tc is observed resulting in a T 3 behaviour below 7K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides.

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The temperature and magnetic-field dependence of the specific heat cp(T, H) in the superconducting (sc) mixed state as well as the upper critical field Hc2(T ) have been measured for polycrystalline YxLu1−xNi2B2C and Y(Ni1−yPty)2B2C samples. The linear-in-T electronic specific-heat contribution γ(H) • T exhibits significant deviations from the usual linear-in-H law resulting in a disorder-dependent negative curvature of γ(H). The Hc2(T ) data point to the quasi-clean limit for (Y, Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni, Pt)-substitutions. The γ(H)-dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of γ(H) data only, d-wave pairing cannot be ruled out.

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