The thermal conductivity of YBa2Cu3O6.9 was measured at low temperatures in untwinned single crystals with concentrations of Zn impurities from 0 to 3% of Cu. A linear term κ0/T = 0.19 mW K −2 cm −1 is clearly resolved as T → 0, and found to be virtually independent of Zn concentration. The existence of this residual normal fluid strongly validates the basic theory of transport in unconventional superconductors. Moreover, the observed universal behavior is in quantitative agreement with calculations for a gap function of d-wave symmetry.

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The thermal conductivity of the heavy-fermion superconductor UPt 3 was measured down to T c /10, along the b and c axis of a single crystal. The anisotropy ratio c / b as T→0 is shown to be a powerful new probe of the gap structure in this compound. Our observation of a nonvanishing value for this ratio establishes a property of the gap in the B phase: the presence of nodes along the c axis. Furthermore, recent calculations by Fledderjohann and Hirschfeld strongly suggest these cannot be point nodes with a linear k dependence.

A neutron-diffraction experiment was performed to investigate the effect of a magnetic field on the antiferromagnetic order in the heavy-fermion superconductor UPt 3 . Our results show that a field in the basal plane of up to 3.2 T, higher than H c2 ͑0͒, has no effect: it can neither select a domain nor rotate the moment. This has a direct impact on current theories for the superconducting phase diagram based on a coupling to the magnetic order.

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