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We report the first measurements of the magnetic-field penetration depth )~ in the heavy electron superconductor UBe13, performed using a SQUID magnetometer. We find the temperature dependence of 2(T)-2(0) to follow a T 2 law at low temperatures, giving further evidence of extreme gap anisotropy in this compound. We calculate the temperature dependence expected for a variety of anisotropic states, including those representing certain classes of "exotic" pairing. In general situations, the supercurrent is not parallel to the vector potential, and a more complicated field penetration takes the place of the normal Meissner effect. We argue that the data are consistent with an energy gap with point nodes on the Fermi surface but inconsistent with the large value of the Landau parameter F~ expected for a translationally invariant Fermi liquid with large effective mass.

Ferromagnetism, antiferromagnetism, or superconductivity has been discovered in most hexa- and dodecaborides.

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tration possesses the same activation enerqy a11 the conductance . When the Hall mobility arises from an excitation process the logarithmic corrections are modified by the lifetime which can be varied simply by varying the Fermi energy. *Sul:lnitted by v . Heine

The magnetic properties of all known sulfides, selenides, and tellurides of Pr and Tm (as representative of the light and heavy rare earths, respectively) have been studied. In most cases, Pr and Tm compounds exhibit Van Vleck paramgnetism at low temperatures owing to crystal-field singlet ground states. Splittings have been derived in several cases by specific-heat measurements. In the case of Pr, X4 (X = S, Se,Te) compounds, specific-heat and susceptibility measurements reveal exchange-induced ferromagnetism. This is further supported by the study of the magnetic phase diagrams of Pr, Se4-Pr, Se3, La3Se4-Pr3Se4, and of superconductivity in the (La, "Pr"), (S4, Se4, Te4) series.Superconductivity persists to very high Pr concentrations, as in the Lal "Pr"Sesystem, indicating a nonmagnetic ground state of Pr. The higher chalcogenides appear to exhibit in general semimetallic or semiconducting behavior, while most of the metallic La compounds are usually superconductors.

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