| Summary: | Heavy fermion (HF) compounds are well known for their unique properties, such as narrow bandwidths, loss of coherence in a metal, non-Fermi-liquid behaviour, unconventional superconductivity, huge magnetoresistance etc. While these materials have been known since the 1970s, there is still considerable uncertainty regarding the fundamental mechanisms responsible for some of these features. Here we report transverse-field muon spin rotation ( μ ^+ SR) experiments on the canonical HF compound UBe _13 in the temperature range from 0.025 to 300 K and in magnetic fields up to 7 T. The μ ^+ SR spectra exhibit a sharp anomaly at 180 K. We present a simple explanation of the experimental findings identifying this anomaly with a gap in the spin excitation spectrum of f -electrons opening near 180 K. It is consistent with anomalies discovered in heat capacity, NMR and optical conductivity measurements of UBe _13 , as well as with the new resistivity data presented here. The proposed physical picture may explain several long-standing mysteries of UBe _13 (as well as other HF systems).
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