Connections between magnetism and superconductivity in UBe13 doped with thorium or boron (invited)

Magnetism and superconductivity appear to be intimately connected in the heavy electron (HE) superconductors. For example, it has been conjectured but not proven that the exchange of antiferromagnetic spin fluctuations are responsible for pairing in HE superconductors. In this paper we review recent results in U1-xThxBe13, where specific heat, lower critical field, and zero-field muon-spin-resonance (μSR) measurements reveal another second-order phase transition (below the superconducting transition) to a state which possesses small-moment magnetic correlations for 0.019≤x≤0.043. We present a new phase diagram for (U,Th)Be13 which indicates that the superconducting and magnetic order parameters are closely coupled. A discussion of the nature of the lower phase is presented, including the consideration of a possible magnetic (time-reversal-violating) superconducting state. When UBe13 is doped with B (UBe12.97B0.03) the Kondo temperature is decreased and the specific heat jump at the superconducting transition temperature is significantly enhanced. However, μSR measurements reveal no magnetic signature in UBe12.97B0.03, unlike the case for Th doping. The correlation between changes in the Kondo temperature and changes in the superconducting properties induced by B doping provide evidence for the importance of magnetic excitations in the superconducting pairing interaction in UBe13