Tracking aluminium impurities in single crystals of the heavy-fermion superconductor UBe13

The influence of Al incorporation on the heavy fermion superconductor UBe13 was investigated to explain the sample dependence of physical properties. Clear evidence for incorporated Al in flux-grown UBe13 single crystals is presented by results from X-ray diffraction, nuclear magnetic resonance and X-ray spectroscopy. The increase of the lattice parameter and the concomitant change of the superconducting properties are caused by substitution of Be in the compound by 1-2 at.% Al. The minute amounts of Al in the structure were located by atomic resolution transmission electron microscopy. Specific heat measurements reveal the strong influence of incorporated Al on the physical properties of UBe13. Upon long-term annealing, Al incorporated in single crystals can leave the structure, restoring properties of Al-free polycrystalline UBe13.