Aluminium impurities in single crystals of the heavy-fermion superconductor UBe₁₃

We studied the influence of Al incorporation on the heavy fermion superconductor UBe₁₃ to explain the sample dependence of physical properties. Al which substitutes Be in the crystal structure of flux-grown UBe₁₃ single crystals can be detected by combining X-ray diffraction, nuclear magnetic resonance and X-ray spectroscopy. The minute amounts of Al (1-2 at. %) in the structure are located by atomic resolution transmission electron microscopy. Specific heat measurements complement this study and reveal strong influence of the incorporated Al on the physical properties of UBe₁₃.