Tracking aluminum impurities in the heavy-fermion superconductor UBe13

16. Juli 2018

Alfred Amon, Iryna Zelenina, Paul Simon, Matej Bobnar, Marcel Naumann, Eteri Svanidze, Frank Arnold, Horst Borrmann, Ulrich Burkhardt, Walter Schnelle, Elena Hassinger, Andreas Leithe-Jasper, and Yuri Grin*

Figure 1. Incorporated aluminum atoms (red) can leave the crystal structure of UBe13-xAlx upon annealing, as shown by WDX spectroscopy and the change in low-temperature specific heat.

The heavy fermion superconductor UBe13 has defied the attempts by the physics community to clarify the true nature of its ground state for several decades. The highest reported electronic specific heat coefficient among U-based heavy fermion compounds and a large specific heat anomaly at the superconducting transition are indicative of the observed unconventional behavior. The investigations of single-crystals and polycrystalline samples of UBe13 are ridden by a strong variation of the physical properties. Scientists from the Max-Planck-Institute for Chemical Physics of Solids were now able to reveal the mechanism behind this sample dependency.
 
 

Figure 2. High-resolution transmission electron microscopy of UBe13 single-crystals in as-grown (a) and annealed (b) condition. Incorporated Al atoms highlighted by red circles.

By detailed measurements of lattice parameters, X-ray spectroscopy, nuclear magnetic resonance and X-ray diffraction, it was shown that during growth of UBe13 single-crystals, Al atoms of the employed aluminum flux are incorporated in the crystal structure of UBe13. The Al impurities expand the crystal lattice of UBe13-xAlx and lower the critical temperature of the superconducting transition. Upon long-term annealing, aluminum can leave the crystal structure and the physical properties of Al-free UBe13 are restored (Fig. 1). The incorporated Al atoms were located by high-resolution transmission electron microscopy and found to replace Be in the crystal structure (Fig. 2).

Revealing the reason for the sample dependency in UBe13 is one step closer to a final understanding of this peculiar compound.

/AA /YG

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