Superconductivity and quantum criticality in the heavy-fermion system β-YbAlB4

Abstract

A long-sought ytterbium-based heavy-fermion superconductor—a hole analogue of the cerium-based systems—has been found. Moreover, there is evidence for a quantum critical point at ambient conditions and without chemical doping. A long-standing question in the field of superconductivity is whether pairing of electrons can arise in some cases as a result of magnetic interactions instead of electron–phonon-induced interactions as in the conventional Bardeen–Cooper–Schrieffer theory1. A major challenge to the idea of magnetically mediated superconductivity has been the dramatically different behaviour of the cerium and ytterbium heavy-fermion compounds. The cerium-based systems are often found to be superconducting1,2,3,4,5,6, in keeping with a magnetic pairing scenario, but corresponding ytterbium systems, or hole analogues of the cerium systems, are not. Despite searches over two decades there has been no evidence of heavy-fermion superconductivity in an ytterbium system, casting doubt on our understanding of the electron–hole parallelism between the cerium and the ytterbium compounds. Here we present the first empirical evidence that superconductivity is indeed possible in an ytterbium-based heavy-fermion system. In particular, we observe a superconducting transition at Tc=80 mK in high-purity single crystals of YbAlB4 in the new structural β phase7. We also observe a novel type of non-Fermi-liquid state above Tc that arises without chemical doping, in zero applied magnetic field and at ambient pressure, establishing β-YbAlB4 as a unique system showing quantum criticality without external tuning.

Cite this article

Nakatsuji, S., Kuga, K., Machida, Y. et al. Superconductivity and quantum criticality in the heavy-fermion system β-YbAlB₄. Nature Phys 4, 603–607 (2008). https://doi.org/10.1038/nphys1002

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