Letter abstract
Nature Physics 4, 603 - 607 (2008)
Published online: 29 June 2008 | Corrected online: 11 July 2008 | doi:10.1038/nphys1002
Subject Categories: Condensed-matter physics | Statistical physics, thermodynamics and nonlinear dynamics | Materials physics
Superconductivity and quantum criticality in the heavy-fermion system 
-YbAlB4
S. Nakatsuji1, K. Kuga1,2, Y. Machida1, T. Tayama1, T. Sakakibara1, Y. Karaki1, H. Ishimoto1, S. Yonezawa2, Y. Maeno2, E. Pearson3, G. G. Lonzarich3, L. Balicas4, H. Lee4,6 & Z. Fisk5
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 
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 -YbAlB4 as a unique system showing quantum criticality without external tuning.
- Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa 277-8581, Japan
- Department of Physics, Kyoto University, Kyoto 606-8502, Japan
- Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, UK
- National High Magnetic Field Laboratory (NHMFL), Tallahassee, Florida 32310, USA
- Department of Physics and Astronomy, University of California, Irvine, California 92697, USA
- Present address: Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Correspondence to: S. Nakatsuji1 e-mail: satoru@issp.u-tokyo.ac.jp
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