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Observation of anomalous magnetic fluctuations in heavy fermion superconductor
-A large step to understand the mechanism of superconductivity mediated by magnetic fluctuations–

Sep. 15, 2011

Joint research group of Japan Atomic Energy Agency (JAEA, President Toshio Okazaki) and Los Alamos National Laboratory (LANL, in USA) has observed the anomalous magnetic fluctuations in the strongly correlated electrons system at the quite low temperatures by means of nuclear magnetic resonance (NMR) technique. This study was done by Dr. Hironori Sakai (JAEA), Dr. Joe D. Thompson (LANL) and his colleagues.

The research group has noticed the so-called non-fermi-liquid state in the unconventional superconductor CeCoIn5, which is anomalous metallic state, appears just above the superconducting critical field (Hc2) in the low temperatures. Such an anomalous metallic state is found to exist in the high-Tc cuprate superconductors as well, but the origin for this anomaly is still unclear since the Hc2 is too high to produce in laboratory-scale; typically it reaches to 20-100 Tesla. In CeCoIn5, however, a similar anomalous metallic state can be realized in the laboratory; it is just 5 Tesla.

NMR technique is quite unique tool to investigate the electronic states microscopically. Nowadays, NMR technique becomes quite common in medical fields, that is, it is a similar technique as magnetic resonance imaging (MRI). The research group has investigated the electronic states of CeCoIn5 just above Hc2 in the low temperatures by means of NMR. As a result, the 4f electrons in CeCoIn5 are proved to be on the verge of antiferromagnetic ordering, just above Hc2, while the superconducting pairing begins to occur below Hc2. In addition, the research group has succeeded to explain the macroscopic physical properties such as specific heat, thermal expansion, and electrical resistivity by theoretical model of spin fluctuations using the same numerical parameters obtained from the NMR data.

This study has conclusively proved that the quantum critical behavior near Hc2 for the heavy fermion superconductor CeCoIn5 is caused by the antiferromagnetic spin fluctuations. Such kind of field-tuned criticality may exist even in the high-Tc cuprates, although it cannot be verified easily since the Hc2 for these superconductors is extensively higher. Perhaps, this work may provide small-scale, but really identifiable values for building a new theory to design for a new higher-Tc material.

This work has been published in PHYSICAL REVIEW LETTERS 107, 137001 (2011).

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