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  • Title: Anomalous spin correlations and excitonic instability of interacting 2D Weyl fermions.
    Author: Hirata M, Ishikawa K, Matsuno G, Kobayashi A, Miyagawa K, Tamura M, Berthier C, Kanoda K.
    Journal: Science; 2017 Dec 15; 358(6369):1403-1406. PubMed ID: 29242340.
    Abstract:
    The Coulomb interaction in systems of quasi-relativistic massless electrons has an unscreened long-range component at variance with conventional correlated metals. We used nuclear magnetic resonance (NMR) measurements to reveal unusual spin correlations of two-dimensional Weyl fermions in an organic material, causing a divergent increase of the Korringa ratio by a factor of 1000 upon cooling, in marked contrast to conventional metallic behavior. Combined with model calculations, we show that this divergence stems from an interaction-driven velocity renormalization that almost exclusively suppresses zero-momentum spin fluctuations. At low temperatures, the NMR relaxation rate shows an unexpected increase; numerical analyses show that this increase corresponds to internode excitonic fluctuations, a precursor to a transition from massless to massive quasiparticles.
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