Anomalous spin correlations and excitonic instability of interacting 2D Weyl fermions

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.

Source: ScienceNOW - December 14, 2017 Category: Science Authors: Hirata, M., Ishikawa, K., Matsuno, G., Kobayashi, A., Miyagawa, K., Tamura, M., Berthier, C., Kanoda, K. Tags: Physics reports Source Type: news