Anomalous Spin Correlations and Mass-Generating Excitonic Instability of Interacting Weyl Fermions

TL;DR

This paper reveals that in two-dimensional Weyl fermions, long-range Coulomb interactions lead to exotic spin correlations and a transition from massless to massive quasiparticles, demonstrated through NMR measurements and theoretical modeling.

Contribution

It uncovers the interaction-driven spin correlations and excitonic instability in Weyl fermions, highlighting a mass-generating transition driven by Coulomb interactions.

Findings

Divergent increase of Korringa ratio upon cooling

Suppression of zero-momentum spin fluctuations due to velocity renormalization

Evidence of inter-node excitonic fluctuations indicating mass gap formation

Abstract

Recent advances in the study of nodal Weyl fermions (WFs), quasi-relativistic massless particles, constitute a novel realm of quantum many-body phenomena. The Coulomb interaction in such systems, having a zero density of states at the Fermi level, is of particular interest, since in contrast to conventional correlated metals, its long-ranged component is unscreened. Here, through nuclear-magnetic-resonance (NMR) measurements, we unveil the exotic spin correlations of two-dimensional WFs in an organic material, causing a divergent increase of the Korringa ratio by a factor of 1000 upon cooling, in striking contrast with conventional metallic behaviors. Combined with model calculations, we show that this divergence stems from the interaction-driven velocity renormalization that almost exclusively suppresses the zero-momentum spin fluctuations. At low temperatures, the NMR rate shows a remarkable increase, which is shown by numerical analyses to correspond to inter-node excitonic fluctuations, precursor of a transition from massless to massive quasiparticles.