Transition from a metal to a massless-Dirac-fermion phase in an organic conductor investigated by ¹³C NMR

The θ-(BEDT-TTF)₂I₃ system is suggested to be the second organic system of massless Dirac fermions, which emerge from the metallic phase at a pressure exceeding 5 kbar. We have performed ¹³C NMR measurements on a single crystal of θ-(BEDT-TTF)₂I₃ under variable pressure across the critical pressure, 5 kbar. The Knight shift and spin-lattice relaxation rate show that a metallic state with an NMR enhancement factor of 2.2-2.4 at ambient pressure persists up to the critical pressure of 5 kbar. Above that, a marked spectral change indicative of a first-order phase transition is found. The angular dependence of NMR spectra indicates a structural phase transition from the θ-type molecular arrangement to the α-type one, which hosts the tilted Dirac cone.