NMR studies of phase transition form a metallic state to a Dirac-electron state in the organic system, $\theta$-(BEDT-TTF)$_{2}$I$_{3}$

The Dirac electron phase is realized in the bulk organic systems, $\theta$ and $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$. The bulky nature of the system allows one to study the Dirac electrons in the spin degrees of freedom by means of NMR (K. Miyagawa et al. JPSJ {\textbf 79}, 063703 (2010)). Moreover, in $\theta$-(BEDT-TTF)$_{2}$I$_{3}$, the Dirac electron phase neighbors a metallic (superconducting) phase in a pressure-temperature phase diagram. To clarify how the Dirac phase emerges from the metallic state, we performed $^{13}$C NMR measurements for this material at ambient and under pressures. The angular dependence of NMR spectra demonstrates that all the molecules are equivalent (Hirata et al. PRB {\textbf 85}, 195146, (2012)). The temperature dependences of Knight shift and spin-lattice relaxation rate, 1/$T_{1}$, hold the Korringa relation, which signifies metallicity under pressures before the transition to the Dirac phase. However, after the system undergoes a transition to the Dirac electron state, the NMR spectral shape becomes complicated indicating a structural phase transition. The analysis of the angular dependence of the NMR spectra shows the molecular arrangement changes from theta to alpha type.