Electron correlations in the quasi-two-dimensional organic conductor θ-(BEDT-TTF) ₂I ₃ investigated by 13C NMR

We report a 13C-NMR study on the ambient-pressure metallic phase of the layered organic conductor θ-(BEDT-TTF) ₂I ₃ (BEDT-TTF: bisethylenedithio-tetrathiafulvalene), which is expected to connect the physics of correlated electrons and Dirac electrons under pressure. The orientation dependence of the NMR spectra shows that all BEDT-TTF molecules in the unit cell are to be seen equivalent from a microscopic point of view. This feature is consistent with the orthorhombic symmetry of the BEDT-TTF sublattice and also indicates that the monoclinic I ₃ sublattice, which should make three molecules in the unit cell nonequivalent, is not practically influential on the electronic state in the conducting BEDT-TTF layers at ambient pressure. There is no signature of charge disproportionation in opposition to most of the θ-type BEDT-TTF salts. The analyses of NMR Knight shift K and the nuclear-spin-lattice relaxation rate 1/T ₁ revealed that the degree of electron correlation, evaluated by the Korringa ratio [∝1/(T ₁TK2)], is in an intermediate regime. However, NMR relaxation rate 1/T ₁ is enhanced above ∼ 200 K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically.