Electron correlations in the quasi-two-dimensional organic conductorθ-(BEDT-TTF)2I3investigated by13C NMR

Abstract

We report a ${}^{13}$C-NMR study on the ambient-pressure metallic phase of the layered organic conductor $\ensuremath{\theta}$-(BEDT-TTF)${}_{2}$I${}_{3}$ (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 $\ensuremath{\theta}$-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 [$\ensuremath{\propto}1/({T}_{1}T{K}^{2}$)], is in an intermediate regime. However, NMR relaxation rate $1/{T}_{1}$ is enhanced above $\ensuremath{\sim}$ 200 K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically.