Electron Correlations in the Quasi-Two-Dimensional Organic Conductor $\theta$-(BEDT-TTF)${}_2$I${}_3$ investigated by ${}^{13}$C NMR

Abstract

We report a ${}^{13}$C-NMR study on the ambient-pressure metallic phase of the layered organic conductor $\theta$-(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 $\theta$-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 [$\propto 1/(T_{1}T{K}^2$)], is in an intermediate regime. However, NMR relaxation rate $1/T_1$ is enhanced above $\sim$ 200K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically.

19pages, 6figures

References in corpus (4)

• Charge Ordering in alpha-(BEDT-TTF)2I3 by synchrotron x-ray diffraction
• Electronic Properties Close to Dirac Cone in Two-Dimensional Organic Conductor $\alpha$-(BEDT-TTF)${}_2$I${}_3$
• ${}^{13}$C NMR Study on the Charge-Disproportionated Conducting State in the Quasi-Two-Dimensional Organic Conductor $\alpha$-(BEDT-TTF)${}_2$I${}_3$
• Unusual hyperfine interaction of Dirac electrons and NMR spectroscopy in graphene

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• Electronic states and molecular dynamics of single-component molecular conductors [M(tmdt)${}_2$] (M=Ni, Pt) studied by ${}^{13}$C and ${}^1$H NMR
• ${}^{13}$C NMR observation of a nonmagnetic charge-ordered state in the organic conductor $\kappa$-(ET)${}_2$Hg(SCN)${}_2$Cl