Electron Correlations in the Quasi-Two-Dimensional Organic Conductor $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ investigated by $^{13}$C NMR

TL;DR

This study uses $^{13}$C NMR to investigate the electronic correlations in the layered organic conductor $ heta$-(BEDT-TTF)$_{2}$I$_{3}$, revealing intermediate electron correlation levels and signs of charge-order fluctuations at higher temperatures.

Contribution

First NMR investigation of $ heta$-(BEDT-TTF)$_{2}$I$_{3}$ at ambient pressure, showing symmetry, absence of charge disproportionation, and evidence of quantum critical charge fluctuations.

Findings

All BEDT-TTF molecules are equivalent, indicating orthorhombic symmetry.

No charge disproportionation observed in the material.

Enhanced relaxation rate above 200K suggests charge-order quantum criticality.

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 [$\varpropto 1/(T_{1}TK^{2}$)], 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.