$^{13}$C NMR Study on the Charge-Disproportionated Conducting State in the Quasi-Two-Dimensional Organic Conductor $\alpha$-(BEDT-TTF)$_2$I$_3$

TL;DR

This study uses $^{13}$C NMR to analyze the local electronic states and electron correlations in the organic conductor $ ext{α}$(BEDT-TTF)$_2$I$_3$, revealing temperature-dependent charge disproportionation and antiferromagnetic correlations.

Contribution

It provides the first local, site-specific NMR evidence of charge disproportionation and electron correlation effects in $ ext{α}$(BEDT-TTF)$_2$I$_3$, linking experimental results with theoretical $m{k}$-space anisotropy predictions.

Findings

Charge disproportionation ratio of 5:4:6 at 140 K.

Presence of sizable antiferromagnetic spin correlations.

Site- and temperature-dependent variations in spin susceptibility and Korringa ratio.

Abstract

The conducting state of the quasi-two-dimensional organic conductor, $\alpha$-(BEDT-TTF)$_2$I$_3$, at ambient pressure is investigated with $^{13}$C NMR measurements, which separate the local electronic states at three nonequivalent molecular sites (A, B, and C). The spin susceptibility and electron correlation effect are revealed in a locally resolved manner. While there is no remarkable site-dependence around room temperature, the local spin susceptibility gradually disproportionates among the nonequivalent sites with decreasing temperature. The disproportionation-ratio yields 5:4:6 for A:B:C molecules at 140 K. Distinct site- and temperature-dependences are also observed in the Korringa ratio, $\mathcal{K}_i \propto (1/T_{1}T)_iK^{-2}_i$ ($i$ = A, B, and C), which is a measure of the strength and the type of electron correlations. The values of $\mathcal{K}_i$ point to sizable antiferromagnetic spin correlation. We argue the present results in terms of the theoretical prediction of the peculiar site-specific reciprocal-space ($\bm{k}$-space) anisotropy on the tilted Dirac cone, and discuss the $\bm{k}$-dependent profiles of the spin susceptibility and electron correlation on the cone.