1202.1049 (Michihiro Hirata et al.)

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

Michihiro Hirata, Kazuya Miyagawa, Kazushi Kanoda, Masafumi Tamura

We report a $^{13}$C-NMR study on the organic conductor
$\theta$-(BEDT-TTF)$_{2}$I$_{3}$ [BEDT-TTF:
bisethylenedithio-tetrathiafulvalene] at ambient pressure. The lattice symmetry
is consistent with the pseudo-orthorhombic structure by the orientation
dependence of the NMR spectra, in which all molecules in the unit cell are
crystallographically equivalent. There is no signature of charge
disproportionation as observed in most of the $\theta$-type BEDT-TTF salts. The
analysis of NMR Knight shift, $K$, and the nuclear spin-lattice relaxation
rate, $1/T_{1}$, revealed 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 is enhanced above 200 K, which is a possible
indication that the system enters into a quantum critical regime of
charge-order fluctuations as suggested theoretically.

View original: http://arxiv.org/abs/1202.1049