A organic conductor κ-(BEDT-TTF)2Cu(NCS)2 shows a superconducting transition at 10.4 K. This salt, however, behave as a semiconductor above 100 K. To reveal the origin of this behavior, we performed 13C-NMR in κ-(BEDT-TTF)2Cu(NCS)2, in which one side of the central C=C in BEDT-TTF molecules is substituted with 13C nuclei. The broadening in the linewidths in the NMR spectrums and the anomalies of the spin-spin relaxation time (T2) were observed at around 100 K, suggesting a slow dynamics of the local field. This slow dynamics is thought to be caused by the ethylene motion of the BEDT-TTF molecules from 1H-NMR measurement. We also measured T2 of the deuterated sample, in which 1H nuclei of the ethylene groups in BEDT-TTF molecules are substituted with D (2H) nuclei. We observed the T2 anomalies the same as in the non-deuterated sample, and revealed that the T2 anomalies depend on the hyperfine coupling constant. These results indicate the connection between the ethylene motion and the conduction electrons.
The connection between the intramolecular motion and the electrons was proposed when scientists discovered molecular conductors. However, there has not been any experimental evidence of it. Our results provide direct experimental evidence of the connection between the conduction electron and the ethylene motion.
This topic was published in "Low-frequency dynamics of κ-(BEDT-TTF)2Cu(NCS)2 observed by 13C NMR" Phys. Rev. B 83, 144505 (2011).