Temperature effects on the ¹³C high resolution NMR in (TMTSF)₂X salts

We report the results of the ¹³C NMR experiments performed on TMTSF molecules of the organic conductors (TMTSF)₂ClO₄ and (TMTSF)₂ReO₄. This investigation has allowed to resolve all individual sites of the organic molecule. Strong and widely distributed enhanced nuclear relaxation rates on different sites of the molecule suggest a picture where Coulomb correlations play a major role for the sites at the corners of the molecule. On the system (TMTSF)₂ReO₄, continuous variations of the resonance positions are observed when the temperature is decreased down to 182 K, temperature at which the system exhibits a structural transition, leading to a semiconducting state at lower temperature. This experiment constitutes the first observation in direct space of the collapse of the conduction electron density at a metal-semiconductor transition.