Chlorine-nuclear-quadrupole-resonance study of the neutral-to-ionic transition in tetrathiafulvalene-chloranil

Abstract

The temperature dependence of the chlorine-35 nuclear-quadrupole-resonance (NQR) parameters has been studied in the mixed-stack charge-transfer complex tetrathiafulvalene-chloranil and discussed with reference to spectroscopic and neutron-diffraction results. The NQR spectrum changes conspicuously at the neutral-to-ionic transition temperature, ${\mathit{T}}_{\mathit{N}\mathrm{\ensuremath{-}}\mathit{I}}$=79.2 K. The changes have been analyzed in terms of the symmetry breaking occurring at the transition. Below, coexistence of the neutral and ionic phases has been observed down to the limit of metastability of the neutral phase (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}69 K). Spin-lattice relaxation rates reveal the existence of highly anisotropic critical fluctuations with a predominantly one-dimensional character.