Abstract
We have observed the characteristic temperature dependence of the intermolecular phonon spectrum in the organic dimer Mott insulator κ-(ET)Cu(CN) exhibiting a dielectric anomaly at ∼30 K. The anomalous spectral narrowing of the 55 cm phonon peak at ∼30 K was analyzed in terms of motional narrowing within the framework of a stationary Gaussian process, i.e., the phonon frequency is modulated by the ultrafast charge fluctuation. The spectral narrowing occurs because the time constant of the correlation time and the amplitude of the frequency modulation satisfy the relation at 30 K. At temperatures below 30 K, the motional narrowing is disturbed by the increase of , near the charge-glass or the short-range order at 6 K. On the other hand, for temperatures above 30 K, the motional narrowing is disturbed by the increase of with increasing temperature.
- Received 16 May 2013
DOI:https://doi.org/10.1103/PhysRevB.88.125101
©2013 American Physical Society