We have observed the characteristic temperature dependence of the intermolecular phonon spectrum in the organic dimer Mott insulator κ-(ET)2Cu2(CN)3 exhibiting a dielectric anomaly at ∼30 K. The anomalous spectral narrowing of the 55 cm-1 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 τc and the amplitude Δ of the frequency modulation satisfy the relation τc<1/Δ at 30 K. At temperatures below 30 K, the motional narrowing is disturbed by the increase of τc, 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 Δ with increasing temperature.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2013 Sep 3|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics