Phonon-Spectrum Narrowing Induced by Ultrafast Charge Fluctuation in an Organic Dimer Mott Insulator

TL;DR

This study reveals how ultrafast charge fluctuations cause phonon-spectrum narrowing in an organic Mott insulator, linked to dielectric anomalies and charge dynamics at different temperatures.

Contribution

It demonstrates the phonon spectral narrowing mechanism driven by charge fluctuations using motional narrowing theory in an organic Mott insulator.

Findings

Spectral narrowing occurs at 30 K due to charge fluctuation dynamics.

Below 30 K, motional narrowing is disrupted by increasing correlation time.

Above 30 K, increased charge fluctuation amplitude disturbs the narrowing.

Abstract

We have observed the characteristic temperature dependence of the intermolecular phonon spectrum in the organic dimer Mott insulator kappa-(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 tau_c and the amplitude of the frequency modulation delta satisfy the relation tau_c<delta at 30 K. At temperatures below 30 K, the motional narrowing is disturbed by the increasing of tau_c, 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 delta with increasing temperature.