E-mv coupling of vibrational overtone in organic conductors: Relationship to optical nonlinearities and ferroelectricity

A strong electron-phonon coupling effect, referred to as electron-molecular vibration (e-mv) coupling, induces characteristic vibronic signals in the vibrational spectra of organic conductors. This paper discusses a new spectroscopic signal induced by the e-mv coupling effect, and the physical implications of its emergence. This vibronic signal, attributable to an overtone of a molecular vibration, appears with an anti-resonance form in the infrared spectrum of some mixed-valency complexes when the compounds undergo a charge-ordering transition. Based on the results of cluster model calculations, the activation of the overtone signal is interpreted as an indication of the generation of anharmonicity in the electronic potential. This analysis suggests that the series of the complexes should possess potential nonlinear optical abilities, though these compounds have long been viewed as electric conductors having the opposite characteristics of dielectrics that show nonlinear optical properties.