Collective excitation of electric dipole on molecular dimer in organic dimer-Mott insulator

TL;DR

This study investigates the terahertz response of an organic dimer-Mott insulator, revealing collective electric dipole excitations and their photoinduced enhancement, indicating proximity to phase boundary instabilities.

Contribution

It demonstrates the observation of collective intra-dimer electric dipole excitations and their photoinduced growth in an organic dimer-Mott insulator, a novel insight into its dielectric properties.

Findings

Identification of a 30 cm^-1 band as collective dipole excitation

Photoinduced enhancement of the dipole-related band

Evidence of phase boundary instability in the material

Abstract

The terahertz (THz) response in 10-100 cm^-1 was investigated in an organic dimer-Mott (DM) insulator kappa-(ET)_2Cu_2(CN)_3 that exhibits a relaxor-like dielectric anomaly. 30 cm^-1 band in the optical conductivity was attributable to collective excitation of the intra-dimer electric dipoles which are formed by an electron correlation. We succeeded in observing photoinduced enhancement of this 30 cm^-1 band, reflecting the growth of the electric dipole cluster in the DM phase. Such optical responses in kappa-(ET)_2Cu_2(CN)_3 reflect instability near the boundary between the DM-ferroelectric charge ordered phases.