Absence of charge order in the dimerized \kappa-phase BEDT-TTF salts

TL;DR

This study uses infrared vibrational spectroscopy to investigate charge distribution in dimerized ppa-phase BEDT-TTF salts, finding no evidence of charge order or ferroelectricity in these materials down to low temperatures.

Contribution

It provides the first spectroscopic evidence showing the absence of charge disproportionation in these organic salts, challenging previous assumptions about charge order in such systems.

Findings

No charge disproportionation observed down to low temperatures

Charge distribution remains unchanged around T=6 K in ppa-(BEDT-TTF)2Cu2(CN)3

Anisotropic optical response indicates molecular tilt and coupling effects

Abstract

Utilizing infrared vibrational spectroscopy we have investigated dimerized two-dimensional organic salts in order to search for possible charge redistribution that might cause electronic dipoles and ferroelectricity: the quantum spin liquid \kappa-(BEDT-TTF)2Cu2(CN)3, the antiferromagnetic Mott insulator \kappa-(BEDT-TTF)2$Cu[N(CN)2]Cl, and the superconductor \kappa-(BEDT-TTF)2$Cu[N(CN)2]Br. None of them exhibit any indication of charge disproportionation upon cooling down to low temperatures. No modification in the charge distribution is observed around T = 6 K where a low-temperature anomaly has been reported for the spin-liquid \kappa-(BEDT-TTF)2$Cu2(CN)3. In this compound the in-plane optical response and vibrational coupling are rather anisotropic, indicating that the tilt of the BEDT-TTF molecules in c-direction and their coupling to the anion layers has to be considered in the explanation of the electromagnetic properties.