Absence of charge order in the dimerized<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>κ</mml:mi></mml:math>-phase BEDT-TTF salts

Abstract

Utilizing infrared vibrational spectroscopy we have investigated dimerized two-dimensional organic salts in order to search for possible charge redistribution that might constitute electronic dipoles and ferroelectricity: the quantum spin liquid $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$ [BEDT-TTF: bis-(ethylenedithio)tetrathiafulvalene], the antiferromagnetic Mott insulator $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]Cl, and the superconductor $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]Br. None of them exhibit any indication of charge disproportionation. Upon cooling to low temperatures all BEDT-TTF molecules remain homogeneously charged within $\ifmmode\pm\else\textpm\fi{}0.005e$. No modification in the charge distribution is observed around $T=6$ K where a low-temperature anomaly has been reported for the spin-liquid material $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(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.