Bandwidth-controlled Mott transition in $\kappa-(BEDT-TTF)_2 Cu [N(CN)_2] Br_x Cl_{1-x}$ I. Optical studies of localized charge excitations

TL;DR

This study uses infrared reflection to investigate how bandwidth control via Br-substitution influences the Mott metal-insulator transition in organic conductors, revealing details of localized and itinerant charge excitations.

Contribution

It provides a detailed analysis of charge excitations and their temperature dependence in bandwidth-controlled Mott transitions using optical and vibrational spectroscopy.

Findings

Identification of two main absorption processes: Hubbard band transitions and intradimer excitations.

Coupling of intradimer excitations to intramolecular vibrations allows disentangling electronic contributions.

Support from cluster model calculations confirms the interpretation of spectral features.

Abstract

Infrared reflection measurements of the half-filled two-dimensional organic conductors $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br$_{x}$Cl$_{1-x}$ were performed as a function of temperature ($5 {\rm K}<T<300$ K) and Br-substitution ($x=0%$, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant and localized charge carriers. The broad mid-infrared absorption has two contributions: transitions between the two Hubbard bands and intradimer excitations from the charges localized on the (BEDT-TTF)$_2$ dimer. Since the latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both electronic and vibrational features provides a tool to disentangle these contributions and to follow their temperature and electronic-correlations dependence. Calculations based on the cluster model support our interpretation.