Optical probe of carrier doping by X-ray irradiation in organic dimer Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}]$Cl

TL;DR

This study uses infrared spectroscopy to show that X-ray irradiation induces carrier doping in an organic Mott insulator, collapsing the Mott gap and indicating proximity to a bandwidth-controlled Mott transition.

Contribution

First demonstration of carrier doping effects in an organic Mott insulator via X-ray irradiation, revealing spectral weight transfer and phase boundary proximity.

Findings

Spectral weight transfer from mid-infrared to Drude response

Collapse of the Mott gap due to irradiation

Evidence of carrier doping induced by irradiation defects

Abstract

We investigated the infrared optical spectra of an organic dimer Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, which was irradiated with X-rays. We observed that the irradiation caused a large spectral weight transfer from the mid-infrared region, where interband transitions in the dimer and Mott-Hubbard bands take place, to a Drude part in a low-energy region; this caused the Mott gap to collapse. The increase of the Drude part indicates a carrier doping into the Mott insulator due to irradiation defects. The strong redistribution of the spectral weight demonstrates that the organic Mott insulator is very close to the phase border of the bandwidth-controlled Mott transition.