Electronic properties of correlated metals in the vicinity of a charge order transition: optical spectroscopy of $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl)

TL;DR

This study investigates how electronic correlations influence the optical properties of quasi-two-dimensional organic conductors near a charge order transition, revealing charge fluctuations and effective mass changes across different compounds and temperatures.

Contribution

It provides experimental evidence of charge order fluctuations and their impact on electronic properties in quarter-filled metals close to a charge order transition.

Findings

Charge order fluctuations are more pronounced from NH4 to Tl compounds.

Metallic character increases and effective mass decreases at lower temperatures in NH4 and Rb salts.

Signatures of charge order are observed in Tl compound despite dominant metallic behavior.

Abstract

The infrared spectra of the quasi-two-dimensional organic conductors $\alpha$-(BEDT-TTF)$_2$$M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl) were measured in the range from 50 to 7000 \cm down to low temperatures in order to explore the influence of electronic correlations in quarter-filled metals. The interpretation of electronic spectra was confirmed by measurements of pressure dependant reflectance of $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ at T=300 K. The signatures of charge order fluctuations become more pronounced when going from the NH$_4$ salt to Rb and further to Tl compounds. On reducing the temperature, the metallic character of the optical response in the NH$_4$ and Rb salts increases, and the effective mass diminishes. For the Tl compound, clear signatures of charge order are found albeit the metallic properties still dominate. From the temperature dependence of the electronic scattering rate the crossover temperature is estimated below which the coherent charge-carriers response sets in. The observations are in excellent agreement with recent theoretical predictions for a quarter-filled metallic system close to charge order.