Electronic properties of correlated metals in the vicinity of a charge order transition: optical spectroscopy of $\alpha$-(BEDT-TTF)${}_{2}M$Hg(SCN)${}_4$ ($M$ = NH${}_4$, Rb, Tl)

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.

References in corpus (7)

• Mott transition, antiferromagnetism, and unconventional superconductivity in layered organic superconductors
• Similarities between organic and cuprate superconductors
• Superconductivity mediated by charge fluctuations in layered molecular crystals
• Competition between Charge Ordering and Superconductivity in Layered Organic Conductors $\alpha$-(BEDT-TTF)${}_{2}M$Hg(SCN)${}_4$ (M = K, NH${}_4$)
• Dynamical properties of a strongly correlated model for quarter-filled layered organic molecular crystals
• Non-Fermi liquid behavior in nearly charge ordered layered metals
• Superconductivity in the charge-density-wave state of the organic metal α-(BEDT-TTF)2KHg(SCN)4

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