Fig. 1: Evidence of low temperature charge order melting in κ-(BEDT-TTF2)Hg(SCN)2Cl provided by vibrational Raman spectrocopy.
From: Melting of charge order in the low-temperature state of an electronic ferroelectric-like system
a Layered crystal structure of quasi-two-dimensional (quasi-2D) molecular-based Mott insulators. b Eigenvectors of charge-sensitive molecular vibration \({\nu }_{2}\). c Raman spectra of \(\kappa\)-Hg-Cl in the spectral range of the charge-sensitive vibration \({\nu }_{2}\) at different temperatures. Drawings on the right represent the charge distribution on the lattice, black is for BEDT-TTF\({}^{+0.5}\), red for BEDT-TTF\({}^{+0.6}\), and blue is for BEDT-TTF\({}^{+0.4}\); ET = BEDT-TTF on the plots. From the top to the bottom: In the metallic state at 45 K only \({\nu }_{2}\)(BEDT-TTF\({}^{+0.5}\)) is observed; at the temperature of charge order transition T\({}_{{\mathrm{CO}}}\)=30 K a mix of metallic domains with \({\nu }_{2}\)(BEDT-TTF\({}^{+0.5}\)) and charge ordered domains with \({\nu }_{2}\)(BEDT-TTF\({}^{+0.6}\))/\({\nu }_{2}\)(BEDT-TTF\({}^{+0.4}\)) is observed; in the charge ordered state at T = 20 K \({\nu }_{2}\)(BEDT-TTF\({}^{+0.6}\)) and \({\nu }_{2}\)(BEDT-TTF\({}^{+0.4}\)) are observed; at T = 10 K charge order starts to melt, broadened \({\nu }_{2}\)(BEDT-TTF\({}^{+0.6}\))/\({\nu }_{2}\)(BEDT-TTF\({}^{+0.4}\)), and weak \({\nu }_{2}\)(BEDT-TTF\({}^{+0.5}\)) are observed, a fraction of molecules corresponding to BEDT-TTF\({}^{+0.5}\) is about 0.2 of the whole system; at T = 4 and 2 K broadened \({\nu }_{2}\)(BEDT-TTF\({}^{+0.6}\))/\({\nu }_{2}\)(BEDT-TTF\({}^{+0.4}\)), and \({\nu }_{2}\)(BEDT-TTF\({}^{+0.5}\)) are observed, ~0.3 of the system has each charge state. d Temperature dependence of the frequencies (upper panel) and line width (middle panel) of \({\nu }_{2}\) and \({\nu }_{3}\). Note the splitting of \({\nu }_{2}\) band below 30 K, and then broadening and change of frequencies which reduces the difference between \({\nu }_{2}(A)\) and \({\nu }_{2}(B)\). The lower panel shows a temperature dependence of the fraction of the system N associated with different charges on BEDT-TTF molecule. See Methods for the details of calculations and error bars definition.
