Fig. 2: Electronic gap in κ-(BEDT-TTF)2Hg(SCN)2Cl observed by Raman spectrocopy, and a schematic phase diagram suggested for this class of materials as a function of temperature and overlap integral in a dimer (BEDT-TTF)2.
From: Melting of charge order in the low-temperature state of an electronic ferroelectric-like system
a Electronic scattering of \(\kappa\)-Hg-Cl in B\({}_{1g}\) scattering channel in metallic state (T = 38 K), charge ordered state (T = 28 K), and at T = 10 K with ~0.16 of the charge order melted. A gap opens on the transition into the charge ordered state and is preserved at T = 10 K. The arrow in the plot marks the isosbestic point in the spectra, while the gap itself is smaller and is estimated to be of about 300 cm−1. The black dashed line is a guide for the eyes to identify the spectral region where frequency dependence of Raman intensity \(\chi ^{\prime\prime} (\omega )\) flattens due to the gapped density of states. The inset shows a new anisotropic mode which appears in the spectra of the charge ordered state, see text for details. Note the decrease of the intensity of this mode at changing the temperature from 28 to 10 K. b Upper panel: a scheme of orientation of light polarization with respect to the lattice of \(\kappa\)-Hg-Cl with charge stripes along \(c\) axis. Lower panel: polarization dependence of the feature at about 420 cm−1 at 20 K. This feature appears in the spectra in the charge ordered phase. Note the anisotropy of the mode. c Suggested schematic phase diagram where overlap integral in a dimer t\({}_{d}\) controls a quantum phase transition between dipole (charge) order and non-charge-ordered Mott insulator phase. A phase transition on lowering temperature is of the first order (solid line), while a phase transition controlled by t\({}_{d}\) is a second order transition (dashed line).
