Ground states of $\kappa$-(ET)$_{2}$ Hg(SCN)$_{2}$ Br and $\kappa$-(ET)$_{2}$ Hg(SCN)$_{2}$ Cl studied by Raman Spectroscopy and Heat Capacity measurements
Embargo until
2019-12-01
Date
2018-10-26
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Johns Hopkins University
Abstract
This work is aimed at studying two iso-structure materials $\kappa$-(BEDT-TTF)$_{2}$ Hg(SCN)$_{2}$ Br ($\kappa$-Hg-Br) and $\kappa$-(BEDT-TTF)$_{2}$ Hg(SCN)$_{2}$ Cl ($\kappa$-Hg-Cl), specifically elucidating the mechanism by which the electron-electron interaction gives rise to their exotic ground states. Both materials are Quasi-two-dimensional organic Mott Insulators on a triangular lattice which makes them highly frustrated systems. $\kappa$-Hg-Cl and $\kappa$-Hg-Br goes through Metal to insulator transition (MIT) at $30$ K and $80$ K. The two iso-structure materials are studied because replacing one atom is equivalent to applying hydro-static pressure and thus giving some hints on the effect of some of the parameters on the ground states. To elucidate the magnetic ground state we study heat capacity and Raman response of single crystals of these materials. Our low temperature heat capacity measurements suggest a presence of a linear term in the temperature dependence of $\kappa$-Hg-Br, which might indicate the existence of gapless spinons. Such linear term couldn't be detected in $\kappa$-Hg-Cl while both have almost identical phonon contribution. Here we demonstrate experimentally that new charge degrees of freedom emerge in the molecule-based Mott insulator $\kappa$-Hg-Br resulting in quantum dipole liquid state. Electrons localized on molecular dimer lattice sites form electric dipoles which do not order at low temperatures and fluctuate with frequency detected experimentally in our Raman spectroscopy experiments. The heat capacity and Raman scattering response support a scenario where the composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest temperatures. Raman Response of $\kappa$-Hg-Cl was very interesting and perplexing; it showed charge order at 30 K where MIT transition takes place, but at very low temperature it crosses over to another phase that seemingly changes the charge symmetry. Few mechanism are suggested but the mechanism by which this re-entrant takes place it is still under investigations. Low-frequency Raman measurements were conducted to investigate magnetic excitations in both materials. No signature of magnetic excitation were detected in either materials which eliminates the possibility of anti-ferromagnetic ordering. Magnetic state is very interesting is still under investigation by many groups active in the field.
Description
Keywords
Organic Mott Insulators, Raman spectroscopy, charge order, charge fluctuations, quantum spin liquid, quantum dipole liquid, quantum dipole solid, heat capacity