Coupling of charge and lattice degrees of freedoms in θ-type BEDT-TTF compound probed by low-temperature heat capacity measurements

Thermodynamic investigations by heat capacity measurements at low temperature between 700 mK and 60 K with magnetic fields up to 8 T for θ-(BEDT-TTF)₂RbZn(SCN)₄ and θ-(BEDT-TTF)₂CsZn(SCN)₄, where BEDT-TTF is bisethylenedithiotetrathiafulvalene are performed. In θ-(BEDT-TTF)₂RbZn(SCN)₄, the inter-site Coulomb repulsion produces a charge ordered ground state in the case of slow cooling while a kind of charge cluster glass state appears in rapidly cooled case. The observed finite γ term of 14.8 mJK⁻² mol⁻¹ for the rapidly cooled sample demonstrates that the electron density of states remains as metal-like clusters which are localized in the short-range scale. We also discuss the enhancement of the lattice heat capacity and appearance of a kind of Boson peak structure in C_p even in a regular crystal lattice in θ-(BEDT-TTF)₂CsZn(SCN)_4. The low energy phonon modes produced by enhanced charge fluctuations due to the inter-site Coulomb repulsion are the main course of these unusual lattice properties. The features reported here suggest that the coupling of charge degrees of freedom with lattices is important for the systems which do not form rigid charge ordered structures