Experimental and theoretical study of the temperature dependent structural and electronic properties of the spin liquid candidate $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$

The organic charge transfer salt $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ is of great experimental as well as theoretical interest due to the interplay of the effects of electronic correlations, low dimensionality and spin frustration. Besides the spin liquid property at very low temperatures, the material has interesting anomalies at 6K, 60K and 150K. Even though the structure of the material has been determined several times over the past 20 years, complete structural data are only available at room temperature. In our work, we fill this gap by precisely determining the structure at 300K, 250K, 200K, 150K, 100K and 20K. We analyze the structures using density functional theory and tight binding methods. We show that the triangular lattice Hubbard Hamiltonian parameters are temperature dependent, with the interaction strength increasing with decreasing temperature and with the frustration going through a minimum at 150 K. Our results point to the fact that even in the absence of structural phase transitions, experimental determination of structures at various temperatures may be important as a starting point for realistic many-body theoretical investigation of complex materials.