Low-energy models for Molecular Solids: θ- and κ-(BEDT-TTF)2X

Date & Time
26 June 2008 (Thur), 1:00 pm - 2:30 pm
6F seminar room, Sengen site
Dr. Kazuma Nakamura(中村和磨博士)
Department of Applied Physics, University of Tokyo(東京大学大学院工学系研究科物理工学専攻)
Low-energy models for Molecular Solids: θ- and κ-(BEDT-TTF)2X
Aiming at a deeper understanding of physics and chemistry of organic molecular solids, we perform first-principles analyses for two different types of (BEDT-TTF)2X; θ- and κ-type compounds. These compounds have strong two-dimensional anisotropy and the conducting layer relevant to the low-energy physics near the Fermi energy consists of aligned BEDT-TTF molecules. In spite of the structural similarity, the physics of the θ and κ compounds is completely different; in the θ type, puzzling coexistence of two different charge-ordering fluctuations in metallic phase is observed near charge-ordered and spin-Peierls phases. On the other hand, the κ compound shows typical Mott physics exhibiting a phase diagram involving antiferromagnetic Mott insulator, genuine Mott insulator with spin liquid nature, paramagnetic metal, and superconductor phases. The origin of such a diversity and mechanisms of stabilizing each phase themselves are intriguing questions to be answered.
I will present ab initio downfolded low-energy effective models of the θ and κ compounds. By the downfolding procedure, we derive model parameters such as transfer integrals and interaction terms, with the help of the maximally-localized-Wannier-function technique and the constrained random-phase-approximation methodology. Novel expressions for entire three-dimensional data of the screened Coulomb and exchange interactions are first derived. The difference between the resulting models for the θ and κ compounds is discussed with particular focus on a substantial difference in the range of the screened Coulomb interaction, together with their comparisons with the screened interactions of several types of transition metal oxides.
Dr. Hiori Kino(木野日織)