Ordering phenomena in dimerized Mott insulators on a triangular lattice: effects of electronic correlations and anion coupling

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This project aims at the understanding of ordering phenomena in dimerized Mott insulators arranged on a triangular lattice, in particular the effect of electronic interactions and coupling to the anions. While the charge transfer salt kappa-(BEDT-TTF)2Cu2(CN)3 has been established as a reference system and prime candidate for spin liquid behavior, we choose two novel families of kappa-phase BEDT-TTF compounds for a comparative study in order to vary important parameters.1. In the first case of the Hg-family, the relative correlations are significantly varied: the dimers are arranged on a triangular lattice close to frustration but exhibit charge order at low temperatures, implying the importance of inter-site Coulomb repulsion. 2. For the Ag-analogue, the structure of the anion network and its coupling to the BEDT-TTF layer is different compared to kappa-(BEDT-TTF)2Cu2(CN)3. This will allow us to test the hypo¬thesis that frustration and intrinsic disorder of the anion layers affects the charge dynamics within the BEDT-TTF layers. By chemical substitution and physical pressure, we can fine-tune the relevant interactions and monitor how the physical properties are affected. In particular we investigate (i) the metal-insulator transition by transport measurements, (ii) the charge disproprtionation on the dimers by vibrational spectroscopy, (iii) the charge excitations by optical spectroscopy, and (iv) the relaxational behavior by dielectric spectro¬scopy. (v) Accompanying studies probe the susceptibility by SQUID and torque measurements and the magnetic behavior by ESR spectroscopy. (vi) We complement our experiments by numerical work on the electronic and phonon properties. We hope to clarify the collective charge excitations and the presence of spin-charge coupling in kappa--phase com¬pounds. This way we clarify the possibility of electronic multiferroicity in organic compounds and the possi¬bi¬lity of future applications.