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Electronic instabilities and conduction in kagome system at 1/3 filling - the case of (EDT-TTF-CONH2)6 [Re6Se8(CN)6]

EDT-TTF-CONH2)6 [Re6Se8(CN)6] is recently synthesized compound composed of layers with kagome structure.[1] Above 180 K the material is 2D-homogenous bad metal, or semiconductor, depending on the pressure ; [2] below 180 K the electrons charge-order into chains, with the antiferromagnetic correlations developing below 100 K. [1] The physics of (EDT-TTF-CONH2)6 [Re6Se8(CN)6] unites two features of electrons on kagome lattice that have separately appeared in focus of recent research. The first is the effect Dirac-cone electronic dispersion on electronic instabilities. The second is the effect of kagome geometry on charge and magnetic orderings. These two subjects are usually discussed separately, since the first is mainly related to weakly interacting fermions near 1/3-filling, when Dirac point coincides with the Fermi level, while the second is usually linked to frustration towards magnetic ordering at 1/2-filling in the strong coupling limit. In (EDT-TTF-CONH2)6 [Re6Se8(CN)6] the band filling is 1/3 indeed, but the experimental findings imply that electron-electron and electron-phonon interactions are substantial.[1] We model the system through a variety of an extended Hubbard–Holstein model. Its phase diagram exhibits a number of ground states, including the homogenous phase, several charge-ordered and antiferromagnetically ordered phases, the bipolaron-crystal phase, and ordered mixtures of bipolaron and antiferromagnetic structures. From the weak-coupling side, the development of charge and spin-ordered involves the destruction of the tip of Dirac cone, the feature particularly robust in the kagome geometry. As the instability develops, the tips sweep through the Brillouin zone, resisting gap-opening, until the two of them merge at the critical value of the order parameter. From the strong-coupling side, 1/3 filling is the highest concentration where the magnetic frustration may be fully avoided. It is thus the avoiding of the frustration that may lead to the formation of antiferromagnetic chain and ring structures. However, independently of the magnetic interaction, 1/3 is also the concentration for which the short-range coulomb interaction is minimized in charge-ordered state, rendering the spin and charge ordering generally decoupled. The electric conduction in charge ordered state takes place through the formation and movement of polaronic defects. The activation gap observed in the conductivity in the charge-ordered state is the sum of energy related to defect creation, and the polaron binding energy. The first disappears in the homogenous state. These two energy scales determine also the pressure dependence of the transition temperature and the nature of the high-temperature phase. The high temperature metallic state at ambient pressure remains somewhat mysterious, with the resistivity two orders of magnitudes above the Mooji limit.

kagome; bad metal; thermoelectric power; polaron gas

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