J. Phys. IV France
Volume 12, Numéro 9, November 2002
Page(s) 231 - 232

J. Phys. IV France
12 (2002) Pr9-231
DOI: 10.1051/jp4:20020401

The charge ordered state and the possible 2 $k_{\rm F}$-CDW state with Fermi surface nesting in organic conductor $\theta$-(BEDT-TTF) 2MM '(SCN) 4 (MM ' = RbZn, CsCo)

M. Watanabe1, Y. Noda1, Y. Nogami2, K. Oshima2 and H. Mori3

1  IMRAM, Tohoku University, Sendai, Japan
2  GSNST, Okayama University, Okayama, Japan
3  ISSP, University of Tokyo, Kashiwa, Japan

Two-dimensional organic conhctor $\theta$-(BEDT-TTF) 2RbZn(SCN) 4 and $\theta$-(BEDT-TT) 2CsCo(SCN) 4 were investigated by X-ray diffraction measurements. Both salts exhibit an isostructural crystal structure at room temperature (RT). However, the RbZn salt unkrgoes a metal-insulator transition associated with superlattice formation of 2c. The charge ordered state below T $_{\rm MI}$, was observed by aystal structure analysis taking into account of the superlattice. This mohlation of 2c has no relation to the topology of the calculated Fermi surface nesting vector at all. Below T MI, the systematic & formations of BEDT-TTF molecular shape indicatedcharge disproportionation between BEDT-TTFs possibly caused by the electron correlation effect. On the other hand, in the CsCo salt, another incommensurate lattice modulation was found under high pressure of 1 GPa This incommensurate mohlation vector of (2/3 0 0.29) is in agreement with the nesting vector of Fermi surface under high pressure. In this system, it seems that small change caused by chemical substitution can vary the electronic state from the charge ordered state to the CDW state.

© EDP Sciences 2002