F.Nad 1,2, P.Monceau 1 and H.M.Yamamoto 3 1 Institute Neel, CNRS-UJF, Batiment E, 25 rue des martyrs, BP 166, Grenoble Cedex 9, France 2 Institute of Radioengineering and Electronics, RAS, Mokhovaya 11/7, Moscow, Russia 3 Condensed Molecular Materials Laboratories RIKEN, 2-1 Hirosawa Wako-shi, Saitama , Japan Charge ordered state and its stabilization in organic compounds

Stabilization of CO in 1D organic compounds Studies show the splitting of the NMR lines at T<T CO [S.Brown et al, 2000]. X-ray studies show no changes in the molecular lattice. “Structureless transition”. [J.P.Pouget and S.Ravy, 1996]. Stabilization of CO in 1D organic compounds occurs by shift of anions as a whole with wave vector q=0. [P.Monceau, F.Nad and S.Brazovskii, 2001] TMTTF anion

Stabilization of CO in  (BEDT-TTF) 2 RbZn(SCN) 4 on slow cooling At T>T CO diffuse rods with q=(1/3, k, 1/4) [M.Watanabe et al, 2003]. At T<T CO space group changes from orthorhombic I222 to P Formation of the lattice superstructure c*/2. Stabilization of CO in RbZn salt is due to development of the lattice superstructure with doubling period along c axis

Properties of  (BEDT-TTF) 2 RbZn(SCN) 4 on fast cooling Slow cooling: 0.1 K/min Intermediate : 4-5 K/min Fast: 9-10 K/min

Behavior of  -(BEDT-TTF) 2 RbZn(SCN) 4 on heating after fast cooling In a quenched state the diffuse rods with q 1 =(1/3, k, 1/4) observed previously at T > T CO becomes weaker but still exist below T CO [M.Watanabe et al, 2003]. At T < T CO the another diffuse rod with q 2 =(0, k, 1/2) appears. The modulation of q 2 type is disordered in the interlayer direction. After annealing the q 2 grows into the satellite reflection of c * /2. Thus, the tendency to formation of CO state, exhibited by electron subsystem but not supported by the appropriate long-range modifications in the molecular lattice, does not lead to formation of real stable CO state at the fast cooling

Properties of θ-(BEDT-TTF) 2 CsZn(SCN) 4 No optical gap down to 10 K [H.Tajima et al,2000]. Below 180 K NMR line broadening occurs that is associated with the development of partial charge disproportionation (CD) [T.Takahashi et al, 2005]. In the range K in CsZn (as well as in CsCo) diffuse sheets were observed along 2a*+c* and 2a*-c* directions [Y.Nogami et al, 1999; M.Watanabe et al, 1999]. Below 90 K two-dimensional diffuse rods with q 1 =(2/3, k, 1/3) and q 2 =(0, k, 1/2) develop from these diffuse sheets.. With further cooling the peak intensity of q 2 increases in comparison with intensity of q 1 peak, though q 1 charge modulation are also retained. The correlation lengths of q 1 and q 2 ranges (domains) are not very large (50 – 70 Ǻ) [Y.Nogami et al, 1999]. In spite of the formation of the partial CD no long-range charge ordering was stabilized.

Temperature dependence of conductance of θ-(BEDT-TTF) 2 CsZn(SCN) 4 In the range K G ~ T 2.9 Theory shows that for variable range hopping on the presence of electron correlations G ~ T 3 [B.Shklovskii and A.Efros]

Temperature dependence of dielectric permittivity ε' of θ-(BEDT-TTF) 2 CsZn(SCN) 4 Frequencies f are indicated in MHz Forms of ε'(T,f) dependences are very similar to known slowing down behavior which is characteristic for various systems with certain degree of disorder, so- called, relaxors and different types of glasses. At low temperatures an existence of two types of domains with different structures (two fold (q 2 ) and tree fold (q 1 )), competing to each other, leads to development of frustration. As a result, glassy-like CO state develops which corresponds to frozen some degree of disorder on long-range scale with conservation of the short-range order with small coherence length [K.Kanoda et al, 2005].

Conclusion For stabilization of forming in 1D and 2D organic compounds CO state, owing to electron-electron correlated interactions, the certain modifications are needed in anion and lattice subsystems. In 1D compounds there is the shift of anion chains as a whole with q=0. In 2D compounds the stabilization is realized by development of superstructure with doubling of the lattice period. If with decreasing temperature this superstructure is not realized on long enough coherence length, but instead of that, the conglomerate of many domains with different structures turns out to be frozen, the glass-like CO state is developed in such 2D compounds.