F.Nad1,2 , P.Monceau1 and H.M.Yamamoto3

 1Institute Neel, CNRS-UJF, Batiment E, 25 rue des martyrs, BP 166, 38042

Grenoble Cedex 9, France2Institute of Radioengineering and Electronics, RAS, Mokhovaya 11/7, 125009

Moscow, Russia3Condensed Molecular Materials Laboratories RIKEN, 2-1 Hirosawa Wako-shi,

Saitama 351-0198, 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<TCO [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)2RbZn(SCN)4 on slow cooling

• At T>TCO diffuse rods with q=(1/3, k, 1/4) [M.Watanabe et al, 2003].

• At T<TCO space group changes from orthorhombic I222 to P212121.

• 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)2RbZn(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)2RbZn(SCN)4 on heating after fast cooling

• In a quenched state the diffuse rods with q1=(1/3, k, 1/4) observed previously at T > TCO becomes weaker but still exist below TCO [M.Watanabe et al, 2003].

• At T < TCO the another diffuse rod with q2=(0, k, 1/2) appears. The modulation of q2 type is disordered in the interlayer direction. After annealing the q2 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)2CsZn(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 300-120 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 q1=(2/3, k, 1/3) and q2=(0, k, 1/2) develop from these diffuse sheets.

.• With further cooling the peak intensity of q2 increases in comparison with intensity of q1

peak, though q1 charge modulation are also retained. The correlation lengths of q1 and q2 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)2CsZn(SCN)4

• In the range 20 - 4.3 K G ~ T 2.9

• Theory shows that for variable range hopping on the presence of electron correlations G ~ T3 [B.Shklovskii and A.Efros]

Temperature dependence of dielectric permittivity ε' of θ-(BEDT-TTF)2CsZn(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 (q2) and tree fold (q1)) , 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.