Spiral Charge Frustration in the Molecular Conductor(DI−DCNQI)2Ag

Abstract

We theoretically study the effect of spiral-type charge frustration in a quasi-one-dimensional molecular conductor (DI-DCNQI)2Ag. We clarify how the spiral frustration in the interchain Coulomb repulsion is relieved and leads to a self-organization of complex charge-lattice ordered chains, in agreement with the recent synchrotron x-ray study [T. Kakiuchi, Phys. Rev. Lett. 98, 066402 (2007)10.1103/PhysRevLett.98.066402]. In addition, we find that a keen competition between charge and lattice degrees of freedom under the frustration gives rise to a characteristic temperature within the ordered phase, below which a drastic growth of molecular displacements occurs. Our results enlighten the relevance of the spiral frustration and provide a possible reconciliation among puzzling experimental data.

Finite-Temperature Properties across the Charge Ordering Transition –Combined Bosonization, Renormalization Group, and Numerical Methods–

“…34,35) A difference between our model and the situation in the actual TMTTF compounds is that, the present calculation does not include the intrinsic lattice dimerization along the chains while it exists in the materials, which leads to another non-linear term in the bosonized Hamiltonian. 22) In (DI-DCNQI) 2 Ag, even though recent studies 36,37) revealed that the system undergoes a more complex chargelattice ordering than the simple CO we investigate in this paper, the T dependence in ρ(T ) across T = T CO observed at high pressure resembles our calculated data, whereas the kink at T = T CO is smeared out at ambient pressure. 38,39) All these Q1D molecular conductors show no anomaly in the bulk χ σ (T ) at T = T CO , while NMR measurements show the appearance of atomic sites showing different Knight shifts and relaxation rates: 40,41) these are also consistent with our analysis.…”

Achievements and Challenges in Molecular Conductors

“…In parallel to, and independent of this theoretical indication, CO was observed by NMR [30] in another quasi-one-dimensional CT, (DI-DCNQI) 2 Ag (DI-DCNQI =2,5-diiodo-N,N'-Dicyanobenzoquinonediimine2,5-diiodo-N,N'-Dicyanobenzoquinonediimine), which is without dimerization at high temperatures. (Once CO sets in, a particular type of dimerization is stabilized [31,32].) A little later CO was also identified by NMR in an experiment with (TMTTF) 2 X (X = PF 6 and AsF 6 ) with finite dimerization [33].…”

Theoretical Studies on Phase Transitions in Quasi-One-Dimensional Molecular Conductors

“…However, it is rather rare to have such an example in the existing compounds; Many have intrinsic dimerization and/or complex interchain coupling. DI-DCNQI 2 Ag [34,50] was formally considered to be the typical system, while recently it has been proposed that the ordered phase below T < 220 K might be a complex CO, DM mixed state [51,52]. TMTTF 2 X [35,53,54] family have been providing an ideal stage for such comparison, for systems with dimerization from the outset.…”