Interchain coupling

Recently, Prigodin and Efetov have developed a theoretical model for interchain interaction at the M-I transition in a random network of coupled metallic chains [94]. In this model, the interchain disorder due to intrinsic defects and the randomness in the distribution of interchain contacts induce localisation. The M-I transition in such a system is determined by the critical concentration of interchain crosslinks, which in turn depends on the localisation lengths and interchain coupling. Moreover, a metallic state can exist in such a random network of coupled metallic chains only if the concentration of interfibril contacts is large enough to overcome the percolation threshold. 

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The temperature of the metal-to-insulator transition in TTF—TCNQ is 53 K. For systems with increased interchain coupling, the transition temperature for the onset of metallic conduction increases roughly as the square of the interaction between the chains. This behavior is tme as long as the coupling between chains remains relatively weak. For compounds with strong interactions between stacks, the material loses its quasi-ID behavior. Thus, the Peieds distortion does not occur even at low temperatures, and the materials remain conductive. 

Chain conformation (gauche vs. Irons) interchain coupling Surface topography Chain dynamics... 

The difference spectra of the sample without cholesterol show that the interchain coupling (reflected in the two band minima) is removed between 40 and 43 C coinciding with Tc. Above T0, X-ray diffraction studies (33) have demonstrated the lack of interdigitation in DHPC bilayers. It is also at this temperature, that the parameter I f increases demonstrating a direct relationship between the physical state of the bilayer and the properties of the guest ketone. A similar observation is made for the sample containing 8 mol % cholesterol. 

As mentioned in Section II, LRO in two dimensions can exist only for a real order parameter, that is, for CDW in a half-filled band. This would be the case for BOW in the polymers or the Peierls state, which would be stabilized by transverse hopping or interchain coupling. This is also the case of the CDW state of the n = 1 two-dimensional Hubbard model. All other types of instabilities, such as those treated in the RPA previously in Section V, require three-dimensional coupling to stabilize any LRO. 

There are two groups of organic conductors which, contrary to onedimensional TCNQ salts, show considerable interchain coupling. They are both of single-stack type, with donor molecules of tetramethyltetrathia-fulvalene (TMTTF) or tetramethyltetraselenafulvalene (TMTSF) and BEDT-... 

HMTSF-TCNQ, which is believed to have strong coupling between chains, does not show ESR signal at room temperature [37]. The linewidth is presumably on the order of 4000 Oe. On the contrary, TMTSF-DMTCNQ, a Se compound with a narrow line ( 70 Oe at 300 K), has been taken as evidence of weak interchain coupling [38]. 

It may be recalled here that the Peierls transition is basically a onedimensional effect coming from the divergent response in one dimension of the electron system at 2kF. However, because of the fluctuations, any transition is possible only at 0 K in one dimension and not at the temperature Tup predicted by mean-field theory. It is then an effect of the (small) interchain coupling to restore a transition temperature lower than TMF but finite. When the interchain coupling becomes too large (under high pressure, for instance) the one-dimensional character is lost and the Peierls transition is suppressed [2,3]. 

The existence of a finite interchain interaction in a quasi-one-dimen-sional system is a prerequisite for a phase transition. However, there exists no necessary relation between the strengths of intra- and interchain couplings. It is thus conceivable to observe for SDW and SC instabilities the existence (if not the predominance) of magnetic fluctuations above the superconducting transition or the reverse as well. Such situations are encountered in the study of the (TMTSF)2X series. 

As far as (TMTTF)2Br is concerned the c-axis parameter is the smallest within the TM2X series [164,165]. The dominant role of interchain coupling... 

In a second step interchain coupling is taken into account. Interchain transport can now take place. Two limiting cases have to be considered. In the case of weak disorder (the chains are reasonably parallel to each other), interchain coupling, by reintroducing some three-dimensional features, can prevent one-dimensional effects, such as one-dimensional localization, which reduce conductivity. If Ae (the mean-square deviation of the on-site energy) is a measure of the disorder, the condition for the onedimensional localization to be removed is... 

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