Temperature interchain coupling

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. 

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. 

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 small upper limit of the Davydov splitting established by the low temperature piezomodulation spectra of PTS indicates that the interchain coupling of electronic transitions is negligible for that system. While the sidechains of the phenylurethane series may change this somewhat, it is unlikely that the interaction will exceed the weak coupling case. This is confirmed by the bandwidth studies of the reflection spectra where the coupling may approach the intermediate case. 

It can be easily seen that T >Tq, Tp>Tp in both limits of strongly and weakly coupled CDWs. Our reasoning is valid even if among the temperatures (6,7) only those characterizing the actual structural instabilities are consistent with the appropriate limit of the interchain coupling. 

A large e-e interaction with respect to the bandwidth leads to a Mott-Hubbard magnetic insulator at low temperature, irrespective of the interchain coupling /12/. Finally, there is some hope to maintain the conducting state at low temperature whenever the interchain coupling is large and e-e inter action small /1/. 

One of the features of reduced interchain coupling is the suppression of three-dimensional ordering and this may be responsible for the lower transition temperature seen in this material. 

In the case of TTF-TCNQ, T measurements show a frequency independence in the frequency range about 14 MHz moreover, we have seen that is temperature independent. Only the first assumption leads to conclusions in agreement-with experimental results. So, we can conclude that in TTF-TCNQ, R(w ) = R(iot). This result confirms that the interchain coupling is... 

Since long-range order at finite temperatures cannot occur in a strictly one-dimensional system, a phase transition (Peierls, superconducting, etc.) can take place only as a result of finite interchain coupling. Therefore, the actual transition temperature T. is not the same as the "mean-field" temperature Tmf (which is the temperature, at which the transition would have occurred, if the interchain interactions had been equal to the intrachain interactions). If we define Tj as the characteristic temperature for the interchain interactions, then T is approximated by the geometric mean Thus, fluctuations or precursor events... 

The weak interchain coupling orders the modulated structure three-dimen-sionally at the Peierls transition temperature, Tp. The force due to an applied electric field, within regions where the CDW is coherent, is counterbalanced by the pinning force of impurities or other defects. For high-purity crystals the application of small electric fields may depin the CDW from the lattice, and the modulated structure slides as a whole. 

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