Peierls-like distortion

As we will show in the sequel, this very structure may be understood to arise from a flzree-dimensional Peierls distortion away from a simple cubic structure. Although Peierls theory (see Section 2.6.2) is one-dimensional in its original formulation, there have been extensions to real systems in which two-and three-dimensional cases of Peierls-like distortion were observed. For ex-... 

The phase transition consists of a cooperative mechanism with charge-ordering, anion order-disorder, Peierls-like lattice distortion, which induces a doubled lattice periodicity giving rise to 2 p nesting, and molecular deformation (Fig. 11c). The high temperature metallic phase is composed of flat EDO molecules with +0.5 charge, while the low temperature insulating phase is composed of both flat monocations... 

In the systems with diamagnetic M(mnt) units, only a tetramerisation of the perylene chains, corresponding to the 2kp Peierls transition, is observed. In the other cases, except for M = Fe, where the dimerisation is of stronger chemical nature, a dimerisation of the M(mnt) units takes place simultaneously with the distortion in the perylene chains. This distortion in the M(mnt)J chains with 5=1/2 can be associated with a spin-Peierls-like transition, and this association is especially tempting in view of the typical spin-Peierls exponential decrease of the susceptibility below f for directions parallel and perpendicular to the chain axis. However for this transition to be spin-phonon driven, it lacks the usual criteria for a spin-Peierls transition requiring an antiferromagnetic interaction J k Tsf, where T p° is the mean field transition temperature [80] for instance in the case of M = Pt with a Tg = 8.2 K, J/ks was estimated as 15 K while Tgp° should be 25 K, the temperature at which one-dimensional... 

In what follows we should bear in mind that the generation of a diamagnetic metallic state (irrespective of whether it is a superconductor or not) will not be favored by a half-filled band of electrons. Either a Peierls distortion or the generation of an antiferromagnetic insulating state will result, with a ferromagnet being less likely for the reasons discussed. Superconductivity in these materials is in fact only observed if electrons are removed, or (less commonly to date) added to the half-filled band. Considerable effort is underway to theoreti-... 

What would the band structure of a chain of p orbitals oriented head-on (Fig. 10.5) look like after a Peierls distortion ... 

From more recent optical data it is proved that Qn(TCNQ)2 is a semiconductor up to 300 K, with an energy gap due mainly to a Peierls distortion on the conducting TCNQ chains [57]. However, this gap Ec = 2A is not constant, as is simply assumed in the model of Epstein et al. In fact, A appears to decrease significantly from = 1200 K at T = 0 K to = 300 K at T = 300 K, somewhat like in the salt TEA(TCNQ)2 (see above). This requires a modified approach in which the existence of a conductivity maximum cjm still implies a T-dependent mobility p, but not so steep as would require a constant gap. 

One of the initial motivations for pressure studies was to suppress the CDW transitions in TTF-TCNQ and its derivatives and thereby stabilize a metallic, and possibly superconducting, state at low temperatures [2]. Experiments on TTF-TCNQ and TSeF-TCNQ [27] showed an increase in the CDW or Peierls transition temperatures (Tp) with pressure, as shown in Fig. 12 [80], Later work on materials such as HMTTF-TCNQ showed that the transitions could be suppressed by pressure, but a true metallic state was not obtained up to about 30 kbar [81]. Instead, the ground state was very reminiscent of the semimetallic behavior observed for HMTSF-TCNQ, as shown by the resistivity data in Fig. 13. One possible mechanism for the formation of a semimetallic state is that, as proposed by Weger [82], it arises simply from hybridization of donor and acceptor wave functions. However, diffuse x-ray scattering lines [34] and reasonably sharp conductivity anomalies are often observed, so in many cases incommensurate lattice distortions must play a role. In other words, a semimetallic state can also arise when the Q vector of the CDW does not destroy the whole Fermi surface (FS) but leaves small pockets of holes and electrons. Such a situation is particularly likely in two-chain materials, where the direction of Q is determined not just by the FS nesting properties but by the Coulomb interaction between CDWs on the two chains [10]. 

The details of what actually happens are presented elsewhere.16 The situation is intricate the observed structure is only one of several likely ways for the parent structure to stabilize—there are others. Diagram 95 shows some possibilities suggested by Hulliger et al.72 CeAsS chooses 95c.75 Nor is the range of geometric possibilities of the MAB phases exhausted by these. Other deformations are possible many of them can be rationalized in terms of second order Peierls distortions in the solid.16... 

An important consideration,in deciding what compounds to synthesise,in order to stabilise the superconducting state versus the CDW or SDW in molecular lattices, is the effective dimensionality. A first priority is to make the lattice less purely one-dimensional, so as to make it stable to Peierls distortion. In that context, the compound (HMTSF)(TCNQ) was a significant early example.HMTSF (hexa-methylene-tetraselenofulvalene) was synthesised to explore the consequence of introducing more bulky substitution of the TSF moieties,with the expectation that the system would be rendered more purely one-dimensional. Yet, it was found that, unlike TMTSF-TCNQ, which like TTF-TCNQ has a major metal-insulator transition near... 

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