Peierls transition temperature

A couple of theoretical studies [5,19] have hitherto attempted to estimate the Peierls transition temperature (Tp) for metallic CNT. A detailed theoretical check with respect to the stability of metallic wavefunction in tube (5, 5) has also... 

Fig. 10. Illustration of the molecular displacements occurring in the a,-Cp plane in [TTF] [Cu(tfd)2] below the spin-Peierls transition temperature of 12 °K. Only the [TTF]+ units are shown for clarity however, the translation of the center of mass of the [Cu(tfd)2] units is indicated (Ref. 51)...

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]. 

Anomalous behavior was observed in the ESR spectra of the TMTTF-C14TCNQ LB film shown in Fig. 8 [61,62] a decrease in spin susceptibility with decreasing temperature, a prominent maximum linewidth, and a maximum g value. These features are similar to those observed in TTF-TCNQ crystals around the Peierls transition temperature [60,63,64]. The results can be reconciled if the system is an organic metal the observed anomalies... 

The occurrence of the energy gap 2A below the Peierls transition temperature allows in principle the collective motion of the electrons under the influence of an applied electric field. This holds as long as the energy hkpvpr of the moving electrons is less than A, where upr is the velocity of the collectively moving electrons. However, this so-called Frbhlich mode [4] is very sensitive to lattice imperfections because it is a true ID movement. 

The measurement and analysis of the magnetic susceptibihly can give further independent information on the electronic stracture of the (Fa)2Pp6 crystals. In particular, from the temperature dependence of the susceptibility, the effective energy gap 2 Aeff above the Peierls-transition temperature Tp can be directly derived. 

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. 

It will be intriguing to theoretically examine the possibility of superconductivity in CNT prior to the actual experimental assessment. A preliminary estimation of superconducting transition temperature (T ) for metallic CNT has been performed considering the electron-phonon coupling within the framework of the BCS theory [31]. It is important to note that there can generally exist the competition between Peierls- and superconductivity (BCS-type) transitions in lowdimensional materials. However, as has been described in Sec. 2.3, the Peierls transition can probably be suppressed in the metallic tube (a, a) due to small Fermi integrals as a whole [20]. 

As already discussed in Chapter 1, this kind of mixed valence salt becomes conductive due to the transfer of one electron from two BEDT-TTF molecules to the anion layers. However, at the surface, the charge can become unbalanced, resulting is an incomplete CT. This leads to differentiated surface vs. bulk nesting vectors and to the existence of surface CDWs (Ishida et al, 1999). The Peierls transition has also been observed on the a -planes of single crystals of TTF-TCNQ with a variable temperature STM (Wang et al, 2003) and will be discussed in Section 6.1. 

Figure 6.36. Detail of the /o(T ) curve for 3 showing the Peierls transition. The dashed line corresponds to a linear interpolation of the higher temperature region. Reprinted from Journal of Solid State Chemistry, Vol. 168, J. Fraxedas, S. Molas, A. Figueras, I. Jimenez, R. Gago, R Auban-Senzier and M. Goffman, Thin films of molecular metals TTF-TCNQ, 384-389, Copyright (2002), with permission from Elsevier.

Highly conducting 1-D system. Undergoes a Peierls transition at low temperature. Nearly superconducting. Stack of super-positioned, square-planar Pt(CN)4 groups. 

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 true 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 Peierls distortion does not occur even at low temperatures, and the materials remain conductive. 

According to the factor-group analysis, there are 18 Raman and 9 IR active new vibrational modes in the low-temperature structure of CuGe03, below the temperature of the spin-Peierls transition Tc=14 K. While two of the Raman-active folded modes were clearly observed in the very first optical experiments, no traces of the IR folded modes could be found for a long time. We have observed IR folded modes for the first time, measuring... 

An additional feature of the temperature dependent X-ray scattering is the persistence above Tc of intensity at the superlattice positions51). This is consistent with a soft phonon mode at a wave vector commensurate with the changes that occur on dimerization. It has been suggested that this low frequency lattice mode may be a requirement for the observation of a spin-Peierls transition. 

The conductivity of Ni(tmp)I at room temperature is high (o 100 Q 1cm 1) and metal-like, increasing until it reaches a rounded maximum at T 115 °K, then decreasing in an activated manner. The spin susceptibility is temperature independent down to a transition temperature of 28 °K, well below the conductivity maximum. At temperatures below the transition, the susceptibility decreases in an activated fashion, with A/k = 60 °K. Similar observations with organic conductors (TMTTF)2Y, Y = CIO4, BF4, have been attributed to a Peierls transition212, but in the present instance the mechanism has yet to be elucidated. 

At high temperature, TTF TCNQ is metallic, with a(T) oc T-2 3 since TTF TCNQ has a fairly high coefficient of thermal expansion, a more meaningful quantity to consider is the conductivity at constant volume phonon scattering processes are dominant. A CDW starts at about 160K on the TCNQ stacks at 54 K, CDW s on different TCNQ chains couple at 49 K a CDW starts on the TTF stacks, and by 38 K a full Peierls transition is seen. At TP the TTF molecules slip by only about 0.034 A along their long molecular axis. 

Structural correlations of Tc with chemical formula or structure type are limited. For the / -(ET)2X salts with linear anions there is a linear dependence of Tc on anion length (but this correlation fails for very long anions, as other phases form) [33]. The (TMTSF)2X salts with tetrahedral anions X show a linear dependence of the Peierls metal-to-insulator phase transition temperature with tetrahedral anion radius [33]. 

Kaiser et al.119 have studied 19F NMR in (Pyrene)i2(SbFg)7 cation salt. The rotational motion of those SbFg anions of this salt can be best discriminated by the analysis of the temporal evolution of the 1H Overhauser shift of the conduction electron ESR line. Temperature dependence of the Overhauser shift detected proton-SLR rate recorded at 9.46 GHz electron spin and 14.4 MHz proton NMR frequency. It is important to note that the proton relaxation reflects only the low temperature BPP-peak of SbFg anion rotation, in addition to conduction electron contribution. This salt undergoes a 3D-ordered Peierls transition at 113 K, which is due to the freezing of the anion motion. 

Recently, the spectral study of DMTM(TCNQ)2 phase transition was performed [60]. The salt is a quarter-filled organic semiconductor containing segregated chains of TCNQ dimers and DMTM counterions. This material undergoes an inverted Peierls transition, which has tentatively been explained in terms of a crystal-field distortion. It was shown that the experimental values of unperturbed phonon frequencies and e-mv coupling constants are nearly independent of temperature. The dimer model fails to reproduce the phonon intensities and line shapes and underestimates the coupling constants, whereas the CDW model produces better results... 

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