Nesting vectors

Fig. 3 (a) Crystal structure of (DMET)2FeBr4. The dotted and dashed lines denote the intermo-lecular anion—anion and donor-anion contacts, respectively, (b) Fermi surfaces obtained for a donor layer around z = 1/2 using the tight-binding approximation. The solid arrow represents the nesting vector Q (a b )/2... 

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. 

We conclude this subsection with several remarks on the interpretation of the anisotropy of Hc2. The largest in-plane anisotropy reported by Metlushko et al. (1997) coincides with the direction of the nesting vector (0.55,0,0). Another manifestation of strong local anisotropy effects is provided by deviations from the 9 (angular) dependence due to anisotropic effective masses (Fermi velocities)... 

Figure 7 Nesting of the Fermi surface of quasi-one-dimensional conductors. At high temperature (a), the thermal fluctuations hide the warping of the surface and the conductor has a one-dimensional nesting vector Q0 = 2kf. At low temperature (b), the warping is felt and there is coherent interchain tunneling with Q0 IkjA + (it b)f>.

The electronic properties of organic conductors are discussed by physicists in terms of band structure and Fermi surface. The shape of the band structure is defined by the dispersion energy and characterizes the electronic properties of the material (semiconductor, semimetals, metals, etc.) the Fermi surface is the limit between empty and occupied electronic states, and its shape (open, closed, nested, etc.) characterizes the dimensionality of the electron gas. From band dispersion and filling one can easily deduce whether the studied material is a metal, a semiconductor, or an insulator (occurrence of a gap at the Fermi energy). The intra- and interchain band-widths can be estimated, for example, from normal-incidence polarized reflectance, and the densities of state at the Fermi level can be used in the modeling of physical observations. The Fermi surface topology is of importance to predict or explain the existence of instabilities of the electronic gas (nesting vector concept see Chapter 2 of this book). Fermi surfaces calculated from structural data can be compared to those observed by means of the Shubnikov-de Hass method in the case of two- or three-dimensional metals [152]. 

The quasiplateau for Tf1 which is observed for C104 and also for PF6 salts under pressure (Fig. 10b) is therefore related to 2kF correlations in a transient temperature regime when the nesting vector evolves from a purely one-dimensional situation (2fcF,0) to a vector that provides the best nesting of the two (three)-dimensional Fermi surface. Below the crossover temperature Tx a Fermi liquid behavior is recovered with an enhanced Korringa... 

A metallic state predicted by one-electron band theory is not stable when its Fermi surface is nested, and becomes susceptible to a metal-to-insulator transition under a suitable perturbation. We now examine the nature of the nonmetallic states that are derived from a normal metallic state upon mixing its occupied and unoccupied band levels. For simplicity, consider the 2D representation of the nested Fermi surface shown in (100), where the vector q is one of many possible nesting vectors. The occupied and unoccupied wave vectors are denoted by k and k, respectively. Each unit cell will be assumed to contain one AO x Suppose we choose the k and k values to satisfy the relationship... 

Fig. 2.4. (a) Schematic representation of a quasi ID Fermi surface with nesting vector fcp = (2A F,7r/6,0). The dash-dotted line is the resulting new Brillouin zone, (b) Opening of the Peierls gap 2A at ifep in the dispersion relation... 

NMR experiments it was even possible to estimate the SDW nesting vector Q — (Qa> Qb, Qc)> with Qo = 2fep, Qb 0.26, and <5c — 0> aud the amplitude a 0.08/iB of the SDW modulation, where /xb is the Bohr magneton [60, 61]. The value of a less than one means that the moments are not well localized but retain still an itinerant character. The vector Q is in good agreement with the nesting vector of the calculated band structure (see Fig. 2.3). 

---