Fermi nesting

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... 

The Fermi surface must have large regions of flat parallel areas, i.e. it must exhibit strong nesting properties. 

Detailed electronic energy-band calculations have revealed the existence of appropriate surface states near the Fermi energy, indicative of an electronically driven surface instability. Angle-resolved photoemission studies, however, showed that the Fermi surface is very curved and the nesting is far from perfect. Recently Wang and Weber have calculated the surface phonon dispersion curve of the unreconstructed clean W(100) surface based on the first principles energy-band calculations of Mattheis and Hamann. ... 

Besides magnetic perturbations and electron-lattice interactions, there are other instabilities in solids which have to be considered. For example, one-dimensional solids cannot be metallic since a periodic lattice distortion (Peierls distortion) destroys the Fermi surface in such a system. The perturbation of the electron states results in charge-density waves (CDW), involving a periodicity in electron density in phase with the lattice distortion. Blue molybdenum bronzes, K0.3M0O3, show such features (see Section 4.9 for details). In two- or three-dimensional solids, however, one observes Fermi surface nesting due to the presence of parallel Fermi surface planes perturbed by periodic lattice distortions. Certain molybdenum bronzes exhibit this behaviour. 

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 14 A section of the Fermi surface of LuNi2B2C showing a nesting feature characterized by the wave vector q (0.55,0,0), and contour plot of the magnitude of the Fermi velocity (in 106 m/s) in the (001) plane through the r point (from Yamauchi and Harima, 2005). 

Thus, measurements of k and Cp provided first hints for possible nodes in the energy gap. More detailed investigations, however, point to different scenarios, in particular those based on Fermi-surface nesting. Also there are discrepancies between experimental data and the (s + g)-wave description. 

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