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Fig. 1.11. The calculated hole Fermi surface of paramagnetic Tb in
the Brillouin zone of Fig. 1.4. The extension of the 'webbing' between
the 'toes' near the zone boundary is believed to give rise to a peak in the
conduction-electron susceptibility χ ( q ), which determines the Q -vector
characterizing the helical structure.
The Fermi surface of paramagnetic Lu, in which the 4 f states are all
filled, has also been studied by the dHvA effect (Johanson et al. 1982)
and found to be in semi-quantitative agreement with the calculations of
Tibbetts and Harmon (1982). Since the results of band structure calcu-
lations have been confirmed experimentally at the Fermi level in widely
separated elements in the rare earth series, it is reasonable to suppose
that they will also be successful in accounting for other ground-state
properties. Characteristic band energies for the trivalent lanthanides,
calculated by Skriver (1983) at a common atomic volume close to the
equilibrium value for Gd, are shown in Fig. 1.12. In this figure, the effect
of the change in potential is thus separated from that of the interatomic
spacing. The most notable feature is the fall in energy of the s band
relative to the d band with increasing atomic number, which results in a
decrease of the occupation of the latter, with consequences, as we shall
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