Hybridization with conduction electrons

The dynamics of correlations within the 5f band (spin fluctuations) was taken into account by Takaoka and Moriya (1983). If the 5f-electron states are hybridized with conduction-electron states, they contribute to the scattering. The thermal dehybridization reduces the scattering rate and even a maximum in the temperature dependence of p can develop. 

In the magnetic transitional-metal compounds to be described in subsequent chapters the conduction electrons lie wholly in a d-band. This does not of course mean that there is no hybridization with 4s-electrons such hybridization must always be present to some extent, and will be responsible for any observable Knight shift. It does mean, however, that the d-band remains separate from the conduction band and contains an integral number of electrons per atom. As we... 

As can be seen in Figure 6, Tc of RM2B2C compounds also much depends on the lattice parameters. As an example the dashed line represents the variation of Tc in a series with non-magnetic elements R. It should be noted that the effects of lattice parameters in Figure 6 cannot be explained by only taking into account the variation of N( p) in the expression (3), caused by the variation of the lattice parameters. In particular in CeN E C and YbN E C superconductivity is suppressed by strong hybridization of 4f electrons with conduction electrons (see Sections 4.2 and 4.12). 

Photoemission and neutron diffraction measurements are limited only to UBe13. The combined XPS-BIS experiments of Wuilloud et al. (1984) displayed a picture similar to other very narrow-band compounds with 5f states extended to 2 eV below Ep, while the 5f intensity is spread above 5 eV about EF. Pronounced satellites corresponding to poorly screened final states accompany the core 4f lines of U, but the XPS spectrum of the Be Is level remains unaffected by hybridization with U electron states. A temperature-dependent narrow feature was distinguished at EF by means of high-resolution studies (Arko et al. 1984). The existence of a low intensity 5f-tail extended far below EF, which can be resolved by resonant photo-emission, is taken as an indication for the hybridization of 5f states with Be-derived conduction-band states (Parks et al. 1984). 

The metal centers are chosen from those that typically exhibit, in the frame of the complexes, reversible redox behavior, being stable in two oxidation states. Such a feature is required in order to sustain electrical conduction, either purely redox in character or somehow hybridized with the electronic one, proper of the a -electron chain fragments. Moreover, it also constitutes an evident necessity whenever one partner of the redox couple is called to play the role of redox mediator in... 

Contrary to what happens at large U, higher V tends to enhance the hybridization of 4f electrons with conduction electrons, thus accelerating the delocalization of the 4f electrons (Koelling et al. 1985). The delocalization of 4f electrons tends to make the 4f band wide. When Ef > V, we have still better localization and expect the Kondo regime in the Ce (or Yb) compounds. 

For concentrated Kondo systems orbitally driven anisotropic hybridization of the f electrons with conduction electrons is crucial (Cooper et al. 1988, Levy and Zhang 1989). The anisotropic part of the mixing interaction causes a substructure within the rather narrow f band, which leads to crystal-field effects even in highly itinerant-electron systems. On account of this crystal-field dressing a localized 5f electron in NpSnj might develop. 

In diamond, each carbon atom is sp3 hybridized and linked tetrahedrally to its four neighbors, with all electrons in C C cr-bonds (Fig. 14.30). Diamond is a rigid, transparent, electrically insulating solid. It is the hardest substance known and the best conductor ol heat, being about five times better than copper. These last two properties make it an ideal abrasive, because it can scratch all other substances, yet the heat generated by friction is quickly conducted away. 

However, in oxides, e.g. that of aluminium, AI2O3, the sp band of the aluminium hybridizes with the p orbitals of the oxygen to form a new band below the Fermi level, which leaves a gap of 7 eV to the antibonding part of the band. The lower part is the valence band, the upper part the conduction band, and the separation between them is the band gap. This material is an insulator, as it will be hard for an electron to become excited to the conduction band so that it can move through the oxides. 

In open shell metals, these empty states can be d- or f-states somewhat hybridized with band states (see Chap. A). In a metal, these states may be pulled down into the conduction band (as a virtual state, see Chap. A) in a compound, presenting a ligand valence band (insulator or semiconductor), they may be pulled down to an energy position coinciding with or very near to this valence band (as a true impurity level). The two possible final states (Eqs. 22 a and 22 b) explain the occurrence of a split response one of the crystal band electrons occupies either the outer hole level P (Eq. 22 a) or the more bandlike hole B " (Eq. 22 b). 

In light lanthanides (La, Ce, Pr, Nd) the pulled down 4f state is nearly localized and hybridizes only weakly with conduction states. The bandwidth W4f will be very narrow, U high and negative, and the occupation probabiUty by conduction electrons rather low. This results in the occurrence of shake-down satellites at a lower binding energy for lanthanides, accompanying a poorly screened main peak (Fig. 7 a). When proceeding to heavier lanthanides, the occupation probability and the intensity of the shake-down satellite are depressed the symmetric, poorly screened core level is left, i.e. the 4f states are completely localized. 

In d-metals, the opposite is true the d-wavefunctions hybridize easily with conduction band states. The main peak can in this case be coordinated with the well screening outer d s, and the shake-up satellite, when observed, is due to the poorly screening process (Fig. 7c). For d-metals, furthermore, the very high density of d-states at Ep is the cause of many secondary electron excitation from just below Ep to empty states just beyond Ep which results in the asymetric high energy tailing of the main peak. Final state multiplet splitting, explained above, can in addition overlap the split response. 

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