Electron pockets

The Fermi surface of WC was proposed based on magnetoresistance and de Haas-van Alphen data taken under high magnetic fields at low temperatures. WC is a semimetal with equal numbers of electron and hole carriers of 1.5 X 1021/cm3. The Fermi surfaces consist of two electron pockets located at the point A and four hole pockets located at the point L, and at the point K or along the T A axis. These results indicate that the spin-orbit interaction is very important in WC. 

In fact it is even possible that a 3D superlattice could be responsible for the electron pockets at low Tjand the change in sign of R i In HMTSF-TCXQ. the transverse transfer integral, t say, may well be large enough i satisfy the con-02) a... 

At energies (mostly) above Ep we find a set of six bands corresponding to the antibonding hybrids of the p orbitals of the two B1 atoms In the unit cell with the 02 and 03 p states. These bands form electron pockets near the L point and at the midpoint between F and Z (which will be referred to as H). Their dispersion across the BZ Is quite different (note their double periodicity) from that of the Cu-0 dpa bands, as a consequence of the different bonding character (ppa versus dpa) and local coordination (rock-salt versus perovsklte-llke). The doubly periodic dispersion of Bl-0 ppo bands can be understood on the basis of simple tight-binding arguments. 

The total density of states at the Fermi level, N(Ep), is 3.03 states/(eV-cell). Large contributions to N(Ep) come from both the Cu-Ol and the Bi-02 layers. The B1-02 contributions are from the pa bands which create small electron pockets around L (and li). 

Despite these common features, tlie T1 systems present some Interesting new points. In both the Tl/2212 and Tl/2223 compounds, there exists the presence of electron pockets around the F and Z points. A careful analysis of the cliaracter of these states, however, reveals important differences with respect to the Bl2Sr2CaCU20g case. While the Bi-0 bands at Ep in Bi2Sr2CaCu20g originate mainly from the In-plane ppa Bl-0 hybrid, the Tl-0 bands at Ep in Tl/2212 and Tl/2223 are mostly from oxygen p states hybridized (anti-bonding) with the T1 orbitals. In fact, the major T1 6s bands are located at about 7 eV below Ep. 

Fig. 36. Ulustration of the effect of an orthorhombic strain on electron pockets at the three X-points in LaAg. Full lines, unstrained state dashed lines, strained state, (a) E(k) for the three X-points, (b) X-point Fermi surface pockets (Niksch et al. 1987).

Fig. 12. Cross-sections of the pocket-Fermi surfaces in LaB , CeBj and PrBe. These electron pockets are flat in character (Onuki et al. 1989a).

The first nonself-consistent APW calulations of the WC monocarbide electronic structure were carried out by Alekseev, Arkhipov and Popova (1982). More correct results for hexagonal WC were obtained later by Mattheiss and Hamann (1984) using the LAPW method. It was shown that the band structure of WC contains much broader (as compared with B1 monocarbides) bands of hybridised C2p-W5d states with a larger energy interval between these bands and their antibonding counterparts. This is evidence of stronger covalency in WC. The Fermi surface of hexagonal WC shows several small hole and electron pockets near the T, K, L points in the Brillouin zone. For WC the total DOS at the Fermi... 

Fermi surface for ThC and UC using the relativistic APW method. The calculated results are in good agreement with the de Haas-van Alphen measurements. The UC Fermi surface appears to consist of three hole pockets in the region of the valence C2p states and six electron pockets in the region of the U5/ states. It was shown that UC is a semimetal and contains almost the same number of holes and electrons. 

---