Fermi surface studies

Besides the organic metals and superconductors based on the donor molecule ET numerous metallic organic charge transfer salts composed of different donors and acceptors have been synthesized. However, only a small number of these salts become superconducting, usually at rather low temperatures, and even less have been investigated in the same detail as the ET compounds. Apparently, the generally low crystal quality of the non-ET materials permitted only a few Fermi surface studies by SdH or dHvA experiments. 

The above information concentrates mainly on UX3 compounds while the experimental data on the analogs with Th or heavier actinides are very scanty. A number of spectroscopic and Fermi-surface studies available for UX3 compounds together with results of band-structure calculations and the systematic behaviour observed across the series of light actinides offer the possibility to draw more general conclusions for this system of AnX3 compounds. 

As mentioned in section 2.3.1, the energy bands of Gd are polarized when the 4f moments are ferromagnetically ordered. There are a number of experiments on the spin polarization effects. These studies have more in common with each other than with the band structure and Fermi surface studies using the same methods. We will review here a positron annihilation experiment (Hohenemser et al., 1968), two field emission experiments (Hofmann et al., 1967 Chrobok et al., 1968), and a UPS experiment (Busch et al, 1969). 

This basic quantity may be related to the true electron momentum distribution when the positron wave function is known through calculations. The many-body factor y f) is known to play some role but it has been established that the correlations have no effect on the position of the Fermi surface breaks (Majumdar 1965). Equation (1) shows that positron annihilation is well suited for Fermi surface studies as only ni(k) has breaks. This can be further seen if one expresses the positron and electron wave functions as Bloch wave functions. One obtains... 

Sm compounds have interesting magnetic behaviors. dHvA measurements have been performed on some compounds such as Smins, SmSb, SmGa2, and SmCu. They are, however, not valence-fluctuation compoimds but f-localized ones. Therefore, the situation is similar to the Nd system. Fermi surface studies for valence-fluctuating Sm compounds are open to future studies. 

The recent Fermi surface study for Laln3 presents a sUghtly different picture (Umehara et al. 1991a) of the Fermi surface. That study concludes from their magnetoresistance measurements that there is no saturation with the implication that the open orbits arising from bridging of the hole surface from F to R (and also X) are not present in Lainj. This is not consistent with the band calculations... 

There are no direct Fermi surface studies of the transition metal Laves phases -which is unfortunate in light of the part they have played, and continue to play, in our understanding of f electron behavior. The Np Laves phases have played a special role because they appear to span the critical separation between localized and itinerant behavior (Aldred et al. 1974). The U and Ce transition metal Laves phases occur on the itinerant side of the Hill (1970) plots, but some do approach, and just cross, the critical separation. The transition to magnetic behavior can be very closely approached by considering NpRu Osz-x alloys (Aldred et al. 1975). Because their properties are consistent with the Hill correlation, it would initially appear that one has a nice simple picture based on a direct f-f overlap analysis. Certainly, a Hill plot analysis was part of the motivation for the extensive studies of the Np materials. However, it appears that these materials heavily involve interaction with the ligands. 

A second class of experiments relates to effects which take place at or about the Fermi level, particularly in metals. Some of these are reviewed in Section 5.4. There are recent reviews of such Fermi surface studies by Lee and Cracknell. ... 

Rajput, S.S., Prasad, R., Singru, R.M., Triftshauser, W., Eckert, A., Kogel, G., Kaprzyk, S. and Bansil, A. (1993) A study of the Fermi surface of lithium and disordered lithium-magnesium alloy theory and experiment, J. Phys. Condens. Matter, 5,6419-6432. 

Sakurai, Y., Tanaka, Y., Bansil, A., Kaprzyk, S., Stewart, A.T. Nagashima, Y., Hyodo, T., Nanao, S., Kawata, H. and Shiotani, N. (1995) High-resolution Compton scattering study of Li asphericity of the Fermi surface and electronic correlation effects, Phys. Rev. Lett., 74, 2252-2255. 

Note that this choice is not unique actually we are now studying another possibility of quark pair between different Fermi surfaces [16]. 

The mean values (A ) begin to split with each other at a density where U a becomes finite. We d like to make a comment here. One may be surprised to see their value of O(GeV), coming from our parameter choice. However, what we d like to reveal here is not their realistic values but a possibility of color magnetic superconductivity and its qualitative features. More realistic study, of course, is needed by carefully checking our approximations, especially the contact interaction and the sharp cutoff at the Fermi surface. 

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. ... 

The Fermi surfaces of these salts have been studied by measuring the quantum oscillations [183] such as SdH (Shubnikov-de Haas) and dHvA and geometrical oscillations (AMRO, angle-dependent magnetoresistance oscillation) ([4], Appendix, pp 445 48). The Fermi surface of k-(ET)2Cu(NCS)2 (Fig. 14c) calculated based on the crystal structure is in good agreement with those observed data [225]. 

Feenstra, R. M., and Martensson, P. (1988). Fermi level pinning at the Sb/GaAs(110) surface studied by scanning tunneling spectroscopy. Phys. Rev. Lett. 61, 447-450. 

The NFE behaviour has been observed experimentally in studies of the Fermi surface, the surface of constant energy, F, in space which separates filled states from empty states at the absolute zero of temperature. It is found that the Fermi surface of aluminium is indeed very close to that of a spherical free-electron Fermi surface that has been folded back into the Brillouin zone in a manner not too dissimilar to that discussed earlier for the simple cubic lattice. Moreover, just as illustrated in Fig. 5.7 for the latter case, aluminium is found to have a large second-zone pocket of holes but smaller third- and fourth-zone pockets of electrons. This accounts very beautifully for the fact that aluminium has a positive Hall coefficient rather than the negative value expected for a gas of negatively charged free carriers (see, for example, Kittel (1986)). 

A very convenient method of investigating electronic effects in catalysis is to study the activities within a series of metal alloys. Changes in alloy composition will alter the energy density of electron levels at the Fermi surface. Moreover, in the particular case of transition metals, it has been seen that alloying with a metal of Group 16 decreases the number of holes in the d-band. These effects should profoundly alter the catalytic activities of the alloys. It is important to keep in mind that within such... 

Here N(sQ is the electron density of states on the Fermi surface for one direction of spin, is the effective volume of phonon generation, is the point contact form factor, averaged over the Fermi surface. It should be noted that point contacts of sizes d > l, d l can work also in diffusive or thermal current regimes [5] and are used for the study of EPI, phase transitions, superconductivity and other interesting physical phenomena. 

---