Excitonic insulators

Finally, we mention that there is some doubt whether the transition in mercury is of normal Anderson type Turkevich and Cohen (1984) have proposed a model in which an excitonic insulator is formed, for conductivities below 102 ft"1 cm 1. 

Of these eight interchain two-particle correlation functions, only the q=0 excitonic insulator (El) responses are divergent. There are both symmetric and antisymmetric forms of El (from the sum and difference of the two operators), and the magnitude of the symmetric form is larger. The regions in which the various two-particle correlation functions are divergent are shown in Fig. (2). We note that El diverges for g a 0 and - g /2 + < k/5 (for finite but... 

In contrast, Kulikov (1982) calculates the band structures of LaHj and LaHj, using self-consistent local-density functional theory. He stresses the importance and sensitivity of the choice for the crystal potential, finding incipient overlap between the conduction and valence bands for LaHj. The concept leads to an excitonic insulator phase at low temperatures. The low-temperature phase is semiconducting, and the higher-temperature phase is metallic i.e., the interpretation is opposite from that of Fujimori and Tsuda (1981). 

The excitonic model was carried further by Turkevich and Cohen (1984a. 1984b) who proposed the stabilization of an excitonic insulator state. This is an insulator in which the hybridized (6s-6p) state actually becomes the ground state so that the system acquires a macroscopic electrical polarization due to the dipoles. Under these conditions, the fluid... 

The bandlike aspects of the excitonic insulator model can be replaced by a more localized description developed from liquid-state theory (Hall and Wolynes, 1986 Lx>gan and Edwards, 1986 Xu and Stratt, 1989). In essence, the delocalized Wannier) exciton is replaced by a localized Frenkel) exciton. Such calculations exhibit a sharp transition in the degree of hybridization as the density of the system is varied. When the mercury density reaches a critical value, the degree of -character in the ground state drops sharply from the 100% (pure s-) atomic value. In common with other exciton models, the electronic transitions associated with electric dipole interactions in the localized limit are strongly enhanced by clustering. 

Electronic properties. The alloy system TmSei j(Tej( is one of the most important alloy systems in the field of intermediate valence since it keeps the important cation untouched and contains intermediate-valent metals, intermediate-valent semiconductors, semiconductor-metal transitions, ferro- and antiferromagnets and a new ground state of condensed matter, the excitonic insulator. The Fermi level generally will be in a hybridization gap, but not in some cases, especially when the material exhibits spontaneous magnetic moments. For the sake of completeness and consistency the entire alloy system will be treated in this section. 

To explain these unusual results Neuenschwander and Wachter (1990a,b) and Bucher et al. (1991) have used the concept of an excitonic insulator. The model of an excitonic insulator was proposed for the first time about 30 years ago by Sir Nevil Mott (1961), later it has been used by Knox (1963) and Kohn (1968), however, up until the present day there was no experimental confirmation. 

In summary the alloy systems TmSei cTe c and Tmi cEuj.Se have proven to belong to one of the most fruitful areas of research where new phenomena such as exchange-induced valence transitions or the excitonic insulator have been discovered. This only became possible by systematically varying the composition and using only well characterized large single crystals, which were never handled in air. 

Why then would this phenomenon be observed in YbS and YbO too In TmSei [Tei we had a small semiconductor gap to close which means the pressure scale is an order of magnitude smaller than in YbS or YbO. With pressure we not only reduce the gap and start valence mixing, but we also increase the electron-hole Coulomb interaction, but only when there are not too many free electrons because they would screen the Coulomb interaction. Thus in TmSei- fTci the alleging of TmSe with TmTe serves to reduce the free carrier concentration in TmSe from about 10 cm to about lO cm at 300 K (see fig. 86). So when the gap is reduced to the meV range and the 4f state acquires a dispersion as in fig. 91 the Coulomb electron-hole interaction wins and the excitonic insulator is formed. In YbS and YbO, which have a large gap of about 1 eV to be closed with pressure, we have to consider that at 400 kbar the volume is reduced to about 70% and thus the electron-hole interaction is substantially increased, much more than in TmSci xTe,( with... 

Further possibilities for long-range effects of chemical carcinogens are provided by the change or occurrence of different collective states in these complex polymers. These collective states can be vibrational or conformational solitons, Mott insulator states, Peierls instabilities, plasmon-type states, charge and spin density waves, excitonic insulator states, etc. Here only one example (which has been worked out in some detail), namely a conformational soliton caused by carcinogen binding, will be discussed. 

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