Fermi glass

We discuss in this section the effect of short-range interaction on the Anderson-localized states of a Fermi glass described in Chapter 1, Section 7, and in particular the question of whether the states are singly or doubly occupied. Ball (1971) was the first to discuss this problem. In this section we consider an electron gas that is far on the metal side of the Wigner transition (Chapter 8) the opposite situation is described in Chapter 6, where correlation gives rise to a metal-insulator transition. We also suppose that Anderson localization is weak (cca 1) otherwise it is probable that all states are singly occupied. 

Fig. 2.17 Density of states and singly and doubly occupied states for a Fermi glass...

The basic construct of the Fermi-glass model is a spatially averaged density of donor and acceptor states which is a function... 

V. N. Savvate ev and Z. Ovadyahu, Slow Relaxation in a Fermi Glass, in Hopping and Related Phenomena Proceedings of the 6th International Conference on Hopping and Related Phenomena, edited by O. Millo and Z. Ovadyahu, p. 44 (1995). 

C104 [286] which can be fitted with the same idea as the PPP case [322], it was conected by the authors of [278] that it was not reproducible but that in this case the origin of the non-Curie behaviour is due to missing thermal contact between the sample and sample tube without Helium gas that is effective even at liquid Helium temperature. On the temperature-independent susceptibility, there i.s not enough data to conclude if it is an evidence for the degenerate Fermi electrons with the definite Fermi level ora Fermi glass system, and the extent of disorder would be a crucial parameter for these materials. 

Dirty" conductors with electrical conductivities of the order of 10 S/cm or less Such systemsscan be achieved with relative ease all that is required is a moderately high density of carriers. Carrier delocalization is neither required nor implied by such values. In such cases, the heavily doped polymers are highly disordered systems which can be considered as examples of Ae "Fermi glass" concept. Indeed, rc-conjugation is not even needed to achieve this low level of conductivity in polymers. ... 

The data obtained allows us to conclude that highly doped PANI-SA and PANI-HCA are Fermi glasses with electronic states locahzes at the Fermi energy due to disorder, whereas PANI-TPSA and PANI-CSA are disordered metals on the metal-insulator boundary, so that the metallic quality of the emeraldine form of RANI grows in the series PANI-HCA —> PANI-SA —> PANI-PTSA —> PANI-CSA. 

For a Fermi glass, in which the states at Ep are Anderson localized, a photon of frequency co can activate the hopping near the Ep, resulting in finite o(w). In this case, Mott and Kaveh [1125,1157,1177] show that o(w) can be derived from the Kubo-Greenwood expression given in Eq. (4.10) in the localization regime... 

Role of Disorder in Conducting Polyaniline Evolution from Fermi Glass to Disordered Metal Although the IR spectrum of PANI-H2SO4 indicates that the low-energy response arises from intraband excitations in the conduction band, the optical spectra are not those of a typical metal. Instead, the charge carriers are local-... 

On the contrary, the optical spectra for the metallic regime indicate delocalized electronic states near Ef, as already explained in the previous section. Thus, on the metallic side of the M-I transition, PPy-PFg is a disordered metal and can be described well by the LMD model [1162], while, on the insulating side, PPy-PF5 is characterized as a Fermi glass, which is attributed to disorder in the context of Anderson IcKaliza-tion. 

The optical spectra of structurally improved materials are compared with those obtained from conventionally prepared materials. While the structurally improved materials show distinct metallic signatures indicative of delocalized electronic states at the Fermi level, the optical spectra of the conventional samples indicate that the states near the Fermi level are localized due to severe disorder in the context of Anderson localization. Thus, conventional samples can be characterized as a Fermi glass. In this case, the two categories of samples show different charge dynamics in the far-infrared consistent with theoretical predictions. 

In 1970, Anderson [51] proposed that a degenerate electron gas in a random disorder potential tends to localize if the magnitude of the disorder potential is large compared with the bandwidth. In such a case all the states become localized, and the system is a Fermi glass an insulator with a continuous density of local-... 

As noted above, in Fermi glass insulators, the Fermi level lies in an energy interval in which all states are localized [42,43,51]. The M-I transition occurs when the disorder is sufficiently weak that the mobility edges move away from the center of the band toward the band tails such that Ef lies in a region of extended states. 

Fig. 3.6 (a) The Anderson transition and (b) the form of the localized wave function in an Anderson metal-insulator transition, (c) The Fermi glass state where the Fermi level lies in the region of localized states, (From Refs. 64,65,80.)... 

C.B. Duke, Organic Solids Traditional Semiconductors or Fermi Glasses in Festkorperprobleme, J. Treusch, ed.. Advances in Solid State Physics, Vieweg, Braunschweig, 1982, Vol. 22, pp. 21-34. 

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