Polaron band narrowing effects

This concept of polarons and, in particular, excitonic polarons has been used to explain observed features of the one-dimensional conducting organic materials based on 7,7,8,8-tetracyano-p-quinodimethane, TCNQ (85). It indicates that a way to reduce the Coulomb repulsion between electrons in the chain is to surround each chain by a highly polarizable medium. However, a limit may be reached beyond which, if the surrounding medium were made more polari zable, the effects due to band narrowing would outweigh the benefits of reduced Coulomb repulsion (85). 

Another experimental evidence against the polaron lattice model for the metallic state of heavily doped trans-(CH)j comes from Electron-Energy-Loss Spectroscopy (EELS) data [21]. These data show levels spread well across the gap, which is more in agreement with the disordered incommensurate state than with the picture of narrow polaron bands in the gap. Band structure calculations using the Valence Effective Hamiltonian (VEH) technique [22] support this conclusion since it is shown that a large energy gap exists between the polaron bands in the band structure of the polaron lattice. On the other hand, experimental and theoretical results have been presented that support the polaronic metal state for doped polyaniline (emeraldine salt) [23]. 

Although measurements continue to be made, it seems quite clear that native DNA is certainly not the band-type conductor first suggested by Eley and Spivey [35]. Disorder effects, polaron coupling, and the narrow band nature of the transport all suggest that A-DNA, or any native DNA, should be an insulator at long length scales. Quoted measurements indeed support this conductivity of less than 10 S/cm [76], or l/g>10 2 for lengths 40 nm [65], -4-10 ° Q for poly(GC) [77], or >10 2 for 1.8-/im DNA [52]. 

The energy E will necessarily have this minimum, but its value at this point can be positive or negative only in the latter case will a stable self-trapped particle (i.e. a small polaron) form. This is most likely to occur for large effective mass, and thus for holes in a narrow valence band or for carriers in d-bands. If the polaron is unstable then there is practically no change in the effective mass of an electron or hole in equilibrium in the conduction or valence band. 

The term mixed valence is widely used in the literature to describe a phenomenon rather different from that considered here. Typically it refers to a metal (or a rare-earth alloy, or a compound such as SmS) which features a broad conduction band (formed by the overlap of s, p, or d orbitals), and a very narrow band such as an /-band, slightly above the Fermi level. This band becomes partially populated, hence the mixed, or fractional, valence. Fluctuations with time, and various degrees of localization in space, result from electron-phonon interaction. A useful review appeared recently.Many of the ideas used in this field parallel those used in other more chemical types of electron transfer. Recent articles on mixed-valence as a polaronic effect and on local polaronic effects exemplify this, and the dynamical properties have been discussed. Other recent reviews of this area deal with spectroscopic techniques and with mixed valency in rare-earth compounds. ... 

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