Polaronic metal

As noted above, the free carrier contribution which extrapolates to the measured a(0) must be in the far-IR below 450 cm"1.15 Nevertheless, most of the 7C-electron oscillator strength remains in the broad absorption band above 0.2 eV. An alternative which appears to be in agreement with the essential experimental facts is that polyacetylene is an example of a polaronic metal. The polaron lattice with a half-filled polaron band is certainly consistent with the observed susceptibility, hi the case of a polaron lattice, the IRAV modes are expected, and would be r -shifted from the Raman modes provided that the pinning is weak. Although the intensity of the IRAV modes was initially csdculated to be much too weak, this calculation ignored the effect of the counter-ions the counter-ion Coulomb potentials may lead to sufficient nonuniformity in the charge density, to yield the observed IRAV mode intensities.For the polaron lattice, a((o) would have two contributions with a "gap" in between ... 

The concept of the polaronic metal has been applied to polyaniline as well, and may be a more general feature of the metallic state in conducting polymers. In the context of the discussions presented in Section I through Section IV, the polaronic metal is indeed a "metal" and the analysis given is appropriate. In particular, the importance of the selforganization of the doped polymer (to avoid counter-ion scattering) and the need to avoid localization etc. are of clear importance. 

We conclude that the available band structure calculations provide the basis for a semiquantitative understanding of the absorption spectra. There are, however, quantitative discrepancies. In addition, the importance of the 1.5 eV interchain transition in [lS] (A )n indicates that more extensive three-dimensional band calculations will be required for a complete understanding of the electronic structure of the polaronic metal. 

Zhao, G., Smolyaninova, V., Prellier, W., and Keller, H. (2000) Electrical transport in the ferromagnetic state of manganites small-polaron metallic conduction at low temperatures. Phys. Rev. Lett, 84, 6086. 

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]. 

Kivelson, S. and Heeger, A. J. (1985) "First Order Transition to a Metallic State in Poly acetylene A Strong-coupling Polaronic Metal." Phys. Rev. Lett. 55,308-311. 

Upon protonation of [(1A)(2A)]n to the emeraldine salt, there is a structural change (with no change in the number of electrons) leading to a half-filled band and a metallic state (described as a "polaronic metal) [36-39]... 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

CO adsorption on electrochemically facetted (Clavilier), 135 Hamm etal, 134 surfaces (Hamm etal), 134 Platinum group metals in aqueous solutions, 132 and Frumkin s work on the potential of zero charge thereon, 129 Iwasita and Xia, 133 and non-aqueous solutions, 137 potentials of zero charge, 132, 137 preparation of platinum single crystals (Iwasita and Xia), 133 Platinum-DMSO interfaces, double layer structure, 141 Polarization time, 328 Polarons, 310... 

Transport in DNA samples with all bases the same could be either by free carriers, i.e., band transport, or by polarons. As will be further discussed in the next section, the polarons are expected to be large polarons, not small. In the conducting polymers there is overwhelming evidence that electrons (holes) from a metal contact are injected directly into polaron states in the polymer, because the polaron states have lower energies than the LUMO (HOMO) or conduction (valence) band edge. As has recently been shown theoretically [30], the injection takes place preferably into a polaron state made available when a polaron-like fluctuation occurs on the polymer chain close to the interface, rather than into a LUMO state, with subsequent deformation to form the polaron. It could also be expected for DNA that injection... 

The behaviour of polarons is of importance for the oxides and similar materials to be discussed in this book, and we summarize some of their properties here, as applied chiefly to transitional-metal oxides. (For more detailed discussions see Mott and Davis (1979, Chap. 3), Austin and Mott (1969a, b), Appel (1968), Emin (1973, 1975) and Mott (1973b). 

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