Excitonic mechanism, high-temperature

Here E is the frequency of the pairing-mediating excitation, N(Ef) the density of states at the Fermi level, and V the electron-excitation coupling strength. In view of the high transition temperature of 28 K and the polarizable jv system of the Ceo molecule, excitonic mechanisms of the type envisaged by Little, where E is an electronic excitation energy, may be of relevance. 

Such a mechanism would be consistent with the absence of reaction by a two-photon absorption process, because such a process does not produce excitons in high concentration and, therefore, reduce blexcitonic process. The temperature effect is simply a manifestation of reduced exciton hopping at lower temperature. 

The present model for nerve impulse resembles closely the exciton mechanism of high-temperature superconductivity, as put forward by Little and Ginzburg. Little s polymer consists of a polyene spine with polarizable dye side chains, the latter forming the exciton band. Ginzburg proposes high-temperature superconductivity to be found in thin metallic films placed between highly dielectric layers. " ... 

In the high-temperature region one can expect peculiarities of c. due to variation of the Lorentz number near an electron zone boundary of a material (see section 2) and to other scattering mechanisms of electrons. Peculiarities can arise in k, , due to bipolar, photon and exciton contributions and possible heat transport by induced vacancy diffusion. Known experimental data on k of rare earth compounds confirm the regularities observed in other types of materials. However, many effects are original, have their own distinctive features. We will separate these results into two groups ... 

Also the formalism of a model of a high-temperature superconductor based on the exciton mechanism is presented. 

In the next section i/e will outline a Green s function formalism to obtain the exciton energies. The calculations of S. Suhai on two typical polydiacetylene crystals (PTS with an acetylene-like structure and TCDU with a butatriene-like structure) are presented in section III. In section IV the formalism of a model of a high-temperature superconductor based on the exciton mechanism is presented, and in the final section a conclusion is given. 

Figs. 2.9,2.10, recorded at very low temperatures and in high resolution. For a crystal of very high quality, the broadening of these transitions is mainly due to phone- >. We briefly analyze their influence on the reflectivity in order to assure that no spurious structures are considered. These spectra have been recorded for the first time and analyzed by Turlet et al.1-67. We simply summarize a few points necessary to test the KK transformation, to point out the specificity of the intrinsic relaxation mechanisms related to exciton-phonon couplings, and to evaluate quantitatively the corresponding coupling parameters. 

Our estimation shows that the radiative and nonradiative intraband transition rates may be comparable at certain temperatures, while at high or low temperature one of these mechanisms dominates. In particular, at room temperature nonradiative transitions are significantly faster. On the contraiy, at helium temperatures the radiative emission (with the energies to be order of 1 eV) corresponding to the intraband transitions can be observed. We suppose that the fast emission band with energies less than the exciton band observed at low temperatures [7] corresponds to the intraband optical transitions. 

Chemical substitution has been used as a probe of the origin of the high transition temperature in YBa2Cu30y. The properties of this material are best understood when the material is considered to be a semiconductor doped to metallicity. When oxygen doped, the material becomes a 1 dimensional excitonic superconductor. The intrinsic semiconducting energy gap then provides the mechanism for the extremely... 

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