Electron phonon coupling energy

The PL spectrum and onset of the absorption spectrum of poly(2,5-dioctyloxy-para-phenylene vinylene) (DOO-PPV) are shown in Figure 7-8b. The PL spectrum exhibits several phonon replica at 1.8, 1.98, and 2.15 eV. The PL spectrum is not corrected for the system spectral response or self-absorption. These corrections would affect the relative intensities of the peaks, but not their positions. The highest energy peak is taken as the zero-phonon (0-0) transition and the two lower peaks correspond to one- and two-phonon transitions (1-0 and 2-0, respectively). The 2-0 transition is significantly broader than the 0-0 transition. This could be explained by the existence of several unresolved phonon modes which couple to electronic transitions. In this section we concentrate on films and dilute solutions of DOO-PPV, though similar measurements have been carried out on MEH-PPV [23]. Fresh DOO-PPV thin films were cast from chloroform solutions of 5% molar concentration onto quartz substrates the films were kept under constant vacuum. 

Thus, as far as the overall electron-phonon coupling energy is concerned, we have reasonably good correlation here between optical and thermal processes. Recently, owing particularly to the work of Taube and of Meyer, there is beginning to be an increasing body of data of correlations of this type, which are of... 

Electron-Phonon Coupling Energies and Frequencies from Experi-mental Data... 

Another interesting application of the total energy approach involves superconductivity. For conventional superconductors, the 1957 theory of Bardeen, Cooper and Schrieffer [26] has been subject to extensive tests and has emerged as one of the most successful theories in physics. However, because the superconducting transition temperature Tc depends exponentially on the electron-phonon coupling parameter X and the electron-electron Coulomb parameter p, it has been difficult to predict new superconductors. The sensitivity is further enhanced because the net attractive electron-electron pairing interaction is proportional to X-p, so when these parameters are comparable, they need to be determined with precision. 

The successful prediction of superconductivity in the high pressure Si phases added much credibility to the total energy approach generally. It can be argued that Si is the best understood superconductor since the existence of the phases, their structure and lattice parameters, electronic structure, phonon spectrum, electron-phonon couplings, and superconducting transition temperatures were all predicted from first principles with the atomic number and atomic mass as the main input parameters. 

Measurement of the specific heat at Tc should yield quite informative data. Information can be gained about the binding energy of the electrons in the Cooper pairs as mediated by electron-phonon coupling (in the BCS theory). 

Besides electron-phonon coupling, the shape of the Fermi surface is important in deciding CDW formation. Fermi surfaces of systems containing linear or planar arrays of atoms reflect their one- or two-dimensional character. Since the Fermi surfaces connect many states with the same wave vector, Q, a periodic distortion having the wave vector Q will produce gaps at those portions connected by Q. The energy gained... 

These equations must be solved numerically. In our first calculations [46] the value of to was taken as 0.3 eV, the value obtained from an ab initio calculation of the energy of a pair of Gs as a function of the distance between them [47] for a separation of 3.4 A. The electron-phonon coupling constant a, the derivative of t with respect to displacement, was obtained as 0.6 eV/A from the results of [47]. Subsequently, when a value of to other than 0.3 eV was used a was scaled accordingly. Although to and a values depend on the particular pair of neighboring bases, the calculations were simplified by using the same value for all pairs of bases. The value of the elastic constant K was taken as 0.85 eVIA, derived [46] from the measured value of the sound velocity in DNA. 

As a further point, we mention that the vibrational energy loss AE (Fig. 16) is also often expressed in terms of the strength of the electron-phonon coupling or of the Huang-Rhys factor S (see, for example, Stoneham, 1975,1977,1981), where... 

The magnitude of the off-diagonal Hamiltonian (i.e. the energy transfer rate) thus depends on the strengths of the electron-phonon and Coulombic couplings and also the overlap of the two exciton wavefunctions[53]. Energy transfer rates from states to state jx, are calculated via the golden rule [54] and used as inputs to a master equation calculation of the excitation transfer kinetics in PSI, in which the dynamical information is included in the matrix K. 

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