Electron-phonon Coupling Coefficient Research Articles

Resonance fluorescence spectra of dye-doped polymers

Site-selected fluorescence spectra have been measured for Mg-octaethylporphyrin doped in polystyrene and PMMA films at low temperatures by a time-correlated single-photon counting method under CW mode-locked dye-laser excitation into the lowest optical absorption band. A narrow resonance line accompanied by a broad sideband has been observed by rejecting the scattered laser light using a time gate. A single-site fluorescence spectrum has been obtained by the use of saturation effect of the laser-induced fluorescence. The density of phonon states weighted by the electron-phonon coupling coefficient has been determined, for the first time for a dye-polymer system, from the analysis of the obtained spectrum. Temperature dependence of the laser-induced fluorescence spectrum is compared with the simulation based on the weighted phonon-state density.

Microscopic theory of electron-phonon interaction and superconductivity of BaPb 1−xBi xO 3

Electron-phonon coupling coefficients in BaPb 1−xBi xO 3 are calculated microscopically on the basis of the tight-binding band model which utilizes energy band structures obtained with the use of the self-consistent LAPW method by Mattheiss and Hamann. Assuming the rigid-band model we calculate, as a function of Bi concentration x, the dimension-l less electron-phonon coupling constant λ in McMillan's strong-coupling theory of superconductor. We obtain large values of λ such as λ>1 for x>0.1. The origin of this large electron-phonon coupling is ascribed to: (i) the hybridization between 2pσ orbitals of 0 atoms and 6s orbitals of Pb or Bi atoms is large in the conduction band, (ii) this σ-bonding is strongly affected by a vibrational motion of 0 atoms in the direction connecting the 0 and Pb (Bi) atoms, and (iii) the mass of 0 atoms is light. We also evaluate superconducting transition temperature T c as a function of x on the basis of the McMillan-Allen-Dynes equation. The results explain semi-quantitatively the observed x-dependence of T c.

Electron-phonon coupling and temperature coefficient of resistivity in Ni-Zr glasses

A simple and direct relationship has been claimed between the electron-phonon coupling constant, λep and the temperature coefficient of resistivity, p. We present results on a wide range of melt-spun Ni-Zr glassy alloys to show that this relationship is not generally valid, even when due allowance is made for the influence of spin-fluctuations. Specifically, λep is almost constant while ϱ′ varies from negative values at the Zr-rich end to positive values at the Ni-rich end. The behavior can be explained, however, within the framework of the Faber-Ziman formalism, using λep provided account is taken correctly for the partial structure factors.

Quenching of stimulated phonon emission in Al xGa 1− xAs-GaAs quantum-well heterostructures

Data are presented on MO-CVD Al x Ga 1− x As- GaAs multiple quantum-well heterostructures (QWH's) consisting of a large GaAs well (500 Å) coupled to one or two arrays of smaller wells (50 Å) that act, when photopumped, as phonon sources and resonators. These data show that the induced phonon-sideband laser operation of the larger well is quenched when the phonon-generating array, but not the larger well, is destroyed by thermal annealing. An estimate of the electron-phonon coupling coefficient for indirect optical transitions is presented, indicating enhanced coupling with increased phonon density and establishing a basis for stimulated phonon emission in QWH's.

Exciton-phonon interaction and energy transfer in benzene and isotopically impure deuterobenzene crystals

Absorption and fluorescence of pure and isotopically substituted benzene crystals are reported for the range 4.2–20°K. The energy of electron- and vibron-phonon interactions and the probability of respective optical transitions are determined. The pronounced asymmetry of phonon sidebands of absorption and luminescence spectra is found. The kinetic analysis of isotopically impure crystal spectra gives the trap-trap energy transfer probability and the energy dependence of the electron-phonon coupling coefficient.

Photon-Induced Electron Pairing

The earlier work on the possibility of interband electron pairing in the presence of a strong radiation field has been further extended. Some additional terms, neglected earlier, have been taken into account and generalized to a situation where the electron-phonon coupling coefficients for the two conduction bands (valleys) are different. It is found that the pairing interaction is attractive and the strength depends on the photon density.

Optical Properties of Zinc Telluride

The optical absorption of ZnTe single crystals grown by the Bridgeman method has been studied from visible wavelengths to 50\ensuremath{\mu}, at temperatures from 1.4 to 300\ifmmode^\circ\else\textdegree\fi{}K. Lattice combination bands are observed from which assignments of zone-boundary phonon energies are made: LO=TO=22, LA=16, TA=7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ eV. The temperature dependence of the absorption is discussed. Absorption due to inter-valence-band transitions has been observed, which indicates the presence of two bands degenerate at $k=0$ and a split-off band lying 1 eV below. Infrared absorption is observed for As impurity in the range 0.05 to 0.1 eV and for a residual impurity in undoped material, probably a zinc vacancy, in the range 0.1 to 0.9 eV. For each impurity, absorption due to transitions to the split-off valence band is also observed near 1 eV. Sharp excitation lines are seen in the spectrum of the residual impurity. The lines are repeated with emission of optical phonons of energy 0.026 eV, from which the electron-phonon coupling coefficient is estimated. Some structure is observed also in the spectrum of As impurity. The intrinsic absorption edge has been measured at 1.7, 80, and 300\ifmmode^\circ\else\textdegree\fi{}K. The energy gap is determined to be 2.385, 2.37, and 2.25 eV, respectively. A tail is observed at the absorption edge which shows the effect of various impurity levels in the undoped material and in the samples doped with As, In, or Fe. In two samples grown from the vapor phase, some sharp absorption lines are present in the range 2.2 to 2.35 eV. These lines are apparently due to exciton-impurity complexes.