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We have performed semiempirical tight-binding calculations of the electronic structure of YBa2Cu3O7, with dand s orbitals included for all the metal atoms and p and sorbitals for oxygen. Here we report studies of oxygen vacancies on the O(1) chain sites in YBa2Cu3O7-y. The modification of the density of states ρ(E) and the shift of the Fermi energy Ep were calculated for 0 < y ≤ 1.0. The Fermi energy is found to increase monotonically with y, confirming the expectation that oxygen vacancies act as donors. Also, ρ(EF)is found to decrease with y.
One potential objection to virtual “sandwich excitons” as a pairing mechanism is that the transition densities may not be large enough to overcome the inherent disadvantage of a large energy denominator . In model calculations, we find that some of the transition densities and matrix elements can be large even if the dielectric constant is sizeable for, e.g., LaO excitons in La2‐xSrxCuO4
We report new theoretical studies of the electronic and structural response of materials to fast intense laser pulses, with durations ∼ 10–100 feintoseconds and intensities up to ∼10 terawatts/cm2. The results provide still stronger evidence that GaAs undergoes a true nonthermal phase transition as the intensity is varied at constant pulse duration. For C60, there are also different regimes of behavior as a function of pulse intensity. At low intensity, various optically-active modes are observed. At high intensity, the breathing moode is by far the most dominant. At still higher intensities, there is photofragmentation, with the evolution of dinmers and other products. These results were all obtained in simulation-. using tight-binding electron-ion dyanamics (TED).
Using the technique of tight-binding electron–ion dynamics,
we have calculated the response of crystalline GaAs when a
femtosecond laser pulse excites 1–20% of the valence
electrons. Above a threshold fluence, which corresponds to
promotion of about 12% of the valence electrons to the conduction
band, the lattice is destabilized and the band gap collapses
to zero. This result supports the conclusion that structural
changes on a subpicosecond time scale observed in pump-probe
experiments are of a nonthermal nature.
The energy levels of antisite defects at a GaAs/Ge (110) interface are calculated and shown to be essentially unaltered with respect to the GaAs valence band maximum by different choices of the valence band offset.
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