Instantaneous Intervalley Transition just at the Franck-Condon State in the Conduction Band of GaAs

TL;DR

This paper investigates two quantum microscopic mechanisms for rapid intervalley electron transitions at the Franck-Condon state in GaAs, highlighting their ultrafast timescales of tens of femtoseconds.

Contribution

It introduces and theoretically analyzes two mechanisms—electron-electron Coulombic transitions and electron-phonon elastic scattering—that explain ultrafast intervalley transitions in GaAs.

Findings

Both mechanisms produce transition times of a few tens femtoseconds.

Elastic electron-phonon scattering occurs without creating new phonons.

The proposed mechanisms explain rapid intervalley transitions at the Franck-Condon state.

Abstract

We propose two possible quantum microscopic mechanisms for intervalley transitions just at the Franck-Condon state in GaAs. The first is the simultaneously combined coulombic elastic transitions of two electrons,from the originally photo-excited $\Gamma$ valley to the mutually opposite L and -L valleys in the Brillouin zone. The second is the "elastic" scattering of an electron from the $\Gamma$ valley to the L one by the frozen phonon, which is not newly created in the excited state, but already exists, being inherited from the starting ground state, according to the Franck-Condon principle of the photo-excitation. Surprisingly, we can theoretically show that such the elastic electron-phonon scattering gives extremely fast time constant of the order of a few tens femtoseconds, as well as the above elastic electron-electron scattering does.