Abstract
We theoretically study the short-time Coulomb dynamics of electrons in the conduction subbands of quantum wires in a quantum kinetic model. In our multi-subband density matrix approach we express the hierarchy of equations of motion for the density matrices in terms of many-particle density correlations, which enables us to obtain a closed set of quantum kinetic equations by neglecting all n-particle density correlations with n>2. It is shown that in one-dimensional systems the time-dependent broadening of transitions on ultra-short time scales has a pronounced effect on both intrasubband scattering (leading to a broadening of the electron distribution) and intersubband impact ionization processes (through `smearing out' of the semiclassical impact ionization threshold). Comparison of semiclassical and quantum kinetic simulations of the Coulomb dynamics in a two-subband system shows that the quantum kinetic impact ionization rate is smaller than the semiclassical one for excitation or initial electron distribution above the subband spacing, while it is larger for excitation or initial electron distribution below the subband spacing.
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