T. Piorek

Spontaneous symmetry breaking of excitons in multiple-quantum-wells

The results of self-consistent exciton energy calculations in finite multiple-quantum-wells with differing numbers of wells and varying barrier widths are described. The calculations show that even for a perfect system, the exciton localizes strongly in one well only. For a system with an even number of wells this leads to a spontaneous symmetry breaking.

Electron tunnelling in CdTeCdMnTe double barrier structures

Pump and probe nonlinear transmission experiments with femtosecond resolution are used to investigate electron tunnelling between the central CdMnTe well and the outer CdTe wells of a CdTeCdMnTe double barrier structure. The time dependence of the reduction in oscillator strength resulting from bandfilling at the exciton resonance is used to measure population changes in the outer wells. A tunnelling time of 8 ps is measured when a carrier density of 1 × 1013cm−2 is used. Monte Carlo simulations show that the timescale of tunnelling via LO phonon scattering is 10 ps. A pump induced increase in absorption at energies smaller than that of the exciton resonance is observed at early times and assigned to band-gap renormalisation.

Magnetic field induced transitions in diluted magnetic semiconductor quantum wells

Abstract The purpose of this paper is to present a detailed theoretical study of the behaviour of excitonic transitions which are forbidden in a zero magnetic field but whose oscillator strength increases with the applied field. The calculation utilizes a two parameter excitonic wave function which permits the relative motion term to assume the symmetry of either a prolate or an oblate spheroid should it be energetically favourable to do so. In addition the effect of the coulombic interaction of the electron and hole is treated in a self-consistent manner and the calculation includes enhanced paramagnetic effects at the interface. A detailed study is given of experimental observations [Ribayrol et al., Phys. Rev. B 51 (1995) 7882] in which the behaviour of the e1-hh3 transition is explained in terms of a magnetic field induced type-I–type-II transition in a diluted magnetic semiconductor quantum well system.