Exciton spin dynamics in diluted magnetic semiconductor quantum wells
Final Report Abstract
We have developed a microscopic, quantum-kinetic theory for the spin dynamics in diluted magnetic semiconductor (DMS) quantum wells accounting for both, correlations due to the exchange coupling with magnetic dopants as well as the Coulomb interaction, which gives rise to excitonic correlations. We demonstrated a rich variety of significant quantum kinetic effects in the exciton spin dynamics in optically excited DMSs. Our studies have overcome the long standing issue that predictions of decay times for the optically induced spin polarization on the basis of Fermi’s golden rule both for free carries as well as for excitons strongly deviate from experiments. Furthermore, applying an external magnetic field we find a non-monotonic dependence of the spin decay on the magnetic field where the trends are completely reversed compared with the golden rule results. Many quantum kinetic features are due to the build-up of a noticeable correlation energy which is accompanied by a massive redistribution of excitons in K-space that eventually increases the exciton kinetic energy. For example, at sufficiently strong magnetic fields an efficient transfer to higher exciton states (2s, 2p) sets in which speeds up the 1s spin decay and may be used to store spin information in dark states that are protected against radiative decay. In a quantum kinetic treatment this transfer is efficient already at magnetic fields well below the threshold set by single-particle energy conservation because the missing energy is provided by the correlation energy. Interestingly, an important contribution to the correlation energy stems from non-magnetic impurity scattering. The latter process thus has a significant impact on the exciton spin dynamics although in the Markovian limit its contribution would be zero. Moreover, we have discovered that the correlation energy per exciton can be directly extracted from linear absorption spectra since the correlation leads to an additional feature in the spectra above the exciton line separated by almost exactly the correlation energy. Furthermore, we have demonstrated that overshoots appearing in the exciton spin dynamics of DMSs are almost completely due to quantum kinetic correlations. Other origins such as e.g. the competition between spin relaxation and radiative decay can be ruled out because radiative decay has only a marginal effect on the spin dynamics in DMSs and such competition processes would exhibit noticeably different dependencies on parameters like temperature and doping fraction. Overall, quantum kinetic effects are most pronounced for optical excitation of excitons at the bottom of the 1s parabola while they become marginal for hot excitons. In the latter case the spin dynamics tremendously slows down due to the K-dependent excitonic form factors. We have also analyzed the impact of acoustic phonons. It turns out that acoustic phonons contribute noticeably to the K-space redistribution. Their contribution is, however, typically much less and builds up much slower than the redistribution evoked by the exchange interaction between excitons and magnetic impurities. Surprisingly, despite a sizeable impact of phonons on the exciton dynamics that leads, e.g., to a strongly temperature dependent contribution to the mean excitonic kinetic energy, it turns out that without magnetic field phonons have almost no influence on the exciton spin dynamics. This changes when a magnetic field is applied. Here, our studies reveal a massive trapping of carriers in dark states with spins opposite to the initially prepared spin which in the course of time leads to an almost complete reversal of the spin polarization that can be maintained over rather long times. This effect is absent in a Markovian theory without phonons. Phonons provide a ratchet type mechanism for the trapping that in the Markovian theory enables and in a quantum kinetic treatment strongly boosts the above spin reversal. A further result of this project concerns situations where exchange interactions between carriers and dopants can efficiently compete with spin orbit couplings. Such a competition is found to significantly speed up the spin decay. For some parameters it can even change the spin dynamics qualitatively by turning a monotonic decay into a damped oscillation. Applying a moderate magnetic field turns out to be sufficient to almost completely suppress the influences of spin orbit couplings. Altogether, the present project has pushed the description of exciton spin dynamics in DMSs towards a higher level by enabling a quantum kinetic description as well as by accounting simultaneously for nonmagnetic scattering, phonon influences and couplings to higher excitonic states. The so advanced description has deepened the understanding of exciton and exciton spin dynamics in DMSs and paves the way to a more unified description of the dynamics in magnetically doped quasi-two dimensional materials in general.
Publications
- Quantum kinetic equations for the ultrafast spin dynamics of excitons in diluted magnetic semiconductor quantum wells after optical excitation. Phys. Rev. B 95, 245203 (2017)
F. Ungar, M. Cygorek and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.95.245203) - Many-body correlations brought to light in absorption spectra of diluted magnetic semiconductors. Phys. Rev. B (Rapid Communication) 98, 161201(R) (2018)
F. Ungar, M. Cygorek and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.98.161201) - Trend reversal in the magnetic-field dependence of exciton spin-transfer rates in diluted magnetic semiconductors due to non-Markovian dynamics. Phys. Rev. B 97, 045210 (2018)
F. Ungar, M. Cygorek and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.97.045210) - Origins of overshoots in the exciton spin dynamics in semiconductors. Phys. Rev. B 99, 165308 (2019)
F. Ungar, M. Cygorek and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.99.165308) - Phonon impact on the dynamics of resonantly excited and hot excitons in diluted magnetic semiconductors. Phys. Rev. B 99, 115305 (2019)
F. Ungar, M. Cygorek and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.99.115305) - Phonon-induced quantum ratchet in the exciton spin dynamics in diluted magnetic semiconductors in a magnetic-field. Phys. Rev. B 99, 075301 (2019)
F. Ungar, M. Cygorek and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.99.075301) - Role of excited states in the dynamics of excitons and their spins in diluted magnetic semiconductors. Phys. Rev. B 99, 195309 (2019)
F. Ungar, M. Cygorek and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.99.195309) - Spin dynamics of hot excitons in diluted magnetic semiconductors with spin-orbit interaction. Phys. Rev. B 100, 045306 (2019)
F. Ungar, P.I. Tamborenea and V.M. Axt
(See online at https://doi.org/10.1103/PhysRevB.100.045306)
