The optical Aharonov-Bohm effect and magneto-optical properties in type-II quantum dots
Whiteside, Vincent Ryan
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We present a detailed experimental study of the magneto-optical properties of type-II quantum dots (QDs) in: (1) ZnTe/ZnSe superlattices grown by Molecular Beam Epitaxy (MBE)—these Zn(SeTe) QDs evolve from Te-clustering in the ZnSe matrix during growth; and (2) diluted magnetic semiconductor, (ZnMn)Se, QDs in a ZnSe matrix produced by migration enhanced epitaxy. In case (1) the Zn(SeTe) QDs display large and robust (with temperature) oscillations as a function of magnetic field in both the photoluminescence energy and intensity as a result of the optical Aharonov-Bohm effect. The large strength of these oscillations is attributed to a combination of the type-II symmetry and the columnar geometry of the structures; the oscillations persist until 180K. The type-II diluted magnetic semiconductor, (ZnMn)Te quantum dots display similar oscillatory effects in the emission intensity. Interestingly, the coherence of the Aharonov-Bohm phase in these magnetic dots is strongly related to the spin polarization of the system due to the Mn-exciton exchange interaction as shown by the disappearance of the oscillations at low magnetic fields. The enhanced coherence at high fields, which leads to strong oscillations in intensity, is attributed to removal of magnetic disorder by the applied magnetic field. While the magnetic nature of the QDs is clear from the polarization measurements there is the seemingly contradictory behavior of a very small Zeeman shift for material that has a corresponding large Zeeman shift for the comparable composition of bulk (ZnMn)Te. More importantly, a red shift greater than 30 meV is observed in the peak energy of the PL as function of time after excitation with a picosecond pulse. These results can be explained by postulating formation of bound magnetic polarons in the QDs. The overall red shift is identified as the magnetic polaron binding energy, E MP ; it is roughly independent of temperature, persisting up to 150K. The large MP binding energy is apparently contradictory to the small observed Zeeman splitting and the temperature dependence of the optical polarization in the steady state. These apparently contradictory properties are interpreted in terms of a model that explains the temperature dependence as well as the polarization and Zeeman energy splitting, while fully taking into account the polaron formation energy. The model is based on the hole-Mn and the Mn-Mn exchange coupling and their role in the magnetic polaron formation with a crucial aspect being the formation of an antiferromagnetically ordered state of the Mn spin system in each of the QDs in the absence of photoinjected holes.