Low-temperature electron-spin relaxation is studied by the optical orientation method in bulk n-GaAs with donor concentrations from 10 14 cm Ϫ3 to 5ϫ10 17 cm Ϫ3 . A peculiarity related to the metal-to-insulator transition is observed in the dependence of the spin lifetime on doping near n D ϭ2ϫ10 16 cm Ϫ3 . In the metallic phase, spin relaxation is governed by the Dyakonov-Perel mechanism, while in the insulator phase it is due to anisotropic exchange interaction and hyperfine interaction

We report on the optical manipulation of the electron spin relaxation time in a GaAs-based heterostructure. Experimental and theoretical study shows that the average electron spin relaxes through hyperfine interaction with the lattice nuclei, and that the rate can be controlled by electron-electron interactions. This time has been changed from 300 ns down to 5 ns by variation of the laser frequency. This modification originates in the optically induced depletion of an n-GaAs layer.

Hybrid structures synthesized from di erent materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an e ective p-d exchange that is mediated by elliptically polarized phonons. E xchange interactions are the origin for correlated magnetism in condensed matter with multi-faceted behaviour such as ferro-, antiferro-or ferrimagnetism. In magnetic semiconductors (SCs), the exchange occurs between free charge carriers and localized magnetic atoms 1-4 and is determined by their wavefunction overlap. To control this overlap, hybrid structures consisting of a ferromagnetic (FM) layer and a semiconductor quantum well (QW) are appealing objects because they allow wavefunction engineering. Furthermore, the mobility of QW carriers will not be reduced by inclusion of magnetic ions in the same spatial region in these systems.More specifically, for a two-dimensional hole gas (2DHG, the p-system) in a QW the overlap of the hole wavefunction with the magnetic atoms in a nearby ferromagnetic layer (the d-system) is believed to result in p-d exchange interaction 5-8 . This exchange interaction may cause strong coupling between the SC and FM spin systems 9 , through which the ferromagnetism of the unified system, as evidenced by its hysteresis loop, can be tuned. In particular, the 2DHG spin system becomes polarized in the effective magnetic field from the p-d exchange 5,8 . Recently 10 , it was shown that in addition to this equilibrium 2DHG polarization there is an alternative mechanism involving spin-dependent capture of carriers from the SC into the FM. For ferromagnetic (Ga,Mn)As on top of an (In,Ga)As QW, electron capture induces electron spin polarization in the QW, representing a dynamical effect in contrast to the exchange-induced equilibrium polarization.Here we study a different FM/QW hybrid, consisting of a Co layer and a CdTe II-VI semiconductor QW, separated by a nanometrethick barrier. Owing to the negligible hole tunnelling through the barrier, this hybrid combination shows mostly a quasi-equilibrium proximity effect due to p-d exchange interaction between magnetic atoms and holes in the QW. Surprisingly, howev...

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We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI3 perovskite nanocrystals prepared by colloidal synthesis and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2 K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 120 µeV. Our calculations demonstrate that there is a significant contribution to the splitting due to the nanocrystal shape anisotropy for all inorganic perovskite nanocrystrals.