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A review is given on the recent studies of spin dynamics in diluted magnetic semiconductor quantum wells. The experiments are using time-resolved photoluminescence techniques to study: spin–lattice relaxation of magnetic Mn 2+ ions; spin interaction between free carriers and magnetic ions; spin diffusion in the magnetic ion system.

The work is an experimental study of optical spin polarization in InAs/GaAs quantum dots (QDs) with 2 resident electrons or holes. A capture of a photo-generated electron-hole pair into such a QD creates a negative or positive tetron (double-charged exciton). Spin polarization was registered by the circular polarization of the QD photoluminescence (PL). The spin state was found to differ radically in the dots with opposite in sign charge. Particularly, under excitation in a GaAs barrier, the ground state PL polarization is negative (relative to the polarization of an exciting light) in the negatively charged QDs and positive in the positively charged QDs. With increasing excitation intensity, the negative polarization rises from zero up to a saturation level, while the positive polarization decreases. The negative polarization increases in weak magnetic fields applied in Faraday geometry, but strong fields suppress it. The positive polarization always increases as a function of magnetic field. We propose a theoretical model that qualitatively explains the experimental results.

The behavior of the optical-orientation signal under resonant optical excitation of localized excitons in quantum wells with semimagnetic layers was studied both experimentally and theoretically. The most remarkable experimental observation made in this study is the increase in the degree of polarization of the quantum well radiation in the conditions in which the Hanle effect is observed. The behavior of magnetically induced circular luminescence polarization in a slightly tilted field (quasi-Voigt geometry) also appears unusual. Possible specific contributions to the optical exciton orientation in semimagnetic nanostructures are discussed. A theoretical model is proposed which, while being based on the well-known concepts of collective spin dynamics of magnetic ions in the exchange field of a photoexcited hole, takes into account fluctuations of the local magnetization. The calculations agree quantitatively with experiment for reasonable values of the parameters.

Confinement of vibronic excitations in semiconductor nanocrystal (NC) quantum dots (QDs) has become a subject of intensive studies in recent years. In this paper we demonstrate the possibility of constructing the spherical QD acoustic phonon eigenmodes by use of nothing but symmetry analysis and apply this treatment to the specific case of CuCl NCs.

It is shown that the mixed character of spheroidal vibrational modes of semiconductor quantum dots of spherical shape may lead to the appearance of a line in the low-frequency Raman spectra of nanocrystals whose spec-tral position is independent of the average radius of nanocrystals in the sample over a wide range of sizes. This effect is associated with the rapid saturation of the dispersion dependence for transversal acoustic phonons in the bulk semiconductor. The maximum radius of quantum dots at which the line indicated above is observed in the spectrum has been estimated.

We observed a giant splitting of exciton spin sublevels in CdS nanocrystals, each doped on the average by one Mn ion. The splitting, which exists in zero external magnetic field, is caused by the gigantic internal magnetic field of the Mn ion and results from the enhancement of the short range spin–spin interactions in nanocrystal quantum dots. The splitting is seen in the strong magnetic circular dichroism of the CdS band edge transitions. The magnitude of the observed band edge splitting is in good agreement with a theoretical calculation of the effective magnetic field.

Peculiarities of optical spectra in diluted magnetic semiconductors are used to find out the characteristic size of the localized exciton state in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ alloys. Three methods for evaluation of the exciton localization radius are suggested. Two of them are based on spin-related phenomena. The localization radius is calculated from measured magnetic-polaron energies and from the degree of the magnetic-field-induced circular polarization of luminescence. The exciton localization radius is determined for a set of samples with Mn content varying from 0.1 up to 0.4. The results of the two spin-related methods are in good agreement to each other. Their comparison with the results of the third approach based on the exciton linewidth analysis allows the conclusion that both the electron and hole of the exciton are localized within the same volume. This latter method, verified by comparison with two others, is applicable also to a wider class of semiconductor compounds which do not contain magnetic impurities.

A theory of Raman scattering of light by acoustic phonons in spherical nanocrystals of zinc-blende and wurtzite semiconductors has been developed with the inclusion of the complex structure of the valence band. The deformation-potential approximation was used to describe the exciton-phonon interaction. It is shown that this approximation allows only Raman scattering processes involving spheroidal acoustic phonons with a total angular momentum F=0 or 2. The effect of phonon quantum confinement on linewidth in Raman scattering spectra and scattered polarization is analyzed. An expression for the shape of the spectral line corresponding to nonresonant scattering from F=0 phonons was obtained.

We report on optical studies of exciton magnetic polarons in artificial crystals of digital alloys of diluted magnetic (semimagnetic) semiconductors built of alternating CdTe and ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ layers, each only a few monolayers thick. Specially designed quantum well structures with the digital magnetic alloy in the quantum well region were grown and studied experimentally by magneto-optical methods. Precise comparative studies of the structures grown simultaneously either on (120)- or (100)-oriented substrates were performed by photoluminescence excitation spectroscopy and photoluminescence under selective excitation. While no difference in magnetic properties, as measured by the giant Zeeman splitting of excitonic states, was observed between (120)- and (100)-oriented diluted magnetic structures, the exciton magnetic polaron energy was found to be by $10--40%$ larger in (120)-oriented structures than in (100)-oriented counterparts. This increase is explained by the effect of the heavy-hole effective mass anisotropy: greater hole mass in [120] direction causes an enhancement of an initial exciton localization which, in turn, gives rise to an increase of the magnetic polaron energy. Results of model calculations are in good agreement with experimental data.

Spin dynamics of free carriers and dynamics of the magnetization in diluted magnetic semiconductors are coupled to each other and mostly determined by the spin and energy transfer between the carriers, magnetic ions, and lattice. The efficiency of this transfer is examined here for low-dimensional heterostructures fabricated from II–VI semiconductor-based DMSs containing Mn2 + . We show here that the dynamics depends strongly on Mn content as well as on specific design of those structures. Therefore, it can be controlled by optical and electrical means.

We observed simultaneously two types of Raman spectra in a GaAs/AlGaAs superlattice under high magnetic field: a folded longitudinal acoustic phonon, IFA, and a structureless background, IB. The temperature dependence of the intensities in both spectra was studied in the range 2–100K and it was established that the background spectrum is due to the first-order Raman scattering from acoustic phonons with a wide range of wave vectors. We obtained the expression for the intensities ratio IB/IFA. It depends on the relationship between the homogeneous and the inhomogeneous broadening of the intermediate state of the photoexcited e-h pair. A rapid decrease of IB observed at T 30K is thought to be due to the increase of the homogeneous width with temperature.

Optical pumping of electron spins and negative photoluminescence polarization are observed when interface quantum dots in a GaAs quantum well are excited nonresonantly by circularly polarized light. Both observations can be explained by the formation of long-lived dark excitons through hole spin relaxation in the GaAs quantum well prior to exciton capture. In this model, optical pumping of resident electron spins is caused by capture of dark excitons and recombination in charged quantum dots. Negative polarization results from accumulation of dark excitons in the quantum well and is enhanced by optical pumping. The dark exciton model describes the experimental results very well, including intensity and bias dependence of the photoluminescence polarization and the Hanle effect.

Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses.

Electron spin coherence in self-assembled (In,Ga)As/GaAs quantum dots has been studied by pump-probe Faraday rotation experiments. Several aspects such as creation of spin coherence, spin dephasing, interaction with lattice etc nuclei will be discussed.

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the charge from positive to neutral to negative using a Schottky diode. We observe that photoluminescence polarization memory has the same sign as the net charge of the dot. Optical pumping of ground state electron spins enhances this effect, as demonstrated through the first measurements of the Hanle effect on an individual quantum dot. With the Overhauser effect in a high longitudinal magnetic field, we demonstrate efficient optical pumping of the quantum dot's nuclear spins for all three charge states.

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 the 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 n-GaAs layer.

A giant negative magnetoresistance has been observed in bulk germanium doped with multiply charged deep impurities. Applying a magnetic field the resistance may decrease exponentially at any orientation of the field. A drop of the resistance as much as about 10000% has been measured at 6 T. The effect is attributed to the spin splitting of impurity ground state with a very large g-factor in the order of several tens depending on impurity., 4 pages, 4 figures

While electron and hole size quantisation in spherical semiconductor nanocrystals embedded in glass matrices has been much studied in the last 15 years, the question of acoustic phonon confinement remains open. Only simple models of either bulk-like phonons [1] or vibrational modes of an elastic sphere with free or fixed surface [2–5] were applied to describe the acoustic phonon energy spectrum in nanocrystals. In these two limiting cases the acoustic phonons either do not sense the interface between the nanocrystal and the glass matrix or can not penetrate through it, i.e. the probabilities of either phonon reflection or transmission are neglected. However the low-frequency Raman spectra line shapes are expected to be very sensitive to these probabilities when the excitation is far from the excitonic resonance and thus a more accurate consideration of acoustic phonons in nanocrystals is required.

An approach to the theory of the acceptor ground state in cubic semiconductors is presented. The model has been developed within the framework of the four-band effective Luttinger Hamiltonian and is applicable for both Coulomb and non-Coulomb accepters. The system of integral equations for the ground-state wave functions has been derived and its solution has been numerically computed. We present the general form of the acceptor-ground-state wave function. The wave functions for a set of acceptor dopants in GaAs are calculated with an accuracy of 2%. The obtained wave functions have been used for qualitative and quantitative analysis of the hot photoluminescence (HPL) spectra and linear polarization in GaAs crystals. Analytical expressions for the line shape and anisotropy of the linear polarization degree have been derived. The dependencies of the HPL characteristics on the excitation energy as well as on the acceptor binding energy have been analyzed. The HPL theory presented allows us to describe the wide spectrum of available experimental data.

The work is an experimental study of optical spin polarization in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots (QDs) with two resident electrons or holes. A capture of a photogenerated electron-hole pair into such a QD creates a negative or positive tetron (doubly charged exciton). Spin polarization was registered by the circular polarization of the QD photoluminescence (PL). The spin state was found to be radically different in the dots with the opposite sign of the charge. Particularly, under excitation in a GaAs barrier, the polarization of the ground-state PL is negative (relative to the polarization of exciting light) in the negatively charged QDs and positive in the positively charged QDs. With increasing excitation intensity, the negative polarization rises from zero up to a saturation level, while the positive polarization decreases. The negative polarization increases in weak magnetic fields applied in Faraday geometry; however, it is suppressed in strong fields. The positive polarization always increases as a function of magnetic field. We propose a theoretical model that qualitatively explains the experimental results.