A system of two coupled CdTe quantum dots, one of them containing a single Mn ion, was studied in continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information in the spin state of the Mn ion has been demonstrated, using orientation of the Mn spin by spin-polarized carriers transferred from the neighbor quantum dot. Mn spin orientation time values from 20 ns to 100 ns were measured, depending on the excitation power. Storage time of the information in the Mn spin was found to be enhanced by application of a static magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple rate equation models were found to describe correctly static and dynamical properties of the system.One of important research directions that may influence the future of information processing, especially of spintronics [1], is focused on physical phenomena occurring in nanoscale-size quantum objects. One of such objects, close to the ultimate limit of information storage miniaturization, is a single Mn atom in a semiconductor quantum dot (QD) [2,3]. After intensive studies of semimagnetic QD containing many magnetic ions [4,5,6,7], single Mn atoms in CdTe [8] and InAs [9] QDs have been observed in photoluminescence (PL) experiments. Many experiments supplied substantial knowledge on physical properties of single Mn atoms, especially in CdTe QDs. In particular, they revealed a strong influence of the position of the Mn atom in the QD, reflecting the symmetry of the system [10]. They demonstrated an efficient optical read-out of the Mn spin state [8]. Furthermore, the dynamics of this state has been studied in photon correlation experiments [11], revealing an important influence of photo-created carriers on Mn spin relaxation. The writing and storing of the information in the Mn spin state has received less attention so far. These issues represent the focus of the present work.In particular, we demonstrate optical writing of information in the spin state of a single Mn ion and we test the stability of this state in the time range up to 0.2 ms.CdTe QDs containing single Mn ions were grown by molecular beam epitaxy. A single layer of self-assembled QDs was deposited in a ZnTe matrix. Manganese was added by briefly opening the Mn effusion cell during the growth of the CdTe layer [12]. The opening time and the Mn flux were adjusted to achieve a large probability of growth of QDs with a single Mn ion in each dot. The selection of single QDs was done by spatial limitation of PL excitation and detection to an area smaller than 0.5 micrometer in diameter, with microscope objective immersed in pumped liquid helium. Continuous wave excitation was used either above the ZnTe barrier gap (at 457 nm) or by a tunable dye laser in the range 570 -600 nm. Well separated photoluminescence lines from individual QDs were observed in the low energy part of the PL spectrum. We were able to select numerous lines showing a PL pattern characteristic for the presence of a...

The mechanisms for generation of long-lived spin coherence in a two-dimensional electron gas (2DEG) have been studied experimentally by means of a picosecond pump-probe Kerr rotation technique. CdTe/(Cd,Mg)Te quantum wells with a diluted 2DEG were investigated. The strong Coulomb interaction between electrons and holes, which results in large binding energies of neutral excitons and negatively charged excitons (trions), allows one to address selectively the exciton or trion states by resonant optical excitation. Different scenarios of spin coherence generation were analyzed theoretically, among them the direct trion photocreation, the formation of trions from photogenerated excitons and the electron-exciton exchange scattering. Good agreement between experiment and theory is found.

We provide an experimental evidence and a theoretical substantiation for a strong reduction of the s-d exchange interaction between electrons and Mn-ion spins with increasing degree of the confinement in nanostructures of diluted magnetic semiconductors. By spin-flip Raman scattering a strong (up to 25%) reduction of the exchange parameter is observed in ͑Cd, Mn͒Te͑͞Cd, Mn, Mg͒Te quantum well structures. As a responsible mechanism we suggest switching-on of the kinetic exchange for the conduction electrons with finite k vectors. Quantitative agreement between experimental data and calculations is achieved. PACS numbers: 75.30.Et, 71.70.Ej, 75.50.Pp, 78.20.Ls Semimagnetic or diluted magnetic semiconductors (DMS) are known for their giant magneto-optical effects originating in a strong sp-d exchange interaction between the band electrons and the magnetic (e.g., Mn) ions. Incorporation of semimagnetic layers in quantum-confined heterostructures opened new possibilities for the "spin splitting engineering" [1]. In this context, in addition to a magnetic field, temperature, and Mn molar fraction dependences, also the confinement energy and the size quantization became means of an effective tuning of the spin splittings. While a precise tuning of the spin splitting is of great significance for future-spin-dependent electronics, it is already now of obvious importance in, e.g., studies of samples with zero g-factor in the fractional quantum Hall regime [2]. Also, exciting physical phenomena have been reported recently for semimagnetic heterostructures: coherent spin excitations [3,4], magnetic polaron formation [5], and ferromagnetic hole alignment in p-type doped structures [6].For the electrons from the vicinity of the conduction band edge the exchange interaction with the localized d electrons of magnetic ions is described by the Hamiltonian:where the parameter of the s-d exchange interaction a is positive. In the case of the holes, the exchange energy has the same form but the parameter of p-d exchange interaction b , 0. In Eq. (1) s and r are the spin (total angular momentum in the case of the holes) and coordinate operators of a carrier, and S i and R i are the total spin and coordinate of the ith magnetic ion. A positive sign of a results from the direct (or potential) exchange of the conduction band electron with the d electrons of Mn ions. The negative sign of b is due to a dominant role of the kinetic exchange [7] which occurs because of possible transitions to virtual states in the d shell [8].A possibility of a modification of the exchange parameters under quantum-confined conditions has been recognized already with the first semimagnetic semiconductor quantum wells (QWs) studied. However, experimental evidences showing unambiguously such a modification of the exciton Zeeman splitting, i.e., of ͑a 2 b͒, have been found only recently [9]. A special design of the QW structures was crucial for the latter studies since one had to rule out effects related to a modification of magnetic properties at the magneti...