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...

No abstract

A variety of BeMgZnSe–ZnSe- as well as BeTe-based quantum-well structures has been fabri- cated and investigated. BeTe buffer layers improve the growth start on GaAs substrates drasti- cally compared to ZnSe/GaAs. The valence-band offset between BeTe and ZnSe has been determined to be 0.9 eV (type II). Due to the high-lying valence band of BeTe, a BeTe–ZnSe pseudograding can be used for an efficient electrical contact between p-ZnSe and p-GaAs. BeMgZnSe quaternary thin-film structures have reproducibly been grown with high struc- tural quality, and rocking curve widths below 20 arcsec could be reached. Quantum-well structures show a high photoluminescence intensity even at room temperature.

No abstract

Using angle-resolved synchrotron-radiation photoemission spectroscopy we have determined the dispersion of the valence bands of BeTe(100) along ΓX, i.e. the [100] direction. The measurements are analyzed with the aid of a first-principles calculation of the BeTe bulk band structure as well as of the photoemission peaks as given by the momentum conserving bulk transitions. Taking the calculated unoccupied bands as final states of the photoemission process, we obtain an excellent agreement between experimental and calculated spectra and a clear interpretation of almost all measured bands. In contrast, the free electron approximation for the final states fails to describe the BeTe bulk band structure along ΓX properly.