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A concept of mixed exciton-trion states is formulated theoretically and proved experimentally for II-VI semiconductor quantum wells with a two-dimensional electron gas. The concept considers the resonances of neutral excitons and charged excitons (trions) as mixed (with each other) via their interaction with free electrons. Reflectivity spectra of modulation-doped ZnSe/(Zn,Mg)(S,Se) and CdTe/(Cd,Mg)Te quantum wells are analyzed. A good qualitative agreement of the experimental results with model calculations is achieved.

Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively-(X − ) and positively (X + ) charged excitons are measured as functions of quantum well width, free carrier density and in external magnetic fields up to 47 T. The binding energy of X − shows a strong increase from 1.4 to 8.9 meV with decreasing quantum well width from 190 to 29Å. The binding energies of X + are about 25% smaller than the X − binding energy in the same structures. The magnetic field behavior of X − and X + binding energies differ qualitatively. With growing magnetic field strength, X − increases its binding energy by 35-150%, while for X + it decreases by 25%. Zeeman spin splittings and oscillator strengths of excitons and trions are measured and discussed.

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Spin relaxation of two-dimensional electrons in asymmetrical (001) AlGaAs quantum wells are measured by means of Hanle effect. Three different spin relaxation times for spins oriented along [110], [110] and [001] crystallographic directions are extracted demonstrating anisotropy of D'yakonov-Perel' spin relaxation mechanism. The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well structures. It is shown that the Rashba spin-orbit splitting is about four times stronger than the Dresselhaus splitting in the studied structure.PACS numbers: 73.21. Fg, 73.63.Hs, 72.25.Rb, 76.60.Jx Spintronics is at present time one of the most important areas of the semiconductor physics for both fundamental research and possible device applications [1]. The main problem of spintronics is creation, registration and lifetime control of carrier spin, especially in lowdimensional systems. Therefore investigation of spin relaxation processes is now an actual problem of the physics of semiconductor heterostructures.The main mechanism of spin relaxation in GaAs based quantum wells (QWs) is the D'yakonov-Perel' kinetic mechanism [2]. It is caused by lack of inversion centrum i) in the bulk semiconductor of which the system is made (bulk inversion asymmetry, or BIA), ii) in the heterostructure (structure inversion asymmetry, or SIA) and iii) in the chemical bonds at heterointerfaces (interface inversion asymmetry, or IIA) [2,3,4]. SIA can be caused by an external electric field or by deformation, BIA and IIA depend strongly on a size of carrier confinement. Therefore spin relaxation times can be controlled by gate voltage or by special heterostructure design.In Ref.[5], anisotropy of spin relaxation has been predicted for heterostructures grown along the axis [001]. It has been theoretically shown that lifetimes of spin oriented along the axes [110], [110] and [001] are different. In particular, changing relation between SIA and BIA one can achieve total suppression of relaxation for spin oriented along one of 110 axes. (IIA in (001)-grown structures is equivalent to BIA, therefore we will use a generalized term 'BIA' for both of them.) Detailed calculations [6,7,8] confirmed that spin relaxation anisotropy exists in real semiconductor heterostructures. Realization of such idea to control spin relaxation times gives new opportunities for spintronics [9]. However experimental discovery of this effect is missed so far.In this Letter, spin relaxation anisotropy in the plane of the QW is observed. In order to demonstrate this effect, the structure has been grown so that SIA and BIA are comparable in efficiency. Note that systems where both SIA and BIA take place have been studied in Refs. [10,11,12,13] but spin relaxation times have not been investigated in such structures.The D'yakonov-Perel' spin relaxation mechanism consists in electron spin precession around an effective magnetic field which is caused by lack of inversion centrum in the system. The corresponding Hamiltonia...

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We report a remarkable enhancement of the magnetic moments of excitons as a result of their motion. This surprising result, which we have observed in magneto-optical studies of three distinct zinc-blende semiconductors, GaAs, CdTe, and ZnSe, becomes significant as the kinetic energy of the exciton becomes comparable with its Rydberg energy and is attributed to motionally induced changes in the internal structure of the exciton. The enhancement of the magnetic moment as a function of the exciton translational wave vector can be represented by a universal equation.