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An experimental and theoretical study of the magnetic polaron states of two-dimensional excitons in quantum wells based on semimagnetic semiconductors (Cd,Mn)Te is reported. It is shown that magnetic-polaron formation in in-plane magnetic fields leads to a lowering of the system symmetry, provided the fields are not too strong. The magnetic moment of the polaron thus formed is not parallel to the external magnetic field and contains a component normal to the quantum-well plane. This spontaneous lowering of the symmetry results in a change of the polarization characteristics of the luminescence from magnetic polaron states and in a weakening (compared to the three-dimensional case) in the efficiency of magnetic field-induced polaron suppression.

In zinc‐blende II–VI alloys the thermal‐expansion coefficient for low temperatures is negative and becomes positive at higher temperatures. We investigated the luminescence properties of molecular‐beam‐epitaxy‐grown (CdMnMg)Te layers in the temperature range from 2 up to 200 K and show that the anomalous temperature dependence of the lattice constant is reflected in the luminescence properties of the excitonic recombination and the internal transition of manganese (ITM). The temperature behavior of the ITM energy is nonmonotonic and the existence of a minimum in the photon energy (at the temperature TMn) can be correlated to the change of sign of the thermal‐expansion coefficient. The decay constants of the ITM begins to decrease drastically at TMn, too. Considering a lattice constant dependent energy transfer rate to the infrared emitting state (1.2 eV) of the manganese ion the variation of the lifetimes can also be explained by the temperature dependence of the lattice constant. Furthermore, we have mea...

We study the influence of quantum confinement and spin-lattice relaxation on the dynamics of exciton magnetic polaron formation in specially designed semimagnetic quantum wells. Using low temperature time-resolved photoluminescence with variable pulse separation we show that a decrease of the well width does not only enhance the polaron energy but also shortens its formation time. Comparison with numerical calculations reveal that the latter can be associated with smaller changes of the excitonic wave function during polaron formation. In addition, by measuring the polaron formation time as a function of pulse separation we show that spin-lattice coupling influences the polaron dynamics indirectly rather than directly by transferring the polaron induced energy gain of the spin-system to the lattice.

We have realized a spin superlattice structure based on a Cd 0.9 Mn 0.1 Te Cd 0.9 Mg 0.1 Te heterostructure, where the valence and the conduction band align with zero band offset. Due to the large spin-splitting of the conduction- and valence band-edges in the semimagnetic (CdMn)Te layer and the vanishing spin-splitting in the nonmagnetic (CdMg)Te layer a significant spatial separation of spin-up and spin-down carriers is achieved with increasing magnetic field strength. The formation of the spin superlattice is proved by the Zeeman pattern of the excitonic states. The Zeeman pattern directly reflects the energy shift of spin-up states localized in the nonmagnetic and spin-down states in the semimagnetic layers. In addition, the exciton magnetic polaron is used as a sensitive probe for the small confinement potentials controlled by the externally applied field, demonstrating the exciton localisation in the spin dependent quantum wells of a spin superlattice.

We report on optical studies of exciton magnetic polarons in CdTe/Cd 1−x Mn x Te quantum well structures with Mn contents 0.4 ≤ x ≤ 0.8. The magnetic polaron energies and formation times are measured by cw- and time-resolved photoluminescence under selective excitation. We find an overall increase of the polaron energy with increasing Mn-concentration in the whole range of Mn contents studied and attribute it to diffusion induced changes of the Mn profile at the interfaces between nonmagnetic and semimagnetic layers. In addition, we find that in these quantum structures the formation time of the polaron is predominantly determined by the Mn content and not by quantum confinement effects. The large band gap discontinuity (up to 1.2 eV) causes large splittings between heavy- and light-hole-states and leads to a strong anisotropy in the suppression of the magnetic polaron formation by magnetic fields applied parallel and perpendicular to the structure growth axis. For both orientations of magnetic field the polaron suppression is qualitatively described by model calculations.

The formation of magnetic polarons in short-period semimagnetic CdTe/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te superlattices is studied by low-temperature cw and time-resolved photoluminescence and compared to the polaron formation in single quantum wells with the equivalent well width and manganese content. The dependence of the polaron energy on the superlattice period, i.e., quantum-well width, is identical to the dependence on well width in the corresponding single quantum wells. This implies that in the superlattice structures the polaron formation process does not lead to a spatial shift of the center of the hole wave function into the semimagnetic barrier. Temperature and magnetic-field-dependent measurements of the polaron energy reveal a more efficient polaron suppression in superlattices than in single quantum wells. This is attributed to the modification of the magnetic properties in the thin semimagnetic barrier layers of the superlattices. The different mechanisms of magnetic-field-induced polaron suppression in Faraday and Voigt geometries are discussed.

Using the method of selective excitation of the exciton luminescence in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te epilayers we have measured energies of localized magnetic polarons (LMP's) for alloys with manganese mole fractions x\ensuremath{\le}0.34. The suppression of the LMP energy has been studied in external magnetic fields and with temperature increase. Polaron formation times and exciton lifetimes have been measured by time-resolved photoluminescence. We have found that in alloys with x0.17 the polaron formation process is interrupted by exciton recombination and, as a result, the LMP does not reach its equilibrium energy. This dynamical effect on the polaron energy together with the strong sensitivity of the LMP formation to the conditions of primary exciton localization causes the absence of the LMP formation in layers with x0.1. Antiferromagnetic clustering of Mn ions, which leads to the spin-glass phase formation at low temperatures, affects the polaron energy and results in the increasing stability of LMP's against suppression by temperature increase and magnetic fields. In ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te with xg0.20 a considerable part of the polaron energy is controlled by the input of clusters of antiferromagnetically coupled Mn spins located in the nonuniform molecular field of localized excitons. The comparison of the exciton Zeeman splitting and the LMP magnetic-field suppression provides insight into the internal structure of LMP's.

We report on the optical study of exciton magnetic polarons in structures with semimagnetic Cd 1− y Mn y Te quantum wells sandwiched between nonmagnetic Cd 1− x Mg x Te or semimagnetic Cd 1− x Mn x Te barriers. We have found magnetic polarons with energy of 9 meV formed from quasi-two-dimensional excitons in Cd 0.9 Mn 0.1 Te/Cd 0.7 Mg 0.3 Te quantum wells with thicknesses varying from 18 to 100 A, but no polaron formation has been found for quasi-three-dimensional excitons situated in quantum wells thicker than 300 A. The experimentally established effect of the dimensionality of the system on the stability of exciton magnetic polarons is in qualitative agreement with theoretical predictions. In Cd 1− y Mn y Te/Cd 1− x Mn x Te structures contributions of the carrier exchange with Mn spins located in the well and in the barrier layers are responsible for the process of magnetic polaron formation. Relation of these contributions has been demonstrated to be controlled by the quantum well width.

The presence of magnetic ions in a dilute semiconductor quantum well is shown to decrease dramatically the low-temperature excitonic dephasing time measured by transient four-wave mixing. The decrease is associated with the magnetic nature of the material. Surprisingly, the application of a magnetic field does not significantly change the dephasing time, which indicates that there must exist a fast dephasing mechanism in addition to the spin–spin interactions responsible for magnetic polaron formation.