R. Pittini

Dynamics of excitonic magnetic polarons in nanostructure diluted magnetic semiconductors

Abstract Exciton dynamics has been studied in quantum wells, quantum dots and quantum wires of diluted magnetic semiconductors by time-resolved photoluminescence spectroscopy. The transient exciton photoluminescence with resonant excitation in Cd 1− x Mn x Te/ZnTe ( x =0.1) multiple quantum wells shows the formation of magnetic polarons in the quantum well excitons. Free and localized states of the excitonic magnetic polarons are distinguished from the difference of the lifetimes and the binding energies. The magnetic polaron formation accompanied with emission or absorption of LO phonons was also observed. Quantum dots of Cd 1− x Mn x Se grown by the self-organization mode showed the luminescence of confined excitons in the dots. The Mn ions in the quantum dots induce large Zeeman shifts and the excitonic magnetic polaron formation in the dots. Quantum wires of Cd 1− x Mn x Se showed polarized exciton emissions parallel to the wires. The polarization was decreased by the external magnetic field perpendicular to the wires. The excitonic magnetic polaron states in the quantum wires are also discussed.

Spin-flip rate of excitonic magnetic polarons in Cd1−xMnxTe/Cd1−yMgyTe quantum wells

The lifetime of excitons in Cd1−xMnxTe/Cd1−yMgyTe single quantum wells was investigated by time-resolved photoluminescence spectroscopy. Spin-flips between dark and bright excitons remarkably change the excitonic lifetime yielding a two-exponential decay in the photoluminescence intensity. The temperature dependence of the two decay times shows that the spin-flip rate depends strongly on the formation of excitonic magnetic polarons. By forming polarons, the spin-flip rate from dark excitons to bright excitons is reduced effectively by the spin- barriers of the polarized Mn ions, which extends significantly the luminescence decay time.

Excitonic spin states and radiative recombinations in Cd1−xMnxTe/Cd1−yMgyTe spin superlattices

Abstract The formation of a spin superlattice structure was realized in Cd0.95Mn0.05Te/Cd0.95Mg0.05Te under a magnetic field. The experimental evidence of the spin superlattice structure was found from time-resolved polarized magneto-photoluminescence measurements, in the Zeeman splittings and the exciton magnetic polaron formation. The Zeeman splitting directly indicates the spatial separation of spin-up and spin-down electrons and heavy holes, which are localized in the Cd0.95Mg0.05Te and Cd0.95Mn0.05Te layers, respectively. The formation of exciton magnetic polarons under a magnetic field can be attributed to the spin-dependent confinement where a large polaron binding energy of 3.5 meV has been observed in 2.5 T to be enhanced by the field-induced localization of the excitons.

TIME-RESOLVED MAGNETO-OPTICAL EXPERIMENTS IN CD1-XMNXTE/ZNTE MULTIPLE QUANTUM WELLS

Optical pump-probe experiments were performed in Cd0.9Mn0.1Te/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. Two transient absorptions of opposite sign were observed. The first signal is negative and indicates the decrease of optical absorption, as the excitons are approaching saturation. The second signal is positive and is interpreted as the absorption of “new excitons” after the initial excitons created by the pump pulse have decayed leaving behind a magnetic polarized Mn environment. The spin relaxation time of the Mn ions has been determined to be 601 ps. Finally, we discuss the effect of the polaron–polaron interaction, relevant in our experiment because we saturate the excitons in the quantum wells.

Asymmetric luminescence line shape and exciton energy relaxation in Zn1−x−yMgxCdySe epilayers

The asymmetric line shape of the transient photoluminescence (PL) spectra of high-quality Zn1−x−yMgxCdySe epilayers is deconvoluted to a narrow and a broad Gaussian peak at each time delay. The fitted energy difference between the two peaks corresponds to one longitudinal optical (LO) phonon energy. We assign the narrow peak to the recombination of free excitons (FXs) and the broad peak to the recombination of excitons localized in the broad band tail characteristic for this quaternary compound. The broad PL peak is found to arise from the energy relaxation of the FX at the exciton mobility edge to the localized states by LO phonon emission. Therefore, the population peak of the localized excitons is formed at one LO phonon energy below the FX for all times after the laser excitation.

Formation dynamics of free excitonic magnetic polarons in Cd1−xMnxTe

Formation processes of magnetic polarons from excitons were studied in Cd1−xMnxTe with the Mn composition x varying from 0.02 to 0.1 by transient photoluminescence spectroscopy. Two kinds of excitonic magnetic polarons were observed. One is the magnetic polaron composed of the free exciton, which is interacting with neighboring Mn-ion spins by the exchange interaction. The other is the magnetic polaron composed of the exciton bound to the neutral donor by aligning the surrounding Mn spins. The donor bound magnetic polarons appear for x=0.02–0.05 with the binding energy of 0.5–1.0 meV. The free excitonic magnetic polarons arise for x=0.02–0.1, where the binding energy of the free excitonic magnetic polaron varies in the range of 3.5-8.0 meV depending on the Mn composition. The stability of the free excitonic magnetic polaron state is confirmed by the appearance of the strong transient photoluminescence in the wide Mn composition range.

Transient behavior of the excitonic magnetic polarons in Cd1−xMnxTe epilayers

We present a study of the dynamics of the excitonic magnetic polarons in Cd1−xMnxTe epilayers. The transient photoluminescence measured by a streak camera under resonant excitation is analyzed with a special attention devoted to line shape of the emission signal. A nonradiative spectral region is observed in zero field and is strongly suppressed in a field of 6 T. This observation is understood to be directly related to the alignment of the Mn2+ spins and is therefore closely related to the formation of magnetic polarons. A few Mn spins are found to be aligned already within 10 ps, but the total Mn spin polarization in the sample is found to increase exponentially with a rise time of 37 ps in Cd0.73Mn0.27Te epilayers. Furthermore, the values of 19.2 and 12.4 meV were determined for the energies of the excitonic magnetic polarons in Cd0.73Mn0.27Te and Cd0.79Mn0.21Te epilayers, respectively.

Spin relaxation times of exciton states in ZnCdSe/ZnSe low dimensional heterostructures

Abstract Using the pulsed excitation with a femtosecond laser and streak-camera detection we have studied the processes of spin relaxation in the system of localized excitons in low dimensional superlattices formed by the insertion of CdSe submonolayers into the ZnSe matrices. Polarization of exciton luminescence in the magnetic field arises due to the thermal redistribution of population in the system of splitted exciton spin sublevels. The distribution of polarization across the PL band countour is governed by the complicated interplay of energy and spin relaxation processes. The study of PL decay kinetics in polarized light allows us to determine with the help of a simple two-level model the times of exciton spin relaxation. Spin relaxation times strongly decrease with the increase of magnetic field indicating the phonon assisted mechanism of spin relaxation. The detailed analysis of the dependence of spin relaxation times on the exciton localization energy is not possible with the model used due to the complicated character of exciton population kinetics for deeply localized states. Deeply localized states are to a great extent populated through the tunnelling relaxation of excitons with higher energies. The transfer of polarization in the processes of exciton energy relaxation has been clearly observed in the study of spectrally resolved decay kinetics in polarized light.

Yb4As3 and Yb4(As0.71Sb0.29)3, two ways to heavy-fermion character

Abstract After the observation of heavy-fermion characteristics in the charge-ordered phase of Yb 4 As 3 , it is surprising that Yb 4 (As 0.71 Sb 0.29 ) 3 shows heavy-fermion behavior, though the charge ordering is suppressed. To shed more light onto this enigma, we measured the optical reflectivity of Yb 4 (As 0.71 Sb 0.29 ) 3 at several temperatures down to 15 K in the photon energy range from 1 meV to 5.5 eV. The comparison of the optical functions of Yb 4 As 3 and Yb 4 (As 0.71 Sb 0.29 ) 3 revealed decisive differences in the far-IR region. For the latter material, we found an hybridization gap of 3.3 meV, absent in Yb 4 As 3 . The effective mass of the conduction carriers takes the temperature-independent value of 2 m e in Yb 4 As 3 , while m * shows a logarithmic temperature dependency and reaches 80 m e at 15 K in Yb 4 (As 0.71 Sb 0.29 ) 3 . This proves that two different mechanisms lead to a large γ value in Yb 4 As 3 and Yb 4 (As 0.71 Sb 0.29 ) 3 .

Excitonic properties of Cd1−xMnxTe quantum wells grown by molecular beam epitaxy

Abstract Time-resolved photoluminescence has been studied in molecular beam epitaxy grown Cd 1− x Mn x Te/Cd 1− y Mg y Te and CdTe/Cd 1− x Mn x Te single and multiple quantum wells. The excitonic properties were investigated at several temperatures and magnetic fields. The dynamics of the exciton magnetic polaron are greatly influenced by the quantum confinement of the excitons in the single quantum wells. The formation of a spin superlattice structure was realized in Cd 0.95 Mn 0.05 Te/Cd 0.95 Mg 0.05 Te multi-quantum wells under an external magnetic field.