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ABSTRACT: Spin-flip Raman scattering of electrons and heavy-holes is studied for
resonant excitation of neutral and charged excitons in a
CdTe/Cd$_{0.63}$Mg$_{0.37}$Te quantum well. The spin-flip scattering is
characterized by its dependence on the incident and scattered light
polarization as well as on the magnetic field strength and orientation. Model
schemes of electric-dipole allowed spin-flip Raman processes in the exciton
complexes are compared to the experimental observations, from which we find
that lowering of the exciton symmetry, time of carrier spin relaxation, and
mixing between electron states and, respectively, light- and heavy-hole states
play an essential role in the scattering. At the exciton resonance, anisotropic
exchange interaction induces heavy-hole spin-flip scattering, while acoustic
phonon interaction is mainly responsible for the electron spin-flip. In
resonance with the positively and negatively charged excitons, anisotropic
electron-hole exchange as well as mixed electron states allow spin-flip
scattering. Variations in the resonant excitation energy and lattice
temperature demonstrate that localization of resident electrons and holes
controls the Raman process probability and is also responsible for symmetry
reduction. We show that the intensity of the electron spin-flip scattering is
strongly affected by the lifetime of the exciton complex and in tilted magnetic
fields by the angular dependence of the anisotropic electron-hole exchange
interaction.
01/2013;
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ABSTRACT: We report on magnetic field-induced oscillations of the photon echo signal from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum well. The oscillatory signal is due to Larmor precession of the electron spin about a transverse magnetic field and depends sensitively on the polarization configuration of the exciting and refocusing pulses. The echo amplitude can be fully tuned from the maximum down to zero depending on the time delay between the two pulses and the magnetic-field strength. The results are explained in terms of the optical Bloch equations accounting for the spin level structure of electrons and trions.
Physical Review Letters 10/2012; 109(15):157403. · 7.37 Impact Factor
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ABSTRACT: We report on magnetic field induced oscillations of the photon echo signal
from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum
well. The oscillatory signal is due to Larmor precession of the electron spin
about a transverse magnetic field and depends sensitively on the polarization
configuration of the exciting and refocusing pulses. The echo amplitude can be
fully tuned from maximum down to zero depending on the time delay between the
two pulses and the magnetic field strength. The results are explained in terms
of the optical Bloch equations accounting for the spin level structure of
electron and trion.
06/2012;
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ABSTRACT: Magnetoluminescence experiments in CdTe/CdMgTe single-quantum well with dense two-dimensional electron gases are performed at high magnetic fields up to 35 T. A broad photoluminescence peak, which originates in the Fermi edge singularity, evolves into singlet-charged excitons under the magnetic field in both + and – polarization. A triplet-charged exciton peak, however, appears only in the – polarization, and the intensity of the triplet peak becomes intense above the magnetic field where the singlet peak of the + polarization crosses the triplet peak. The crossing magnetic field is consistent with the recent theoretical calculation of the magnetic field dependence of the charged excitons by Redliski and Kossut, confirming that the nature of the PL peaks around = 1 can be explained from the viewpoint of the charged excitonic transitions even for the dense two-dimensional electron systems.
Journal of Superconductivity 04/2012; 18(2):215-218.
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K. Kukliński,
Ł. Kłopotowski,
K. Fronc,
M. Wiater,
P. Wojnar,
P. Rutkowski,
V. Voliotis,
R. Grousson,
G. Karczewski, J. Kossut,
T. Wojtowicz
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ABSTRACT: We present photoluminescence studies of highly excited single self-assembled CdTe quantum dots under continuous-wave and pulsed excitations. We observe appearance of emission bands related to sequential filling of s-, p- and d-shells. We analyze the inter-shell splitting for five samples, in which the dots were formed from a strained CdTe layer of different width. We find that by increasing the CdTe layer width, the inter-shell splitting increases. In a time resolved measurement, we observe a radiative cascade between transitions involving one, two, and more than two excitons.
Applied Physics Letters 10/2011; 99(14):141906-141906-3. · 3.84 Impact Factor
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G. Bartsch,
M. Gerbracht,
D. R. Yakovlev,
J. H. Blokland,
P. C. M. Christianen,
E. A. Zhukov,
A. B. Dzyubenko,
G. Karczewski,
T. Wojtowicz, J. Kossut,
J. C. Maan,
M. Bayer
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ABSTRACT: A magneto-optical study of the energy and spin structure of charged excitons in a 20-nm-thick CdTe/Cd0.65Mg0.35Te quantum well is performed in strong magnetic fields up to 51 T. The type of resident carriers (holes or electrons) in the quantum well is controlled optically by above-barrier illumination, permitting a direct comparison of positively (T+) versus negatively (T-) charged excitons. The binding energies of the singlet states of these complexes behave qualitatively differently with increasing magnetic field B; namely, the binding energy decreases for T+ and increases for T- with B. The triplet state of T+ is identified in strong fields with a binding energy smaller than that of the T- triplet state.
Physical Review B - PHYS REV B. 01/2011; 83.
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ABSTRACT: The magnetization dynamics in diluted magnetic semiconductor heterostructures
based on (Zn,Mn)Se and (Cd,Mn)Te has been studied experimentally by optical
methods and simulated numerically. In the samples with nonhomogeneous magnetic
ion distribution this dynamics is contributed by spin-lattice relaxation and
spin diffusion in the Mn spin system. The spin diffusion coefficient of
7x10^(-8) cm^2/s has been evaluated for Zn(0.99)Mn(0.01)Se from comparison of
experimental and numerical results. Calculations of the giant Zeeman splitting
of the exciton states and the magnetization dynamics in the ordered alloys and
parabolic quantum wells fabricated by the digital growth technique show perfect
agreement with the experimental data. In both structure types the spin
diffusion has an essential contribution to the magnetization dynamics.
05/2010;
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ABSTRACT: Two temporally non-overlapping linearly cross-polarized 140 fs laser pulses are shown to control the spin polarization in a three-level system. Simultaneous excitation of the two excited states triggers quantum beatings originating from the interference of the wavefunctions corresponding to different spin sublevels of the states. Although the beatings are not seen in the spin densities of the excited states they are clearly observed in the magneto-optical Kerr effect. An analytical expression for the description of the beatings is obtained. Experimental results are in good agreement with theoretical predictions and demonstrate the control of beatings with attosecond resolution.
Journal of Physics Condensed Matter 03/2010; 22(11):115801. · 2.55 Impact Factor
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ABSTRACT: Optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization is studied experimentally and theoretically. For that purpose the coherent electron spin dynamics in a n-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state. The amplitude and phase shifts of the electron spin beat signal in an external magnetic field, that are induced by laser control pulses, depend on the pump-control delay and polarization of the control relative to the pump pulse. Additive and non-additive contributions to pump-induced signal due to the control are isolated experimentally. These contributions can be well described in the framework of a two-level model for the optical excitation of the resident electron to the trion. Comment: 15 pages, 18 figures
12/2009;
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ABSTRACT: The growth of self-assembled CdTe quantum dots embedded in cubic MnTe by molecular beam epitaxy is reported. The dots are forced to form despite a small lattice mismatch of 2.3% between the dot and the barrier materials. Their properties are studied by means of time integrated and time resolved photoluminescence at various temperatures and magnetic fields. We demonstrate a considerable diffusion of Mn ions from the MnTe barrier into nominally nonmagnetic CdTe quantum dot, which is manifested as an enhancement of their magneto-optical effects, such as the Zeeman splitting of excitonic levels in the dots.
Phys. Rev. B. 11/2009; 80(19).
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ABSTRACT: We report the growth of self assembled CdMnTe diluted magnetic quantum dots by molecular beam epitaxy. The Mn content within the dots can be efficiently controlled by the growth conditions. The presence of magnetic Mn ions in the zero-dimensional structures results in a strong enhancement of their spin related properties, in the giant Zeeman splitting of excitonic levels, in particular. The insight into these magnetic properties is obtained by means of micro-photoluminescence measurements at an external magnetic field. We study the evolution of several photoluminescence lines related to the emission from individual quantum dots at an external magnetic field. We determine an approximate number of magnetic ions inside the dots and the sizes of the dots by comparing these dependences to the model calculations.
Journal of Physics Conference Series 02/2009; 146(1):012032.
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ABSTRACT: Photoluminescence spectra taken from modulationdoped CdTe/(Cd0.7Mg0.3)Te quantum well structures with a two-dimensional electron gas of moderate to high density (from ne ∼ 1010 up to ne ∼ 1012 cm–2) in magnetic fields from 0 T to 45 T were studied. The observed spectra were analyzed in terms of combined excitonelectron processes. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 10/2008; 6(2):516 - 519.
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ABSTRACT: We report on the growth and high spatial resolution magneto-photoluminescence of CdMnTe quantum dots embedded in a ZnCdTe matrix with Mn content of about 3%. The giant Zeeman spitting of excitonic emission coming from individual quantum dots and magnetization fluctuations inside them are studied in magnetic fields up to 7 T. We have found that the description of these effects requires the inclusion of antiferromagnetic Mn-Mn interaction in the considerations. Moreover, the values of the Zeeman shift are strongly related to the size of the dot. In particular, the higher spectral position of the line is, i.e., the smaller the dot, the smaller the Zeeman splitting that is observed. This effect is quantitatively explained with model calculations made under the assumption of magnetic polaron formation in diluted magnetic quantum dots and the presence of ion-ion exchange interaction.
Nanotechnology 06/2008; 19(23):235403. · 3.98 Impact Factor
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ABSTRACT: Modulation-doped CdTe/Cd0.7Mg0.3Te quantum well structures with a two dimensional electron gas of low and moderate density (from ne = 3 × 1010 cm–2 up to ne ≈ 1012 cm–2) have been studied. Photoluminescence spectra taken from the quantum well in magnetic fields from 0 T to 45 T at different electron densities were studied. The observed spectra were analyzed in terms of combined exciton-electron processes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 02/2008; 5(7):2404 - 2407.
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ABSTRACT: We present results on the linear polarization of the photoluminescence (PL) of dilute magnetic semiconductor quantum wells which is induced by an applied magnetic field. The PL is found to be partially linearly polarized, but without any correlation with the polarization of the exciting light, whilst the direction of the polarization does not depend on the direction of the magnetic field in the plane although its magnitude does. We present a model for this behavior in terms of a possible reduction of the in-plane symmetry of the quantum well; we conclude that any perturbations giving (i) a finite in-plane hole g-factor and (ii) a mixing of light and heavy holes are sufficient to generate the type of experimental behavior we observe.
Journal- Korean Physical Society 01/2008; 53(5):2782-2786. · 0.45 Impact Factor
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ABSTRACT: 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.
08/2007;
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ABSTRACT: We use a two-color transient Kerr rotation technique to study the spin dynamics in an n-doped CdTe∕Cd0.85Mg0.15Te quantum well. The dynamics displays the interplay between excitons, trions, and the two-dimensional electron gas. The spin relaxation of individual species is resolved by spectral selection. The spin dynamics are quantitatively described by rate equations involving the spin populations of excitons, trions, and the electron gas. Under resonant excitation of excitons, spin polarization of the electron gas is generated through trion formation, with the spin coherence partially lost through exciton spin relaxation. A maximum hole spin-flip time is observed around the trion resonance, with a rapid decrease for increasing excitation energy.
Phys. Rev. B. 03/2007; 75(11).
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13th International Conference on II-VI Compounds13th International Conference on II-VI Compounds, Jeju City, SOUTH KOREA; 01/2007
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ABSTRACT: Magnetic-field-induced second-harmonic generation (MFISH) has been studied in diluted magnetic semiconductors Cd1−xMnxTe (0.001<x<0.22) being primarily paramagnetic. Experiments have been performed in magnetic fields up to 10 T and MFISH signals have been observed in the spectral range near the band gap for temperatures varying from 1.8 to 200 K. Depending on the Mn content, MFISH signals arise from two distinctly different origins related to orbital and spin quantization of the band states in magnetic field. The orbital contribution dominates for zero and very low Mn concentrations, whereas the spin contribution dominates for higher Mn contents. These contributions can be distinguished by their characteristic dependencies of the MFISH intensity on magnetic field. The orbital part of the MFISH scales as square of the field strength in accordance with a phenomenological theory. In the case of the spin quantization, the MFISH intensity scales linearly with the magnetization, a behavior which is not expected from the phenomenological model.
Phys. Rev. B. 12/2006; 74(23).
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ABSTRACT: The spin coherence of a two-dimensional electron gas (2DEG) at different densities in Cd Te / Cd <sub>0.85</sub> Mg <sub>0.15</sub> Te quantum wells has been examined by the time-resolved Kerr rotation technique using resonant excitation of either trions or excitons. The formation of negatively charged trions, either excited resonantly or via exciton states, causes strong spin polarization of the 2DEG. This effect leads to a long lasting exponential decay in the nanosecond regime. Spin dephasing times T<sub>2</sub><sup>*</sup> of the 2DEG measured as a function of electron density up to 2.4×10<sup>11</sup> cm <sup>-2</sup> show a nonmonotonic behavior with a maximum at 8×10<sup>10</sup> cm <sup>-2</sup> .
Applied Physics Letters 12/2006; · 3.84 Impact Factor
