Many conductors, including recently studied Dirac materials, show saturation of coherence length ... more Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities or two-level systems specific to non-crystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) Pb1−xSnxSe thin film with Sn contents x corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.
Many conductors, including recently studied Dirac materials, show saturation of coherence length ... more Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities or two-level systems specific to non-crystalline solids. Here, by considering SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in Pb$_{1-x}$Sn$_x$Se thin film with Sn contents $x$ corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-r...
[1]. Using electrostatic control of exchange we create conductive channels of DWs which, unlike c... more [1]. Using electrostatic control of exchange we create conductive channels of DWs which, unlike conventional edge channels, are not chiral and should contain both spin polarizations. We will present results on the formation of isolated DWs of various widths and discuss their transport properties. [1] A. Kazakov, et al., Phys. Rev. B 94, 075309 (2016).
We present a theoretical and experimental study of the interplay between spin-orbit coupling (SOC... more We present a theoretical and experimental study of the interplay between spin-orbit coupling (SOC), Coulomb interaction and motion of conduction electrons in a magnetized two-dimensional electron gas. Via a transformation of the many-body Hamiltonian we introduce the concept of spin-orbit twisted spin-waves, whose energy dispersions and damping rates are obtained by a simple wave-vector shift of the spin waves without SOC. These theoretical predictions are validated by Raman scattering measurements. With optical gating of the density, we vary the strength of the SOC to alter the group velocity of the spin wave. The findings presented here differ from that of spin systems subject to the Dzyaloshinskii-Moriya interaction. Our results pave the way for novel applications in spin-wave routing devices or for the realization of lenses for spin waves.
Phonon excitations in (CdTe)12/(MnTe)n (100) superlattices (n 2, 4, 8) were investigated at 295 K... more Phonon excitations in (CdTe)12/(MnTe)n (100) superlattices (n 2, 4, 8) were investigated at 295 K and 25 K with the use of Raman scattering. From the "folded" phonon frequencies the elastic constant c 11 vaJue for MnTe was estimated. The strain arising from lattice mismatch (determined by X-ray diffraction) results in shifts of MnTe and CdTe "confined" LO phonon frequencies. For the precise determination of LO phonon dispersions an additional shift due to Mn diffusion at the CdTe/MnTe interface should be taken into account.
Pr oceed in gs o f t he XXX I I n t ern at io n al Sch oo l o f Sem icond uct i ng Co m p ou n ds... more Pr oceed in gs o f t he XXX I I n t ern at io n al Sch oo l o f Sem icond uct i ng Co m p ou n ds, Ja szo wi ec 200 2
Catalytically enhanced growth of ZnMnTe diluted magnetic semiconductor nanowires by molecular bea... more Catalytically enhanced growth of ZnMnTe diluted magnetic semiconductor nanowires by molecular beam epitaxy is reported. The growth is based on the vapor-liquid-solid mechanism and was performed on (001) and (011)-oriented GaAs substrates from elemental sources. X-ray diffractometry, scanning and transmission electron microscopy, atomic force microscopy, photoluminescence spectroscopy, and Raman scattering were performed to determine the structure of nanowires, their chemical composition, and morphology. These studies revealed that the obtained ZnMnTe nanowires possess zinc-blende structure, have an average diameter of about 30 nm, typical length between 1 and 2 µm and that Mn 2+ ions were incorporated into substitutional sites of the ZnTe crystal lattice.
Narrow gap semiconductors (NGS) offer several scientifically unique features important for the fi... more Narrow gap semiconductors (NGS) offer several scientifically unique features important for the field of spintronics. In order to explore these features we are using standard pump-probe and magneto-optical Kerr effect (MOKE) spectroscopy at different excitation wavelengths, power densities, and temperatures. Our goal is measuring and controlling carrier/spin relaxation in a series of InSb-based quantum wells and films, and InMnSb ferromagnetic films. The dynamic effects observed in these structures demonstrate strong dependence on the photo-induced carrier density.
Precise-temperature-dependent measurements of energy of excitons confined in CdTe/CdMnTe quantum ... more Precise-temperature-dependent measurements of energy of excitons confined in CdTe/CdMnTe quantum wells and superlattices were performed between 2 K and 100 K. We show that magnetic fluctuations lead to a variation of the valence band offset, in agreement with a model proposed previously for bulk materials.
While the support for the existence of a Mn-derived impurity band in the diluted magnetic semicon... more While the support for the existence of a Mn-derived impurity band in the diluted magnetic semiconductor Ga 1−x Mn x As has recently increased, a detailed quantitative analysis of its formation and properties is still incomplete. Here, we show that such an impurity band arises as the result of an anticrossing interaction between the extended states of the GaAs valence band and the strongly localized Mn states according to the valence band anticrossing model. The anticrossing interpretation is substantiated by optical measurements that reveal a shift in the band gap of GaAs upon the addition of Mn and it also explains the remarkably low hole mobility in this alloy. Furthermore, the presence of a Mn-derived impurity band correctly accounts for the metal-to-insulator transition experimentally observed in Ga 1−x Mn x As 1−y ͑N,P͒ y with y Յ 0.02.
We use a two-color transient Kerr rotation technique to study the spin dynamics in an n-doped CdT... more We use a two-color transient Kerr rotation technique to study the spin dynamics in an n-doped CdTe/ Cd 0.85 Mg 0.15 Te 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.
Physica E: Low-dimensional Systems and Nanostructures, 2004
CdTe/Cd 1-x Mn x Te is a prototype of a dilute magnetic semiconductor quantum well with its barri... more CdTe/Cd 1-x Mn x Te is a prototype of a dilute magnetic semiconductor quantum well with its barrier containing Mn localized spins. Substantial number of papers has been reported so far for the
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2004
We provide unambiguous experimental evidence that the upper limit of ~110 K commonly observed for... more We provide unambiguous experimental evidence that the upper limit of ~110 K commonly observed for the Curie temperature T C of Ga 1-x Mn x As is caused by the Fermilevel-induced hole saturation. This conclusion is based on parallel studies of the location of Mn in the lattice, the effectiveness of acceptor center, and ferromagnetism on a series of Ga 1-x-y Mn x Be y As layers, in which the concentration of magnetic moments and of free holes can be independently controlled by the Mn and Be contents. Ion channeling and magnetization measurements show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T C with increasing Be concentration. At the same time the free hole concentration remains relatively constant at ~5x10 20 cm-3. These results indicate that the concentrations of free holes as well as of ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors. Based on these results, we propose to use heavy n-type counter-doping of Ga 1-x Mn x As (by, e.g., Te) to suppress the formation of Mn interstitials at high x, and thus improve the T C of the alloy system.
GaAs/GaMnAs core-shell nanowires were grown by molecular beam epitaxy. The core GaAs nanowires we... more GaAs/GaMnAs core-shell nanowires were grown by molecular beam epitaxy. The core GaAs nanowires were synthesized under typical nanowire growth conditions using gold as catalyst. For the GaMnAs shell the temperature was drastically reduced to achieve low-temperature growth conditions known to be crucial for high-quality GaMnAs. The GaMnAs shell grows epitaxially on the side facets of the core GaAs nanowires. A ferromagnetic transition temperature of 20 K is obtained. Magnetic anisotropy studies indicate a magnetic easy axis parallel to the nanowire axis.
The effect of modulation doping by Be on the ferromagnetic properties of Ga1−xMnxAs is investigat... more The effect of modulation doping by Be on the ferromagnetic properties of Ga1−xMnxAs is investigated in Ga1−xMnxAs/Ga1−yAlyAs heterojunctions and quantum wells. Introducing Be acceptors into the Ga1−yAlyAs barriers leads to an increase of the Curie temperature TC of Ga1−xMnxAs, from 70 K in undoped structures to over 100 K with the modulation doping. This increase is qualitatively consistent with a multiband mean field theory simulation of carrier-mediated ferromagnetism. An important feature is that the increase of TC occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, but not when the Be-doped layer is grown first. This behavior is expected from the strong sensitivity of Mn interstitial formation to the value of the Fermi energy during growth.
Many conductors, including recently studied Dirac materials, show saturation of coherence length ... more Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities or two-level systems specific to non-crystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) Pb1−xSnxSe thin film with Sn contents x corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.
Many conductors, including recently studied Dirac materials, show saturation of coherence length ... more Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities or two-level systems specific to non-crystalline solids. Here, by considering SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in Pb$_{1-x}$Sn$_x$Se thin film with Sn contents $x$ corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-r...
[1]. Using electrostatic control of exchange we create conductive channels of DWs which, unlike c... more [1]. Using electrostatic control of exchange we create conductive channels of DWs which, unlike conventional edge channels, are not chiral and should contain both spin polarizations. We will present results on the formation of isolated DWs of various widths and discuss their transport properties. [1] A. Kazakov, et al., Phys. Rev. B 94, 075309 (2016).
We present a theoretical and experimental study of the interplay between spin-orbit coupling (SOC... more We present a theoretical and experimental study of the interplay between spin-orbit coupling (SOC), Coulomb interaction and motion of conduction electrons in a magnetized two-dimensional electron gas. Via a transformation of the many-body Hamiltonian we introduce the concept of spin-orbit twisted spin-waves, whose energy dispersions and damping rates are obtained by a simple wave-vector shift of the spin waves without SOC. These theoretical predictions are validated by Raman scattering measurements. With optical gating of the density, we vary the strength of the SOC to alter the group velocity of the spin wave. The findings presented here differ from that of spin systems subject to the Dzyaloshinskii-Moriya interaction. Our results pave the way for novel applications in spin-wave routing devices or for the realization of lenses for spin waves.
Phonon excitations in (CdTe)12/(MnTe)n (100) superlattices (n 2, 4, 8) were investigated at 295 K... more Phonon excitations in (CdTe)12/(MnTe)n (100) superlattices (n 2, 4, 8) were investigated at 295 K and 25 K with the use of Raman scattering. From the "folded" phonon frequencies the elastic constant c 11 vaJue for MnTe was estimated. The strain arising from lattice mismatch (determined by X-ray diffraction) results in shifts of MnTe and CdTe "confined" LO phonon frequencies. For the precise determination of LO phonon dispersions an additional shift due to Mn diffusion at the CdTe/MnTe interface should be taken into account.
Pr oceed in gs o f t he XXX I I n t ern at io n al Sch oo l o f Sem icond uct i ng Co m p ou n ds... more Pr oceed in gs o f t he XXX I I n t ern at io n al Sch oo l o f Sem icond uct i ng Co m p ou n ds, Ja szo wi ec 200 2
Catalytically enhanced growth of ZnMnTe diluted magnetic semiconductor nanowires by molecular bea... more Catalytically enhanced growth of ZnMnTe diluted magnetic semiconductor nanowires by molecular beam epitaxy is reported. The growth is based on the vapor-liquid-solid mechanism and was performed on (001) and (011)-oriented GaAs substrates from elemental sources. X-ray diffractometry, scanning and transmission electron microscopy, atomic force microscopy, photoluminescence spectroscopy, and Raman scattering were performed to determine the structure of nanowires, their chemical composition, and morphology. These studies revealed that the obtained ZnMnTe nanowires possess zinc-blende structure, have an average diameter of about 30 nm, typical length between 1 and 2 µm and that Mn 2+ ions were incorporated into substitutional sites of the ZnTe crystal lattice.
Narrow gap semiconductors (NGS) offer several scientifically unique features important for the fi... more Narrow gap semiconductors (NGS) offer several scientifically unique features important for the field of spintronics. In order to explore these features we are using standard pump-probe and magneto-optical Kerr effect (MOKE) spectroscopy at different excitation wavelengths, power densities, and temperatures. Our goal is measuring and controlling carrier/spin relaxation in a series of InSb-based quantum wells and films, and InMnSb ferromagnetic films. The dynamic effects observed in these structures demonstrate strong dependence on the photo-induced carrier density.
Precise-temperature-dependent measurements of energy of excitons confined in CdTe/CdMnTe quantum ... more Precise-temperature-dependent measurements of energy of excitons confined in CdTe/CdMnTe quantum wells and superlattices were performed between 2 K and 100 K. We show that magnetic fluctuations lead to a variation of the valence band offset, in agreement with a model proposed previously for bulk materials.
While the support for the existence of a Mn-derived impurity band in the diluted magnetic semicon... more While the support for the existence of a Mn-derived impurity band in the diluted magnetic semiconductor Ga 1−x Mn x As has recently increased, a detailed quantitative analysis of its formation and properties is still incomplete. Here, we show that such an impurity band arises as the result of an anticrossing interaction between the extended states of the GaAs valence band and the strongly localized Mn states according to the valence band anticrossing model. The anticrossing interpretation is substantiated by optical measurements that reveal a shift in the band gap of GaAs upon the addition of Mn and it also explains the remarkably low hole mobility in this alloy. Furthermore, the presence of a Mn-derived impurity band correctly accounts for the metal-to-insulator transition experimentally observed in Ga 1−x Mn x As 1−y ͑N,P͒ y with y Յ 0.02.
We use a two-color transient Kerr rotation technique to study the spin dynamics in an n-doped CdT... more We use a two-color transient Kerr rotation technique to study the spin dynamics in an n-doped CdTe/ Cd 0.85 Mg 0.15 Te 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.
Physica E: Low-dimensional Systems and Nanostructures, 2004
CdTe/Cd 1-x Mn x Te is a prototype of a dilute magnetic semiconductor quantum well with its barri... more CdTe/Cd 1-x Mn x Te is a prototype of a dilute magnetic semiconductor quantum well with its barrier containing Mn localized spins. Substantial number of papers has been reported so far for the
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2004
We provide unambiguous experimental evidence that the upper limit of ~110 K commonly observed for... more We provide unambiguous experimental evidence that the upper limit of ~110 K commonly observed for the Curie temperature T C of Ga 1-x Mn x As is caused by the Fermilevel-induced hole saturation. This conclusion is based on parallel studies of the location of Mn in the lattice, the effectiveness of acceptor center, and ferromagnetism on a series of Ga 1-x-y Mn x Be y As layers, in which the concentration of magnetic moments and of free holes can be independently controlled by the Mn and Be contents. Ion channeling and magnetization measurements show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T C with increasing Be concentration. At the same time the free hole concentration remains relatively constant at ~5x10 20 cm-3. These results indicate that the concentrations of free holes as well as of ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors. Based on these results, we propose to use heavy n-type counter-doping of Ga 1-x Mn x As (by, e.g., Te) to suppress the formation of Mn interstitials at high x, and thus improve the T C of the alloy system.
GaAs/GaMnAs core-shell nanowires were grown by molecular beam epitaxy. The core GaAs nanowires we... more GaAs/GaMnAs core-shell nanowires were grown by molecular beam epitaxy. The core GaAs nanowires were synthesized under typical nanowire growth conditions using gold as catalyst. For the GaMnAs shell the temperature was drastically reduced to achieve low-temperature growth conditions known to be crucial for high-quality GaMnAs. The GaMnAs shell grows epitaxially on the side facets of the core GaAs nanowires. A ferromagnetic transition temperature of 20 K is obtained. Magnetic anisotropy studies indicate a magnetic easy axis parallel to the nanowire axis.
The effect of modulation doping by Be on the ferromagnetic properties of Ga1−xMnxAs is investigat... more The effect of modulation doping by Be on the ferromagnetic properties of Ga1−xMnxAs is investigated in Ga1−xMnxAs/Ga1−yAlyAs heterojunctions and quantum wells. Introducing Be acceptors into the Ga1−yAlyAs barriers leads to an increase of the Curie temperature TC of Ga1−xMnxAs, from 70 K in undoped structures to over 100 K with the modulation doping. This increase is qualitatively consistent with a multiband mean field theory simulation of carrier-mediated ferromagnetism. An important feature is that the increase of TC occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, but not when the Be-doped layer is grown first. This behavior is expected from the strong sensitivity of Mn interstitial formation to the value of the Fermi energy during growth.
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Papers by Tomasz Wojtowicz