Spin Superlattice Research Articles

Spin-resonant splitting in magnetically modulated semimagnetic semiconductor superlattices

We investigate spin-resonant splitting in magnetically modulated semimagnetic semiconductor superlattices by adopting tight-binding model and Green's function method under the influence of an external electric field. Spin-dependent resonant splitting features for both the transmission spectra and the current density spectra are discussed in more detail. Under no influence of the external electric field, the periodic nature of the spin superlattice leads to a regular profile of quantization in the transmission, which is composed of spin-dependent resonant bands separated by nonresonant gaps, where the resonant splitting rule of the transmission for spin-up case is exactly the same as that for spin-down case. The transmission resonances are drastically suppressed by the external electric field, the difference between resonant bands and nonresonant gaps is lessened and the transmission spectra are smoothed out. It is shown that splitting of the current density is more complex. In contrast with the transmission, the number of oscillations in the current density spectra has no simple direct correspondence to the number of unit cells and cannot be summarized in the simple rule.

Excitonic properties of Cd 1− xMn xTe quantum wells grown by molecular beam epitaxy

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 0.95Mn 0.05Te/Cd 0.95Mg 0.05Te multi-quantum wells under an external magnetic field.

Excitonic spin states and radiative recombinations in Cd 1− xMn xTe/Cd 1− yMg yTe spin superlattices

The formation of a spin superlattice structure was realized in Cd 0.95Mn 0.05Te/Cd 0.95Mg 0.05Te 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 Cd 0.95Mg 0.05Te and Cd 0.95Mn 0.05Te 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.

Magneto-optical properties ofZn0.95Mn0.05Se/Zn0.76Be0.08Mg0.16Sequantum wells andZn0.91Mn0.09Se/Zn0.972Be0.028Sespin superlattices

We have performed magnetoluminescence and magnetoreflectance studies on novel semimagnetic semiconductor heterostructures based on $(\mathrm{ZnMn})\mathrm{Se}$/$(\mathrm{ZnBeMg})\mathrm{Se}$. The successful fabrication and various optical properties of that system are demonstrated by ${\mathrm{Zn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Se}$/'' ''${\mathrm{Zn}}_{0.76}{\mathrm{Be}}_{0.08}{\mathrm{Mg}}_{0.16}\mathrm{Se}$ single quantum wells. The exciton spin splitting was used to determine the valence-band offset for $(\mathrm{ZnMn})\mathrm{Se}$/$(\mathrm{ZnBeMg})\mathrm{Se}$ to $0.22$ of the total band-gap discontinuity. Weakly confining multiple quantum wells made of ${\mathrm{Zn}}_{0.91}{\mathrm{Mn}}_{0.09}\mathrm{Se}/{\mathrm{Zn}}_{0.972}{\mathrm{Be}}_{0.028}\mathrm{Se}$ demonstrate a change of the band alignment from type I to type II for one of the exciton spin components in external magnetic fields which results in the formation of a spin superlattice. This spin superlattice formation manifests itself in an asymmetric Zeeman splitting of a spatially direct exciton caused by a spin-dependent change of the exciton binding energy. A pronounced broadening of the exciton in reflectance and photoluminescence excitation spectra was observed when scattering into the (ZnBe)Se barriers becomes possible.

Excitons in extremely shallow quantum wells

Excitons in extremely shallow semiconductor quantum wells are considered in the limit when both the conduction band and the valence band confining potentials are small compared to the binding energy of a three-dimensional (3D) exciton. Under these circumstances it is found that the quantization of the center-of-mass motion can make a sizable contribution to energies of excitonic optical transitions. A simple effective Hamiltonian is derived for describing this situation, with a potential that confines the motion of the exciton center of mass. The shape of the potential is approximated either by a parabolic profile (when quantum wells are narrow compared with the 3D exciton Bohr radius), or by a rectangular potential (for wide quantum wells), and the resultant eigenvalue problem is solved accordingly. The results are compared to experimental data obtained in magnetooptical studies of ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}$ spin superlattices, giving excellent quantitative agreement.

Time-resolved spin dynamics in strained Zn 1−xMn xSe/ZnSe spin superlattices

We use time-resolved spectroscopy to directly compare the spin dynamics in strained ZnMnSe epilayers and strained ZnMnSe/ZnSe spin superlattices. The spin-relaxation in these materials is observed to depend strongly on the concentration of the magnetic impurities. Surprisingly, however, no difference is observed between the spin-relaxation of excitons in the quasi-three-dimensional epilayers or the two-dimensional spin superlattices. Indeed, the spin-relaxation of excitons is also seen to be independent of the applied magnetic field or magnetization of the Mn-impurities.

Spin dependent confinement effects in [formula omitted] spin superlattices

We have realized a spin superlattice structure based on a Cd0.9Mn0.1TeCd0.9Mg0.1Te 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.

Spin dependent carrier localization in Fe-based semimagnetic semiconductor heterostructures

Semimagnetic (or diluted magnetic) semiconductor heterostructures offer many unique opportunities for the study of spin dependent effects, including magnetic field induced quantum confinement and carrier spin lifetimes. In diluted magnetic semiconductors (DMS) such as Zn1 − xFexSe, strong spin exchange interactions significantly modify the band structure in the presence of an applied magnetic field. This allows one to continuously and reversibly modify the band offsets in tailored heterostructures to selectively localize carriers according to their spin state, thereby providing another degree of freedom in tuning the spatial localization and overlap of carrier wave functions after the fact of growth. We review several examples which have been recently demonstrated in Zn1 − xFexSe-based heterostructures grown by molecular beam epitaxy, including field tunable type-I–type-II transitions, the formation of a spin superlattice, and the continuous evolution of band alignment to the realization of a re-entrant type-I structure. Additional control over carrier confinement and lifetimes via carrier spin is possible in structures which consist of alternating layers of Zn1 − xFexSe and Zn1 − yMnySe. In such multiple quantum well systems, the heavy hole exciton simultaneously exhibits both Brillouin and Van Vleck paramagnetic behavior depending upon the spin state probed. Evidence is also observed for the formation of an electron spin population inversion which arises from the spin splitting of the electron states and the simultaneous spin separation of the holes into different layers of the structure.

Optical Properties of Diluted Magnetic Semiconductor Quantum Structures

Conduction and valence band edges in diluted magnetic semiconductors undergo enormous Zeeman shifts when a magnetic field is applied, reach- ing values in excess of 100 meV at low temperatures. These Zeeman shifts can thus have profound consequences on the properties of DMS/non-DMS heterostructures, since they provide the opportunity of tuning their band alignment by varying an applied field. This leads to a variety of entirely new effects, and also provides a powerful tool for probing the effect of band alignment on the properties of semiconductor heterostructures in general. We illustrate this with several examples. First, using the ZnSe/ZnMnSe sys- tem, we discuss the creation of a spatial spin modulation (spin superlattice). Second, we use the drastic differences in the Zeeman splitting occurring in different layers of a DMS/non-DMS superlattice in order to pinpoint the localization in space of the specific electronic states involved in optical tran- sitions. We illustrate this by investigating the localization of above-barrier states in type-I ZnSe/ZnMnSe superlattices, and of spatially-direct (type-I) excitons which occur in ZnTe/CdMnSe and ZnMnTe/CdSe type-II super- lattices. Finally, we exploit Zeeman tuning to demonstrate the confinement effects which occur in a single quantum barrier.

Coexistence of Brillouin and Van Vleck spin exchange in Zn1−xMnxSe/Zn1−yFeySe spin superlattice structures

We have studied a new type of semiconductor quantum well system in which the carrier-ion exchange interactions are determined by the spin of the carrier. The samples consist of alternating layers of Zn1−xMnxSe and Zn1−yFeySe grown by molecular-beam epitaxy (MBE). At zero field, the carriers initially interact randomly with both transition metal species. When a magnetic field is applied, the excitonic wave functions are increasingly localized in one or the other of the magnetic layers according to their spin state as the competing spin exchange interactions define the confining potential. The spin components of the heavy hole exciton are subsequently dominated by different exchange interactions as revealed by their temperature and field dependence: the behavior of the spin-down component (−3/2,−1/2) is described by exchange interactions of the carriers with the Mn2+ ions and exhibits Brillouin paramagnetic behavior, while the spin-up component (+3/2,+1/2) is dominated by interactions with Fe2+ ions and exhibits Van Vleck paramagnetism. These structures are thus characterized by an initial competition and eventual coexistence of Brillouin- and Van Vleck-like paramagnetic behavior for the exciton.

Line-shape analysis of reflectance spectra from ZnSe/Zn0.99Fe0.01Se spin superlattices

Line-shape analysis of magnetoreflectance spectra from ZnSe/Zn0.99Fe0.01Se quantum well structures was performed using a classical dielectric function model. This model explains the spectral dependence on the sample geometry, as well as providing additional evidence of spin superlattice formation in ZnSe/ZnFeSe heterostructures.

Femtosecond spin dynamics in magnetically tunable heterostructures

Ultrafast spin dynamics in diluted magnetic semiconductor heterostructures are probed to observe the effects of dimensionally and quantum confinement on spin scattering. Two novel structures, the spin superlattice and the magnetically-coupled double quantum well, exhibit unusual spin behavior. These experiments resolve energy-dependent electronic and magnetic interactions on a femtosecond time scale in quantum geometries.

Exciton energies in shallow quantum wells and spin superlattices.

We describe a general method of calculating exciton energies in shallow quantum wells. This technique is applicable to both single- and multiple-quantum-well systems, and is valid for both type-I and type-II systems, even under circumstances where the valence- and/or conduction-band offsets may be small compared to the exciton binding energy. Quantitative predictions of excitonic energies and relative intensities are made and compared with experimental data on a number of different shallow-well diluted magnetic semiconductor quantum well and superlattice systems. Based on these comparisons, it is shown that the model indeed provides a detailed description and analysis of the type-I--type-II transition and the behavior of excitons in a spin superlattice. In addition, the model also predicts and describes certain additional phenomena, such as metastable ``above-gap'' excitons which should exist in certain type-II systems. Also, reentrant type-I--type-II--type-I transitions are predicted under some circumstances.

Magneto-optical properties of Mn-based II–VI semimagnetic superlattices

The magneto-optical properties of semimagnetic superlattices based on Mn containing II–VI alloys are reviewed. This covers mainly the two families of materials CdTe/Cd 1− x Mn x Te and ZnSe/Zn 1− x Mn x Se. Inter-band measurements have been made of reflectivity, photoluminescence (PL) and PL excitation (PLE) spectroscopy in both systems, and a subject of particular interest has been the observation of a type I to type II superlattice transition induced by magnetic field and the large sp-d exchange interaction with the Mn d-shell. Conflicting reports of the observation of this effect have been made in the CdTe/Cd 1− x Mn x Te system due to the large change in the excitonic binding energy which also occurs at the transition. By contrast very clear evidence is seen in the Zn family of compounds, and this has been developed to include the observation of a spin superlattice in which the quantum wells are in opposite materials for opposite spin orientations of the holes. Simple envelope function calculations describe the electronic states well for long-period structures butt a detailed description of short-period structures, and cases where a magnetic field is applied in the plane of the layer, are not yet well understood. Both magnetic polaron and LO phonon polaron effects have been observed in the CdTe/Cd 1− x Mn x Te systems due to the strong magnetic coupling and the large polar coupling in these materials.

Femtosecond scattering dynamics in magnetic semiconductor spin superlattices.

Spin-dependent dynamics of polarized carriers in a series of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$SeZnSe superlattice structures are directly observed by time-resolved upconversion photoluminescence spectroscopy at low temperatures. Polarization measurements in small magnetic fields reveal spin-flip scattering of carriers confined within a magnetic quantum well. A marked change in the dynamics is observed with increasing field and subsequent spin superlattice formation. Exciton lifetimes and spin relaxation are seen to be strongly dependent on both the energy and location of spin states in the heterostructure.

Electron and hole spin lifetimes in Zn1−xMnxSe and Zn1−yFeySe based heterostructures

We have studied carrier spin relaxation in diluted magnetic semiconductor (DMS) epilayer and spin superlattice samples using polarization and magnetic field dependent photoluminescence. The giant Zeeman splitting of the DMS materials permits unambiguous identification of electron and hole excitonic spin relaxation times. We find that the heavy hole spin lifetime τhs is comparable to the radiative recombination time τr, and substantially longer than the electron spin lifetime τes, with τhs/τr≊4 and τes/ τr≊0.06. We attribute these long spin lifetimes to the strain induced splitting of the bulk degenerate light and heavy hole bands, which inhibits mixing of the hole spin states and subsequent heavy hole spin flip relaxation processes. In the regime of weak confinement, the spin relaxation times are independent of the degree of quantum confinement, the number of interfaces, the magnetic ion concentration, and the magnetic species. We conclude that spin flip via exchange scattering between the carriers and magnetic ions is not the dominant excitonic spin relaxation mechanism.

Spin lifetimes in diluted magnetic semiconductor spin superlattices

The growth of spin superlattice structures in which spin-up and spin-down carriers occupy alternating layers in the structure has recently been reported. We report here measurements of electron and hole spin lifetimes and spin relaxation processes. In dc photoluminescence spectra, both the higher-energy (+1/2,+3/2) and lower-energy (−1/2,−3/2) heavy-hole exciton interband transitions are observed, even though it would be energetically favorable for the spin-up carriers to first relax to the spin-down state before radiative recombination. From the field dependence of the intensity ratios of these components and a rate equation model, we determine the heavy hole τhs and electron τes spin lifetimes relative to the radiative lifetime τr, with τhs/τr≊4.5 and τes/τr≊0.08, so that the heavy-hole spin lifetime is ≊50 times longer than that of the electron. This is attributed to the strain-induced splitting of the heavy- and light-hole bands, which prevents fast spin relaxation of the holes.

Long hole spin relaxation times in diluted magnetic semiconductor heterostructures

We report the observation of long spin lifetimes for heavy holes in (Zn,Mn)Se and (Zn,Fe)Se based heterostructures. These long spin relaxation times are observed in both simple strained epilayers, as well as in complex spin superlattice structures in which the spin up and spin down carriers occupy alternating layers of the superlattice. In photoluminescence spectra, both the higher energy (+1/2, +3/2) and lower energy (−1/2, −3/2)heavy hole exciton interband transitions are observed, even though it would be energetically favorable for the spin up carriers to first relax to the spin down state before radiative recombination. From the magnetic field dependence of the intensity ratio of these components and a rate equation model, we find that the heavy hole spin lifetime τhs is substantially longer than the electron spin lifetime τes, with τhs /τr ≈ 4 and τes /τr ≈ 0.06, where τr is the radiative recombination time. This is attributed to the strain induced splitting of the heavy and light hole bands, which inhibits mixing of the hole spin states and subsequent dipole-allowed transitions producing fast spin relaxation.

Zeeman-Tuning of Heterostructures Consisting of Semimagnetic and Non-Magnetic Semiconductors

Conduction and valence band edges in semimagnetic semiconductors (e.g., Zn1-xMnxSe) undergo enormous Zeeman shifts when a magnetic field is applied, reaching values in excess of 100 meV at low temperatures. We discuss the consequences of this effect for heterostructures consisting of magnetic and nonmagnetic semiconductors, where such Zeeman shifts provide the ability of tuning band alignments at the interfaces of the constituent materials. As illustrative examples of such Zeeman tuning, we discuss spin superlattices, magnetic-field-induced type I→type II conversion, mapping of probability distributions in semiconductors, and opportunities for spin lifetime determination.

Exciton oscillator strength in magnetic-field-induced spin superlattices CdTe/(Cd,Mn)Te.

Resonance-reflectivity data on CdTe/(Cd,Mn)Te quantum wells and superlattices are analyzed to obtain the exciton longitudinal-transverse splitting or oscillator strength as a function of the layer widths and/or the band offsets altered by externally applied magnetic fields. A nonlocal-dielectric-response theory for single quantum wells and the concept of an optically homogeneous dielectric medium formulated for periodic structures are developed. Strong effects of the magnetic-field-induced type-I--type-II transition and the spin-superlattice formation for holes on the exciton oscillator strength are found to be in agreement with our calculations.