Universal magnetic proximity effect in ferromagnet-semiconductor quantum well hybrid structures.

Hybrid ferromagnet-semiconductor systems possess new outstanding properties, which emerge when bringing magnetic and semiconductor materials into contact. In such structures, the long-range magnetic proximity effect couples the spin systems of the ferromagnet and semiconductor on distances exceeding the carrier wave function overlap. The effect is due to the effective p-d exchange interaction of acceptor-bound holes in the quantum well with d-electrons of the ferromagnet. This indirect interaction is established via the phononic Stark effect mediated by the chiral phonons. Here, we demonstrate that the long-range magnetic proximity effect is universal and observed in hybrid structures with diverse magnetic components and potential barriers of various thicknesses and compositions. We study hybrid structures consisting of a semimetal (magnetite Fe3O4) or dielectric (spinel NiFe2O4) ferromagnet and a CdTe quantum well separated by a nonmagnetic (Cd,Mg)Te barrier. The proximity effect is manifested in the circular polarization of the photoluminescence corresponding to the recombination of photoexcited electrons with holes bound to shallow acceptors in the quantum well induced by magnetite or spinel itself, in contrast to interface ferromagnet in case of metal-based hybrid systems. A nontrivial dynamics of the proximity effect is observed in the studied structures due to recombination-induced dynamic polarization of electrons in the quantum well. It enables the determination of the exchange constant Δexch ≈ 70 µeV in a magnetite-based structure. The universal origin of the long-range exchange interaction along with the possibility of its electrical control offers prospects for the development of low-voltage spintronic devices compatible with existing solid-state electronics.

Low voltage control of exchange coupling in a ferromagnet-semiconductor quantum well hybrid structure.

Voltage control of ferromagnetism on the nanometer scale is highly appealing for the development of novel electronic devices with low power consumption, high operation speed, reliable reversibility and compatibility with semiconductor technology. Hybrid structures based on the assembly of ferromagnetic and semiconducting building blocks are expected to show magnetic order as a ferromagnet and to be electrically tunable as a semiconductor. Here, we demonstrate the electrical control of the exchange coupling in a hybrid consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well, separated by a thin non-magnetic (Cd,Mg)Te barrier. The electric field controls the phononic ac Stark effect-the indirect exchange mechanism that is mediated by elliptically polarized phonons emitted from the ferromagnet. The effective magnetic field of the exchange interaction reaches up to 2.5 Tesla and can be turned on and off by application of 1V bias across the heterostructure.

Polarimetry of photon echo on charged and neutral excitons in semiconductor quantum wells.

Coherent optical spectroscopy such as four-wave mixing and photon echo generation deliver rich information on the energy levels involved in optical transitions through the analysis of polarization of the coherent response. In semiconductors, it can be applied to distinguish between different exciton complexes, which is a highly non-trivial problem in optical spectroscopy. We develop a simple approach based on photon echo polarimetry, in which polar plots of the photon echo amplitude are measured as function of the angle φ between the linear polarizations of the two exciting pulses. The rosette-like polar plots reveal a distinct difference between the neutral and charged exciton (trion) optical transitions in semiconductor nanostructures. We demonstrate this experimentally by photon echo polarimetry of a CdTe/(Cd, Mg)Te quantum well. The echoes of the trion and donor-bound exciton are linearly polarized at the angle 2φ with respect to the first pulse polarization and their amplitudes are weakly dependent on φ. While on the exciton the photon echo is co-polarized with the second exciting pulse and its amplitude scales as cosφ.

Room temperature sputter deposited catalyst-free nanowires with wurtzite/zinc blende ZnO superstructure and their application in electromechanical nanogenerators on polymer and paper substrates.

Catalyst-free growth of ZnO nanowires using reactive magnetron sputtering at room temperature is reported. We discuss the growth of the nanowires using reactive magnetron sputtering as a function of argon and oxygen flow values changing at a set ratio of 10:2. A transition from nanostructured Zn to nanowire ZnO growth is observed at 20 sccm Ar and 4 sccm O2. Densification and improved alignment of the nanowires is visible for increasing flow values up to 50 sccm Ar and 10 sccm O2. Nanowires exhibit stacking fault regions of zinc blende ZnO in wurtzite ZnO. The regions encompass the whole width of the nanowires and their quantum well behavior is manifested in the photoluminescence spectra. The nanowires were subsequently deposited on paper and PET substrates and electromechanical nanogenerators were fabricated. Manual pressing and depressing of the devices induced voltages of 50 µV and 2 µV for the devices on PET and paper substrates, respectively.

Growth and optical investigations of high quality individual CdTe/(Cd,Mg)Te core/shell nanowires.

CdTe nanowires with the average diameter of only 40 nm coated with (Cd,Mg)Te shells are grown using Au-catalyzed vapor-liquid-solid growth mechanism in a system for molecular beam epitaxy. High optical quality of individual nanowires is revealed by means of low temperature cathodoluminescence and micro-luminescence. It is found that, the optical emission spectrum consists mostly of the near band edge emission without any significant contribution of defect related luminescence. Moreover, the importance of surface passivation with (Cd,Mg)Te coating shells is demonstrated.

Spin-Orbit Twisted Spin Waves: Group Velocity Control.

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 and for the realization of lenses for spin waves.

Coexistence of optically active radial and axial CdTe insertions in single ZnTe nanowire.

We report on the growth, cathodoluminescence and micro-photoluminescence of individual radial and axial CdTe insertions in ZnTe nanowires. In particular, the cathodoluminescence technique is used to determine the position of each emitting object inside the nanowire. It is demonstrated that depending on the CdTe deposition temperature, one can obtain an emission either from axial CdTe insertions only, or from both, radial and axial heterostructures, simultaneously. At 350 °C CdTe grows only axially, whereas at 310 °C and 290 °C, there is also significant deposition on the nanowire sidewalls resulting in radial core/shell heterostructures. The presence of Cd atoms on the sidewalls is confirmed by energy dispersive X-ray spectroscopy. Micro-photoluminescence study reveals a strong linear polarization of the emission from both types of heterostructures in the direction along the nanowire axis.

Genotype variation in bark texture drives lichen community assembly across multiple environments.

A major goal of community genetics is to understand the influence of genetic variation within a species on ecological communities. Although well-documented for some organisms, additional research is necessary to understand the relative and interactive effects of genotype and environment on biodiversity, identify mechanisms through which tree genotype influences communities, and connect this emerging field with existing themes in ecology. We employ an underutilized but ecologically significant group of organisms, epiphytic bark lichens, to understand the relative importance of Populus angustifolia (narrowleaf cottonwood) genotype and environment on associated organisms within the context of community assembly and host ontogeny. Several key findings emerged. (1) In a single common garden, tree genotype explained 18-33% and 51% of the variation in lichen community variables and rough bark cover, respectively. (2) Across replicated common gardens, tree genotype affected lichen species richness, total lichen cover, lichen species composition, and rough bark cover, whereas environment only influenced composition and there were no genotype by environment interactions. (3) Rough bark cover was positively correlated with total lichen cover and richness, and was associated with a shift in species composition; these patterns occurred with variation in rough bark cover among tree genotypes of the same age in common gardens and with increasing rough bark cover along a -40 year tree age gradient in a natural riparian stand. (4) In a common garden, 20-year-old parent trees with smooth bark had poorly developed lichen communities, similar to their 10-year-old ramets (root suckers) growing in close proximity, while parent trees with high rough bark cover had more developed communities than their ramets. These findings indicate that epiphytic lichens are influenced by host genotype, an effect that is robust across divergent environments. Furthermore, the response to tree genotype is likely the result of genetic variation in the timing of the ontogenetic shift from smooth to rough bark allowing communities on some genotypes to assemble faster than those on other genotypes. Organisms outside the typical sphere of community genetics, such as lichens, can help address critical issues and connect plant genotype effects to long-established streams of biological research, such as ontogeny and community assembly.

Impact of matrix metalloproteinase-9 overexpression on synaptic excitatory transmission and its plasticity in rat CA3-CA1 hippocampal pathway.

Metalloproteinases (MMPs) have been shown to play a crucial role in synaptic plasticity and cognitive processes. We recently reported that in the mossy fiber - CA3 hippocampal pathway, LTP maintenance required fine-tuned MMP-9 activity, as both MMP-9 excess and absence impaired LTP. Here we used acute brain slices from transgenic (TG) rats overexpressing MMP-9 to investigate the impact of excessive MMP-9 activity on the excitatory synaptic transmission in the CA3-CA1 projection. Using field potential recordings, we have demonstrated that MMP-9 overexpression increased the strength of basal synaptic transmission but had no effect on the short-term plasticity in comparison to the wild-type (WT) group. In attempt to shed light on mechanisms underlying this observation, miniature excitatory postsynaptic potentials (mEPSCs) were recorded from pyramidal CA1 neurons. We found that mEPSCs in the TG group had a significantly slower decaying phase than in WT but amplitudes and frequencies were similar. The lack of differences in mEPSC frequency and short-term plasticity between TG and WT groups suggests that MMP-9 overexpression effect on fEPSPs was mainly postsynaptic. Additionally, we have found that excess of MMP-9 in TG rats was associated with impaired late-phase of LTP in the considered pathway. It seems thus that augmented synaptic strength in TG rats occurred in expense of impaired long-term plasticity induced by tetanization. In conclusion, overexpression of MMP-9 leads to increase in the strength of basal excitatory synaptic transmission and impairs of LTP maintenance phase in the CA3-CA1 pathway in vitro.

Nanoscale morphology of multilayer PbTe/CdTe heterostructures and its effect on photoluminescence properties.

We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures crucially depends on the growth temperature. The two-dimensional layered, superlattice-like character of the structures remains preserved only when grown at low substrate temperatures, such as 230 °C. The samples grown at the slightly elevated temperature of 270 °C undergo a morphological transformation to structures consisting of CdTe and PbTe pillars and columns oriented perpendicular to the substrate. Although the pillar-like objects are of various shapes and dimensions these structures exhibit exceptionally strong photoluminescence in the near infrared spectral region. At the higher growth temperature of 310 °C, PbTe and CdTe separate completely forming thick layers oriented longitudinally to the substrate plane. The observed topological transformations are driven by thermally activated atomic diffusion in the solid state phase. The solid state phase remains fully coherent during the processes. The observed topological transitions leading to the material separation in PbTe/CdTe system could be regarded as an analog of spinodal decomposition of an immiscible solid state solution and thus they can be qualitatively described by the Cahn-Hillard model as proposed by Groiss et al (2014 APL Mater. 2 012105).

Coherent coupling of excitons and trions in a photoexcited CdTe/CdMgTe quantum well.

We present zero-, one-, and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the coherent nonlinear optical response. Distinct peaks in the spectra are manifestations of exciton-exciton and exciton-trion coherent coupling. Excellent agreement using density matrix calculations highlights the essential role of many-body effects on the coupling. Strong exciton-trion coherent interactions open up the possibility for novel conditional control schemes in coherent optoelectronics.

Activation of an intense near band edge emission from ZnTe/ZnMgTe core/shell nanowires grown on silicon.

The absence of luminescence in the near band edge energy region of Te-anion based semiconductor nanowires grown by gold catalyst assisted molecular beam epitaxy has strongly limited their applications in the field of photonics. In this paper, an enhancement of the near band edge emission intensity from ZnTe/ZnMgTe core/shell nanowires grown on Si substrates is reported. A special role of the use of Si substrates instead of GaAs substrates is emphasized, which results in an increase of the near band edge emission intensity by at least one order of magnitude accompanied by a simultaneous reduction of the defect related luminescence. A possible explanation of this effect relies on the presence of Ga-related deep level defects in structures grown on GaAs substrates, which are absent when Si substrates are used. Monochromatic mapping of the cathodoluminescence clearly confirms that the observed emission originates, indeed, from the ZnTe/ZnMgTe core/shell nanowires, whereas individual objects are studied by means of microphotoluminescence.

Terahertz radiation from magnetic excitations in diluted magnetic semiconductors.

We probed, in the time domain, the THz electromagnetic radiation originating from spins in CdMnTe diluted magnetic semiconductor quantum wells containing high-mobility electron gas. Taking advantage of the efficient Raman generation process, the spin precession was induced by low power near-infrared pulses. We provide a full theoretical first-principles description of spin-wave generation, spin precession, and of emission of THz radiation. Our results open new perspectives for improved control of the direct coupling between spin and an electromagnetic field, e.g., by using semiconductor technology to insert the THz sources in cavities or pillars.

Magnetic-field control of photon echo from the electron-trion system in a CdTe quantum well: shuffling coherence between optically accessible and inaccessible states.

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.

Spin-transistor action via tunable Landau-Zener transitions.

Spin-transistor designs relying on spin-orbit interaction suffer from low signal levels resulting from low spin-injection efficiency and fast spin decay. Here, we present an alternative approach in which spin information is protected by propagating this information adiabatically. We demonstrate the validity of our approach in a cadmium manganese telluride diluted magnetic semiconductor quantum well structure in which efficient spin transport is observed over device distances of 50 micrometers. The device is turned "off" by introducing diabatic Landau-Zener transitions that lead to a backscattering of spins, which are controlled by a combination of a helical and a homogeneous magnetic field. In contrast to other spin-transistor designs, we find that our concept is tolerant against disorder.

GABAergic and glutamatergic currents in hippocampal slices and neuronal cultures show profound differences: a clue to a potent homeostatic modulation.

Acute hippocampal slices and primary neuronal cultures are often used with a tacit assumption that basic characteristics of the two models closely resemble each other. The use of the cell cultures, however, may raise controversies because of non-physiological conditions resulting from e.g. glial cells deficit, random neuronal sprouting, lack of specificity in the synaptic connections, impaired homeostasis, etc. Importantly, alteration in neuronal environment, especially when occurring over a prolonged period of time, may give rise to a profound homeostatic modulation. In the present study we have compared the properties of GABAergic and glutamatergic (non-NMDA) currents in pyramidal neurons from hippocampal slices and neuronal cell culture. We show that, most strikingly, amplitude ratio of currents elicited by ultrafast applications of saturating GABA and glutamate was nearly one order of magnitude larger in cultured neurons than that in slices. Miniature IPSCs and EPSCs also showed substantial differences between these two models. In particular, mEPSC amplitudes were larger and more frequent in cultured neurons but their time duration was longer in slices. Miniature IPSCs did not show differences in amplitude when comparing slices and cultures but their time duration was faster and occurrence more frequent in slices. In conclusion, we provide evidence that expression pattern of GABA(A) and glutamate receptors as well as synaptic current properties in the neuronal cell culture show profound differences with respect to that in the physiological conditions.

TEM characterization of MBE grown CdTe/ZnTe axial nanowires.

CdTe/ZnTe axial nanowires were successfully fabricated by molecular beam epitaxy with the use of Au nano-catalysts and vapour-liquid-solid growth mechanism. Nanowires had zinc-blende structure with numerous stacking faults in the bottom ZnTe part and near perfect crystalline structure in the top CdTe part. Energy dispersive X-ray spectroscopy (EDXS) and lattice fringe spacing analysis revealed nonabrupt nature of hetero-interface, whose width was estimated to be 50-70 nm for the nanowires having a diameter in the range from 40 to 50 nm.

Coherence-mediated laser control of exciton and trion spins in CdTe/CdMgTe quantum wells studied by the magneto-optical Kerr effect.

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.

ZnTe-ZnO core-shell radial heterostructures grown by the combination of molecular beam epitaxy and atomic layer deposition.

ZnTe-ZnO core-shell radial heterostructures were grown using a new method of combining molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Zinc telluride nanowires (core) were grown on a GaAs substrate using gold catalyzed vapor-liquid-solid mechanism. An atomic layer deposition technique using diethyl zinc and deionized water as precursors was applied for zinc oxide shell formation. The core-shell ZnTe-ZnO heterostructures thus obtained were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and photoluminescence measurements.

Origin of magnetic circular dichroism in GaMnAs: giant zeeman splitting versus spin dependent density of states.

We present a unified interpretation of experimentally observed magnetic circular dichroism (MCD) in the ferromagnetic semiconductor (Ga,Mn)As, based on theoretical arguments, which demonstrates that MCD in this material arises primarily from a difference in the density of spin-up and spin-down states in the valence band brought about by the presence of the Mn impurity band, rather than being primarily due to the Zeeman splitting of electronic states.