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Achieving metrological precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperatures of the order of 20 mK, while the Curie temperature in the involved material is as high as 20 K. The reason for this discrepancy remains one of the biggest open questions surrounding the effect, and is the focus of this article. Here we show, through a careful analysis of the non-local voltages on a multi-terminal Corbino geometry, that the chiral edge channels continue to exist without applied magnetic field up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator, and that thermally activated bulk conductance is responsible for this quantization breakdown. Our results offer important insights on the nature of the topological protection of these edge channels, provide an encouraging sign for potential applications, and establish the multi-terminal Corbino geometry as a powerful tool for the study of edge channel transport in topological materials., The quantum anomalous Hall effect has so far been limited to temperature of the order of 20 mK. Here, Fijalkowski et al. report the existence of chiral edge channels up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator with a multi-terminal Corbino geometry.

Spin-dependent transport phenomena due to relativistic spin-orbit coupling and broken space-inversion symmetry are often difficult to interpret microscopically, in particular when occurring at surfaces or interfaces. Here we present a theoretical and experimental study of spin-orbit torque and unidirectional magnetoresistance in a model room-temperature ferromagnet NiMnSb with inversion asymmetry in the bulk of this half-heusler crystal. Besides the angular dependence on magnetization, the competition of Rashba and Dresselhaus-like spin-orbit couplings results in the dependence of these effects on the crystal direction of the applied electric field. The phenomenology that we observe highlights potential inapplicability of commonly considered approaches for interpreting experiments. We point out that, in general, there is no direct link between the current-induced non-equilibrium spin polarization inferred from the measured spin-orbit torque and the unidirectional magnetiresistance. We also emphasize that the unidirectional magnetoresistance has not only longitudinal but also transverse components in the electric field -- current indices which complicates its separation from the thermoelectric contributions to the detected signals in common experimental techniques. We use the theoretical results to analyze our measurements of the on-resonance and off-resonance mixing signals in microbar devices fabricated from an epitaxial NiMnSb film along different crystal directions. Based on the analysis we extract an experimental estimate of the unidirectional magnetoresistance in NiMnSb., Comment: 29 pages, 13 figures

Optically induced spin transfer (OISTR) is a pathway to control magnetization dynamics in complex materials on femto- to attosecond timescales. The direct interaction of the laser field with the material creates transient nonequilibrium states, which can exhibit an efficient spin transfer between different magnetic subsystems. How far this spin manipulation via OISTR is a general phenomenon or restricted to a subset of materials with specific properties is an open experimental and theoretical question. Using time-resolved magneto-optical Kerr measurements and time-dependent density functional theory we investigate OISTR in Heusler compounds. We show that the half-Heusler materials NiMnSb and CoMnSb exhibit strong signatures of OISTR, whereas this is less pronounced in the full-Heusler compounds Co2MnSi, Co2FeSi, and Co2FeAl in agreement with ab initio calculations. Our work opens up a systematic path for coherent manipulation of spin dynamics by direct light-matter interaction.

The presence of twins, both tilting and twisting of domains and resulting strain at domain boundaries in magnetically doped topological insulators can significantly modify their band topology and carrier density, and hence their electronic properties. In this paper, we report on a detailed microstructural characterization of Cr-doped (Bi,Se)2Te3 layers grown by molecular beam epitaxy on Si(111). We provide detailed microscopical descriptions of defects present in the films (twins, mosaicity tilt, mosaicity twist), and suggest ways to control their structural quality.

In this letter we report on proximity superconductivity induced in CdTe-HgTe core-shell nanowires, a quasi-one-dimensional heterostructure of the topological insulator HgTe. We demonstrate a Josephson supercurrent in our nanowires contacted with superconducting Al leads. The observation of a sizable $I_c R_n$ product, a positive excess current and multiple Andreev reflections up to fourth order further indicate a high interface quality of the junctions., Accepted for publication in Nano Letters

The half-metal ferromagnet NiMnSb, with its high spin polarization, low magnetic damping and tunable magnetic anisotropy, is a promising material for applications in spin torque devices. We develop the epitaxial growth of NiMnSb/ZnTe/NiMnSb heterostructures, aiming towards the realization of an all-NiMnSb based magnetic tunneling junction (MTJ). Layers are grown in situ by Molecular Beam Epitaxy (MBE) and Atomic Layer Epitaxy (ALE) methods. By tuning Mn content, the magnetic anisotropy of each of the two NiMnSb layers is adjusted in order to achieve mutually orthogonal uniaxial anisotropies. SQUID measurements of the magnetization along orthogonal crystal directions [110] and [ 1 1 ¯ 0 ] confirm that the two layers have mutually orthogonal anisotropy. High Resolution X-Ray Diffraction measurements and simulations confirm the nominal layer stack and demonstrate the high crystalline quality of the individual layers. Such layer stacks provide a potential basis for TMR-based spin-torque devices such as spin-torque oscillators.

The material system (Hg,Cd)Te is a prototype system for topological insulators and Dirac semi-metals. Molecular beam epitaxy is the preferred method for the growth of (Hg,Cd)Te epilayer heterostructures and CdTe-HgTe core-shell nanowires. Unfortunately, the high crystalline quality desired for charge transport investigations is only achieved for a very narrow window of the growth temperature. Additionally, the low growth temperature requires a special thermometry. Here, we demonstrate the improvement of the growth of (Hg,Cd)Te epilayer heterostructures and CdTe nanowires by the use of band edge thermometry. We show that even for narrow-gap materials band edge thermometry provides a suitable method to control the temperature directly at the growth front with a high precision and reproducibility, in contrast to a conventionally used thermocouple.

SnTe belongs to the recently discovered class of topological crystalline insulators. Here we study the formation of line defects which break crystalline symmetry by strain in thin SnTe films. Strained SnTe(111) films are grown by molecular beam epitaxy on lattice- and thermal expansion coefficient-mismatched CdTe. To analyze the structural properties of the SnTe films we applied {\em in-situ} reflection high energy electron diffraction, x-ray reflectometry, high resolution x-ray diffraction, reciprocal space mapping, and scanning tunneling microscopy. This comprehensive analytical approach reveals a twinned structure, tensile strain, bilayer surface steps and dislocation line defects forming a highly ordered dislocation network for thick films with local strains up to 31\% breaking the translational crystal symmetry., 15 pages, 4 figures

(Bi,Sb)${}_{2}$Te${}_{3}$ layers doped with Vanadium are ferromagnetic topological insulators exhibiting the quantum anomalous Hall effect (QAHE) at low temperatures. The Hall resistivity is quantized to h/e${}^{2}$ without an external magnetic field. The authors succeeded in reproducible epitaxial growth of thin layers, with an optimized Sb content in a narrow range around 80% showing the QAHE. Such layers grown on Si or InP substrates and with or without a protecting Te cap show quantization. The QAHE persists independently of the interfaces between cap, layer and substrate, limited crystalline quality, and layer degradation, confirming the robustness of the quantum anomalous Hall effect.

The ferromagnetic topological insulator $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_{2}{\mathrm{Te}}_{3}$ has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_{2}{\mathrm{Te}}_{3}$ thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V $3d$ impurity band was extracted and exhibits a high density of states near the Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.

Magnetoresistance studies are one of the simplest yet powerful tools to investigate the electronic properties of solids. The authors here report on high-field angle-dependent magnetotransport measurements on epitaxial thin films of Bi${}_{2}$Se${}_{3}$, a three-dimensional topological insulator. At low temperature, they observe quantum oscillations that demonstrate the simultaneous presence of bulk and surface carriers. Their key observation is a strong anisotropy in the magnetoresistance in Bi${}_{2}$Se${}_{3}$ that depends on the orientation of the applied current (electric field) with respect to the applied magnetic field. When the magnetic field is applied parallel to the electric field, they find a strong negative longitudinal magnetoresistance (NLMR) that, quite strikingly, persists even up to room temperature. With this finding, the authors demonstrate that the observation of a NLMR is not unique to topological semimetals, such as Weyl semimetals, and therefore cannot by itself be taken as a diagnostic tool and as conclusive evidence for the existence of Weyl fermions. These results could pave the way towards a general understanding of the emergence of the axial anomaly that is suggested to be a universal phenomenon for generic three-dimensional metals in the presence of parallel electric and magnetic fields.

The electrodynamics of topological insulators (TIs) is described by modified Maxwell's equations, which contain additional terms that couple an electric field to a magnetization and a magnetic field to a polarization of the medium, such that the coupling coefficient is quantized in odd multiples of $e^2 / 2 h c $ per surface. Here, we report on the observation of this so-called topological magnetoelectric (TME) effect. We use monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semi-transparent gate to selectively address surface states. In high external magnetic fields, we observe a universal Faraday rotation angle equal to the fine structure constant $\alpha = e^2 / \hbar c$ when a linearly polarized THz radiation of a certain frequency passes through the two surfaces of a strained HgTe 3D TI. These experiments give insight into axion electrodynamics of TIs and may potentially be used for a metrological definition of the three basic physical constants., Comment: Submitted 13 March, 2016; 7 pages, 4 figures

We use time-resolved photoluminescence (TRPL) spectroscopy to unequivocally clarify the microscopic origin of the nanosecond free exciton photoluminescence rise in GaAs at low temperatures. In crucial distinction from previous work, we examine the TRPL of the GaAs free exciton second LO-phonon replica. This enables us to simultaneously monitor the unambiguous time evolution of the total exciton population and the cooling dynamics of the initially hot free exciton ensemble. We demonstrate by a model based on the Saha equation and the experimentally determined cooling behavior that the long-debated slow photoluminescence rise is caused by time-dependent shifts in the thermodynamic quasiequilibrium between free excitons and the uncorrelated electron-hole plasma.

The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization $M_S$, Gilbert damping $\alpha$, and exchange stiffness $A$. $M_S$ and $A$ are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for $M_S$ and $\alpha$. The calculated $A$ shows the same trend as the experimental data, but has a larger magnitude. Additionally to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices., Comment: 3 figures

The GaAs doped with donors manifests long times of spin relaxation, while in the case of acceptors (or magnetic impurities) spin relaxation rate increases markedly, in accordance with theoretical predictions. From the practical point of view, this situation is unfavorable, since the devices based on spin degrees of freedom require long times of the spin memory. Therefore semiconductors such as p-GaAs were not considered as promising materials for spintronics. In the present work this conclusion is refuted by means of investigation of the spin dynamics of electrons in epitaxial layers of gallium arsenide doped with Mn impurities. In spite of the expectations, we have discovered the suppression of the spin relaxation of electrons in GaAs:Mn by two orders of magnitude. This effect is a consequence of compensation of the hole and manganese effective magnetic fields due to the antiferromagnetic interaction. The analogous results obtained for the case of GaAs quantum well doped with Mn [R. C. Myers, et al., Nature Materials 7, 203 (2008)] were interpreted as the result of the spin precession of magnetic acceptors rather than electrons. Through separate measurements of g-factor by means of time resolved spectroscopy it has been proved that long times of spin relaxation in p-GaAs:Mn relate to electrons and not to magnetic acceptors. The oscillation frequency of the angle of Kerr rotation depends linearly on the magnetic field and complies with g=0.46±0.02, i.e. the electronic g-factor.

The prototypical ferromagnetic semi-conductor (Ga,MnAs) is interesting for spintronics devices, largely because of its rich magnetic and transport anisotropy. The lack of local anisotropy control has until recently limited device options to structures where all elements have the anisotropy characteristic of the parent layer. We describe here a novel approach to anisotropy control using lithographically engineered strain relaxation. By patterning the layer to create local strain relaxation, we allow for a controlled deformation of the crystal. The strong spin orbit coupling then leads to a new anisotropy term that can be independently tuned for each element of a compound device. We use this technique to demonstrate a novel non-volatile memory element.

We summarize experimental studies on the optical polarization properties of MBE-grown shape-asymmetric self-assembled CdSe/ZnSe quantum dots (QDs). The zero magnetic field fine structure splitting of excitonic transitions in structurally anisotropic QDs gives rise to an effective linear-to-circular and circular-to-linear optical polarization conversion upon resonant excitation of the QD ensemble. The effect is qualitatively modelled in terms of a pseudospin formalism. Furthermore, we show how an in-plane magnetic field leads to a complex rotation behavior of the axis of the optical polarization. Our analysis based on the exciton pseudospin Hamiltonian unambiguously displays that these effects are induced by isotropic and anisotropic contributions to the heavy hole Zeeman term. Likewise we show that the latter mechanism can be used to tune the optical respone of shape-anisotropic QDs such that the optical polarization properties are equal to QDs of perfect D2d symmetry. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Post-growth thermal annealing, (TA) in combination with mesa-fabrication technologies, was applied to tune the shape and material composition of ZnCdSe quantum dots (QDs). The QDs were studied by means of photoluminescence spectroscopy. An enhancement of QD homogeneity due to TA is demonstrated by substantial narrowing of the QD emission, observed from large QD ensembles. Energy shifts of QD luminescence, exceeding 100 meV after thermal annealing, were observed. Tracing the evolution of single exciton lines, the activation energy of an enhanced diffusion process in small mesas was determined. Distinct changes of quantum dot symmetries were identified through a reduction of the exciton fine structure splitting. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the degree of circular polarization (ρ) in the emission of semimagnetic CdMnSe/ZnSe quantum dots (QDs) with Mn contents of nominally 0, 1 and 2 %. Circularly polarized excitation was used to control the polarization of the excited carriers. The g factors were determined from the dependence of ρ on the magnetic field strength. We demonstrate that in QDs with 1 % Mn the exciton g factor is vanishingly small. We also present measurements on the excitation power dependent changes of the polarization. A direct heating mechanism is identified as origin of the drastic enhancement of the g factor by ramping up the excitation power. For high laser powers the exciton g factor increases by a factor of 30. In addition, by comparing the luminescence polarization of QDs with 2 % Mn and without Mn a sign reversal of the g factor was observed.

We report on low-temperature spatially resolved photoluminescence (SRPL) experiments to study the diffusion of free excitons in a $1.5\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-thick layer of high-purity epitaxial GaAs. Extending previous SRPL experiments, we analyze the stationary diffusion profiles detected on the second LO-phonon replica of the free exciton. This allows us to circumvent the inherent interpretation ambiguities of the free-exciton zero-phonon line. Moreover, a spatially resolved line shape analysis of the $(FX)\ensuremath{-}2\ensuremath{\hbar}{\mathrm{\ensuremath{\Omega}}}_{\text{LO}}$ replica provides direct experimental access to the pump-induced exciton temperature profile. We demonstrate that only resonant optical excitation prevents the buildup of a temperature gradient in the carrier system, which otherwise severely distorts the stationary and time-resolved free-exciton diffusion profiles.

We report on the observation of macroscopic free exciton photoluminescence (PL) rings that appear in spatially resolved PL images obtained on a high purity GaAs sample. We demonstrate that a spatial temperature gradient in the photocarrier system, which is due to nonresonant optical excitation, locally modifies the population balance between free excitons and the uncorrelated electron-hole plasma described by the Saha equation and accounts for the experimentally observed nontrivial PL profiles. The exciton ring formation is a particularly instructive manifestation of the spatially dependent thermodynamics of a partially ionized exciton gas in a bulk semiconductor.

Materials that crystalize in diamond-related lattices, with Si and GaAs as their prime examples, are at the foundation of modern electronics. Simultaneously, the two atomic sites in the unit cell of these crystals form inversion partners which gives rise to relativistic non-equilibrium spin phenomena highly relevant for magnetic memories and other spintronic devices. When the inversion-partner sites are occupied by the same atomic species, electrical current can generate local spin polarization with the same magnitude and opposite sign on the two inversion-partner sites. In CuMnAs, which shares this specific crystal symmetry of the Si lattice, the effect led to the demonstration of electrical switching in an antiferromagnetic memory at room temperature. When the inversion-partner sites are occupied by different atoms, a non-zero global spin-polarization is generated by the applied current which can switch a ferro-magnet, as reported at low temperatures in the diluted magnetic semiconductor (Ga,Mn)As. Here we demonstrate the effect of the global current-induced spin polarization in a counterpart crystal-symmetry material NiMnSb which is a member of the broad family of magnetic Heusler compounds. It is an ordered high-temperature ferromagnetic metal whose other favorable characteristics include high spin-polarization and low damping of magnetization dynamics. Our experiments are performed on strained single-crystal epilayers of NiMnSb grown on InGaAs. By performing all-electrical ferromagnetic resonance measurements in microbars patterned along different crystal axes we detect room-temperature spin-orbit torques generated by effective fields of the Dresselhaus symmetry. The measured magnitude and symmetry of the current-induced torques are consistent with our relativistic density-functional theory calculations.

Exciton photoluminescence in a pair of strongly coupled artificial asymmetric quantum dots (QDs) has been studied in a magnetic field up to 8 T. The QD molecules have been fabricated by a selective interdiffusion technique applied to asymmetric semimagnetic CdTe/Cd(Mg,Mn)Te double quantum wells. The lateral confinement potential within the plane, induced by the diffusion, gives rise to effective zero-dimensional exciton localization. In contrast to a typically positive exciton Lande g-factor, an exciton transition in the non-magnetic QD demonstrates a nearly zero g-factor, indicating a strong electron tunnel coupling between the QDs. The strong coupling results in the formation of an inter-QDs indirect exciton, which is a ground exciton state at high magnetic field, as found in the experiment and confirmed by our calculations.

Resonant photoluminescence and photoluminescence excitation spectroscopy have been applied to CdSe/ZnSe quantumdots where separation between the quantized levels is close to the optical phonon energy. Astrong relaxation enhancement induced by the optical phonon is observed. The fine structure of the phonon replicas is resolved. It is attributed to the optical phonons with large wave vectors from the symmetrical points of the Brillouin zone. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated electroluminescence (EL) characteristics of hybrid II–VI/III–V light emitting diodes (LEDs) at low temperatures and in magnetic fields up to 10 T. Spin-polarized or unpolarized electrons are injected from n-type Cd(Mn)Se layers into a wide quantum well of InAs where they undergo radiative recombination with unpolarized holes injected via p-type InAs/AlAsSb layers. Measurements of the circular polarization properties of the emitted mid-infrared EL have been made to investigate spin-injection from the Brillouin paramagnet CdMnSe into InAs; a “non-magnetic” CdSe injector is used for comparison. To infer spin polarization from the circular polarization degree, details of the InAs band structure in a magnetic field have to be taken into account due to the large electron g-factors and, more importantly, because radiative recombination and spin relaxation of injected carriers occur on similar timescales. As a result optical and spin polarization are not simply related to each other. Experimentally, the circular polarization degrees of magnetic and non-magnetic structures are observed to be very similar. In addition, broad, multi-component EL features, as well as significant carrier heating complicate the quantitative analysis.

We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field- dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1-xMnxAs and observe different Mn constituents: ferromagnetic Mn with an n(d) > 5 lineshape and paramagnetic Mn with distinct n(d) = 5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency. (c) 2005 Elsevier B.V. All rights reserved

In recent years several experiments have successfully demonstrated spin injection into semiconductors. All of these used concepts where the sign of the spin polarization is determined by a magnetic field. In resonant tunneling diodes with semimagnetic semiconductor layers, spin selection can be obtained by changing the applied voltage. In this paper we give a brief explanation for the necessity of spin filters and present first results which demonstrate a giant spin-splitting in an all II–VI semiconductor resonant tunneling diode. Moreover, we present transport experiments performed on ferromagnetic (Ga,Mn)As nanoconstrictions that can be used to trap and investigate domain walls. When operated in the tunneling regime, these constrictions can show a magnetoresistance of up to 2000%.

In this contribution a model for the elastic relaxation of Al0.5Ga0.5As and ZnSe wire structures, respectively, is presented. The wire structures extend along [110] and were characterized by high resolution x-ray diffraction experiments. Based on Monte Carlo simulations, the wire structures, which have been simulated at atomic resolution, were relaxed to reduce the strain caused by the lattice misfit. The x-ray diffraction patterns calculated for the final structures reproduce the observed data. The structures display a two dimensional strain gradient and curved lattice planes. Hence the introduced modeling is a powerful method in particular to resolve the atomic structure of a wet chemically etched or molecular beam epitaxial grown wire by giving a microscopic picture on an atomic scale.

The MBE growth of type III HgTe/Hg 1− x Cd x Te heterostructures will be discussed. Spin–orbit (s–o) coupling is particularily large in HgTe based quantum wells; the observed value of 17 meV is at least two to three times larger than values reported in the literature for III–V heterostructures or any other system. Rashba spin–orbit splitting in n type modulation doped quantum wells (QWs) has been investigated as a function of the 2DEG density and compared with self-consistent Hartree calculations based on an 8×8 k · p model. Furthermore the presence of two periodic SdH oscillations in p type QWs with an inverted band structure has been observed and is the first direct evidence that these heterostructures are indirect semiconductors.

Dynamics of spin–lattice relaxation of the magnetic Mn-ion system is investigated for nanostructures with (Cd,Mn)Te and (Cd,Mn)Se diluted-magnetic-semiconductor (DMS) quantum dots. A nonequilibrium phonon technique with optical detection of the induced heating of the magnetic ion system was exploited. A strongly nonexponential decay of the spin dynamics is found to be specific for the DMS quantum dots. It is explained in the frame of a model suggesting that the spin dynamics in quantum dots is strongly affected by a spin diffusion into/from a DMS wetting layer, where the spin–lattice relaxation rate differs from that in the quantum dots. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.

It is demonstrated by SQUID measurements that (Ga,Mn)As films can exhibit perpendicular easy axis at low temperatures, even under compressive strain, provided that the hole concentration is sufficiently small. In such films, the easy axis assume a standard in-plane orientation when the temperature is increased towards the Curie temperature. The findings are shown to corroborate the predictions of the mean-field Zener model for strained (III,Mn)V ferromagnetic semiconductors.

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.

A review of our recent experiments on the studies of spin-lattice relaxation in semimagnetic semiconductor quantum wells (QWs) is given. Three series of magnetic nanostructures are studied: undoped (Cd,Mn)Te/(Cd,Mg)Te QWs; n-doped (Cd,Mn)Te/(Cd,Mg)Te QWs with two-dimensional electron gas; p-doped (Cd,Mn)Te/(Cd,Mg,Zn)Te QWs with two-dimensional hole gas. In the experiments, we use a technique which combines an injection of nonequilibrium phonons and an optical detection of the induced changes via the exciton photoluminescence. We observe that doping induces a significant acceleration of the spin-lattice relaxation process, which is due to spin-flip transitions between Mn ions and free carriers. We demonstrate experimentally that it is possible to tune the value of spin-lattice relaxation time by changing the intensity of the laser excitation in the doped QWs.

In this paper we review and extend our modelling of the physics governing electrical spin injection into non-magnetic semiconductors. A critical evaluation is given of several approaches to circumvent the impedance mismatch that prohibits spin injection in the diffusive transport regime, i.e. ballistic transport, the use of tunnel barriers and the physics of a Schottky contact. We conclude by discussing an experimental illustration of our modelling. In an all-electrical geometry, we utilize spin injection from a paramagnetic injector to demonstrate a novel magnetoresistance effect.

Magnetic properties of two new classes of highly p-type doped II-VI diluted magnetic semiconductors (DMS): thin layers of Be 1-x Mn x Te:N and bulk Zn 1-x Mn x Te:P are presented. Despite the fact that the samples are insulating, we observe clear indications of the low temperature ferromagnetic correlation, namely: (i) positive Curie-Weiss temperature (T CW 2 K), and (ii) open hysteresis loops at temperatures below T CW . A qualitative analysis of the results favours those models of the carrier-mediated ferromagnetism in DMS which allow for the coupling of only a part of the available spins.

The magnetic anisotropy in NiMnSb has been found to be highly sensitive to layer parameters, such as Mn concentration or thickness. Here, we report on the magnetic anisotropy of thin epitaxial single crystal quality NiMnSb films (3–40 nm) in combination with different capping layer materials, Ru, Pt, and Ta. We observe that the easy axis of the magnetic anisotropy is forced into the [11¯0] crystal direction for Pt capped and into the [110] crystal direction for Ta capped NiMnSb, irrespective of the original anisotropy of the NiMnSb film. This effect is independent of the thickness of the capping layer and even present in multilayers with a Cu interlayer (few nanometers) between the NiMnSb and Pt cap, indicating that it is not related to interface chemistry and possibly pointing to it originating from the strong spin-orbit coupling in the capping layers.

By means of spin- and angle-resolved photoelectron spectroscopy we studied the spin structure of thin films of the topological insulator Bi2Se3 grown on InP(111). For thicknesses below six quintuple layers the spin- polarized metallic topological surface states interact with each other via quantum tunneling and a gap opens. Our measurements show that the resulting surface states can be described by massive Dirac cones which are split in a Rashba-like manner due to the substrate induced inversion asymmetry. The inner and the outer Rashba branches have distinct localization in the top and the bottom part of the film, whereas the band apices are delocalized throughout the entire film. Supported by ab initio calculations, our observations help in the understanding of the evolution of the surface states at the topological phase transition and provide the groundwork for the realization of two-dimensional spintronic devices based on topological semiconductors.

Electrical spin injection into semiconductors has been a challenge during the last years. Although many attempts have been made to realize spin injection from a ferromagnetic metal into a semiconductor two-dimensional electron gas (2DEG), all effects published up to now were small and could easily be explained by stray Hall effects or magnetoresistance. We have developed a theoretical model, which explains these negative results by the mismatch in conductivities and spin scattering in metallic ferromagnets and semiconductors. At the same time, the model indicates the direction towards more promising devices using highly spin polarized II–VI-magnetic semiconductors as a spin aligner. We have demonstrated highly efficient spin injection into a GaAs/AlGaAs light emitting diode. The spin polarization was detected via the optical polarization of the emitted light.

Single diluted magnetic semiconductor (DMS) quantum dots are studied by means of photoluminescence spectroscopy and magnetoluminescence. The sp-d exchange interaction between a single electron-hole pair and roughly 100 Mn spins within the dot is demonstrated to result in (i) a significant enhancement (more than one order of magnitude) of the emission linewidth and (ii) a strongly modified magnetic field dependence of the polarization degree in a single DMS quantum dot as compared to a non-magnetic reference sample.

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn1 − x − yBexMnySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.

We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors the transit time of spin polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance. However, in none of the experiments we do observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle type effects indicates that the magnetoresistance is not based on the injection of spin polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance., Comment: 14 pages, 7 figures

We report on spatially resolved pump-probe MOKE spectroscopy measurements of electron spins in bulk $n$-type GaAs. For low lattice temperatures these measurements are significantly compromised by local changes in the excitonic magneto-optical response which are not related to the electron spin polarization. These local changes in the excitonic Kerr spectrum are due to carrier heating caused by above-band-gap optical excitation. When hot electrons are present, a measurement of the local Kerr rotation at a fixed arbitrary probe wavelength does not necessarily correctly reveal the local electron spin polarization. By analyzing the spatial dependence of the full excitonic Kerr rotation spectrum we determine the local excitonic spin splitting energy from which we obtain the true lateral electron spin polarization profile.

Strained bulk HgTe is a three-dimensional topological insulator, whose surface electrons have a high mobility (30,000 cm^2/Vs), while its bulk is effectively free of mobile charge carriers. These properties enable a study of transport through its unconventional surface states without being hindered by a parallel bulk conductance. Here, we show transport experiments on HgTe-based Josephson junctions to investigate the appearance of the predicted Majorana states at the interface between a topological insulator and a superconductor. Interestingly, we observe a dissipationless supercurrent flow through the topological surface states of HgTe. The current-voltage characteristics are hysteretic at temperatures below 1 K with critical supercurrents of several microamperes. Moreover, we observe a magnetic field induced Fraunhofer pattern of the critical supercurrent, indicating a dominant 2\pi-periodic Josephson effect in the unconventional surface states. Our results show that strained bulk HgTe is a promising material system to get a better understanding of the Josephson effect in topological surface states, and to search for the manifestation of zero-energy Majorana states in transport experiments., Comment: experimental paper; 19 pages, 4 figures

Epitaxial layers of the topological insulator Bi2Se3 have been grown by molecular beam epitaxy on laterally lattice-matched InP(111)B substrates. High resolution X-ray diffraction shows a significant improvement of Bi2Se3 crystal quality compared to layers deposited on other substrates. The measured full width at half maximum of the rocking curve is Delta omega=13 arcsec, and the (omega-2theta) scans exhibit clear layer thickness fringes. Atomic force microscope images show triangular twin domains with sizes increasing with layer thickness. The structural quality of the domains is confirmed on the microscopic level by transmission electron microscopy., 4 pages, 4 figures

Extrinsic disorder strongly affects the performance of graphene-based quantum dots. The standard SiO2 substrate is generally considered to be one major factor besides edge-induced disorder. In this report we present the fabrication of lithographically defined quantum dots on SiO2 with short and narrow constrictions and different central island sizes. Low temperature transport measurements display distinct Coulomb-blockade peaks with amplitudes exceeding what is commonly observed experimentally. The analysis of the normalized Coulomb-blockade peak spacing shows a size dependence, which has not previously been observed for devices on SiO2. Furthermore, a quantitative comparison of the peak spacing distribution to the literature shows that one of the two devices compares favorably to a similar sized dot placed on hexagonal boron nitride, which is known to reduce the substrate disorder. Our findings suggest that the other sources of extrinsic disorder, such as lithography residues, may play an important role ...

We developed a real-time in situ x-ray Bragg diffraction technique for monitoring epitaxial growth. In our setup, the x-ray diffraction requirement of an extremely exact sample adjustment and an angular scan of sample and detector are circumvented by using a slightly divergent x-ray beam and observing an extremely asymmetric Bragg reflection with a multichannel detector. The angular range covered by the stationary multichannel detector corresponds nearly exactly to the qz interval of a conventional ω−2θ scan. The technique is demonstrated by monitoring the molecular-beam epitaxial growth of a ZnSe epilayer on (001)GaAs. The exposure time of each diffraction pattern is only a few seconds, which enables a real-time x-ray diffraction monitoring of the epitaxial growth process.

We have fabricated gated vertical quantum dots made from a II?VI semiconductor heterostructure containing a paramagnetic quantum well. The absence of a known Schottky gate metal compatible with ZnSe based material precludes the traditional method of using a self-aligning shadow evaporated gate. Instead, we make use of a multi-step electron beam lithography process to surround a pillar with an insulating dielectric and gate. This process allows for the processing of dots with diameters down to 250?nm. Preliminary transport data confirming the magnetic nature of the resulting artificial atom are presented.

The thermopower of a quantum dot, defined in the two-dimensional electron gas in a GaAs-AlxGa1-xAs heterostructure, is investigated using a current heating technique. At lattice temperatures kBT much smaller than the charging energy e2/C, and at small heating currents, sawtoohlike thermopower oscillations are observed as a function of gate voltage, in agreement with a recent theory. In addition, a remarkable sign reversal of the amplitude of the thermopower oscillations is found in the non-linear regime at large heating currents.

We perform high resolution X-ray diffraction on GaMnAs mixed crystals as well as on GaMnAs/GaAs and GaAs/MnAs superlattices for samples grown by low temperature molecular beam epitaxy under different growth conditions. Although all samples are of high crystalline quality and show narrow rocking curve widths and pronounced finite thickness fringes, the lattice constant variation with increasing manganese concentration depends strongly on the growth conditions: For samples grown at substrate temperatures of 220 and 270 degrees C the extrapolated relaxed lattice constant of Zincblende MnAs is 0.590 nm and 0.598 nm respectively. This is in contrast to low temperature GaAs, for which the lattice constant decreases with increasing substrate temperature., Comment: pdf only