We introduce a new class of spintronics devices in which a spin-valve like effect results from strong spin-orbit coupling in a single ferromagnetic layer rather than from injection and detection of a spin-polarized current by two coupled ferromagnets. The effect is observed in a normalmetal/insulator/ferromagnetic-semiconductor tunneling device. This behavior is caused by the interplay of the anisotropic density of states in (Ga,Mn)As with respect to the magnetization direction, and the two-step magnetization reversal process in this material.PACS numbers: 75.50. Pp, 85.75.Mm Devices relying on spin manipulation are hoped to provide low-dissipative alternatives for microelectronics. Furthermore, spintronics is expected to lead to full integration of information processing and storage functionalities opening attractive prospects for the realization of instant on-and-off computers. A primary goal of current spintronics research is to realize a device with metal spin-valve like behavior [1] in an all semiconductor-based structure enhancing integration of spintronics with existing microelectronics technologies. An oft proposed scheme for such a device consists of a tunnel barrier between two ferromagnetic semiconductors. As such, (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As structures have previously been studied [2,3] with some promising results. However, realizing the full potential of these systems will require a complete understanding of the physics of tunneling into (Ga,Mn)As, which we have found to be rather different than previously thought.In this spirit, we investigate transport in a structure consisting of a single ferromagnetic (Ga,Mn)As layer fitted with a tunnel barrier and a non-magnetic metal contact. We report some of the rich experimental properties of such a tunneling structure and provide an interpretation of the measured spin-valve like effect as a tunneling anisotropic magnetoresistance (TAMR) due to a two-step magnetization reversal and a magnetization dependent density of states (DOS) in the (Ga,Mn)As layer.The magnetic layer in our sample is a 70 nm thick epitaxial (Ga,Mn)As film grown by low temperature (270• C) molecular beam epitaxy onto a GaAs (001) substrate [4]. High-resolution x-ray diffraction showed that the sample had high crystalline quality comparable to that of the substrate. From the measured lattice constant and the calibration curves of Ref.[5], the Mn concentration in the ferromagnetic layer is roughly 6%. Etch capacitance-voltage control measurements yielded a hole density estimate of ∼ 10 21 cm −3 and the Curie temperature of 70 K was determined from SQUID measurements.After growth, the sample surface was Ar sputtered to remove any potential oxides, and a 1.4 nm Al layer was deposited at a rate of 0.

A review is given on the formation mechanisms and the properties of Si/Ge nanostructures that have been synthesized by self-assembling and self-ordering during heteroepitaxy of silicon-germanium alloys on single-crystal silicon substrates. The properties of electronic subbands in smooth strained Si/SiGe quantum well structures are presented as a basis for characterizing coherent Si/Ge nanostructures with free motion of carriers in a reduced number of dimensions. The low-dimensional band structure of valence band states confined in strained Si/Ge and Si/SiGe nanostructures is analysed by optical and electrical spectroscopy.The nanostructures presented were fabricated by self-assembly induced by elastic strain relaxation without applying any patterning technique. Misfit lattice strain of SiGe material deposited on Si substrates can relax by bunching of atomic surface steps with SiGe agglomeration at the step edges or by nucleation of Ge-rich islands in the Stranski-Krastanow growth mode. The size, density and composition of such Si/Ge nanostructures representing quantum wires and dots, respectively, can be tuned in a wide range by the growth parameters. Local strain fields extending into the Si host influence the nucleation and the lateral arrangement of nanostructures in subsequent layers and can be applied for self-ordering of nanostructures in the vertical as well as the lateral direction.Interband and intra-valence-band photocurrent, absorption and photoluminescence spectroscopy as well as C-V and admittance measurements reveal a consistent view of the band structure in Si/Ge quantum dot structures. This is in good agreement with model calculations based on band offsets, deformation potentials and effective electron masses known from earlier studies of Si/SiGe quantum well structures. The effective valence band offsets of hole states within Si/Ge nanostructures reach about 400 meV. Typical quantization energies of about 40 meV due to lateral confinement and Coulomb charging energies up to about 15 meV were observed for holes confined in 20 nm sized Si/Ge dots. Future applications of Si/Ge nanostructures such as photodetectors with improved performance or novel functionality are discussed.

Topological insulators doped with transition metals have recently been found to host a strong ferromagnetic state with perpendicular to plane anisotropy as well as support a quantum Hall state with edge channel transport, even in the absence of an external magnetic field. It remains unclear however why a robust magnetic state should emerge in materials of relatively low crystalline quality and dilute magnetic doping. Indeed, recent experiments suggest that the ferromagnetism exhibits at least some superparamagnetic character. We report on transport measurements in a sample that shows perfect quantum anomalous Hall quantization, while at the same time exhibits traits in its transport data which suggest inhomogeneities. We speculate that this may be evidence that the percolation path interpretation used to explain the transport during the magnetic reversal may actually have relevance over the entire field range. 73.43.Fj, 75.45.+j, 75.50.Pp The recent report on the experimental observation of a quantum anomalous Hall effect (QAHE) in Cr-doped (Bi,Sb) 2 Te 3 [1] generated significant interest in this material system for its potential as a magnetic topological insulator and as a test bed for the study of the Quantum Hall effect without the need for an external magnetic field [2][3][4][5][6]. This original report showed that the anomalous Hall contribution [1] appeared to saturate to a value of one conduction quantum as the sample was cooled to mK temperatures, but did not yet provide evidence that the transport takes place in edge states. In order to convincingly verify that the transport takes place in quantum Hall like edge states, non-local geometries are required. Such measurements were first reported in [3], albeit in configurations where the signals were very small, and where their interpretation required invoking some loss mechanism in the edge channels, and subsequently in [4], where convincing evidence of edge state transport was reported. This last paper also observed some unusual temperature and sweep rate related features in their data, which were at least in part interpreted as additional cooling through adiabatic demagnetization mechanisms. Shortly after the first reports on Cr-doped layers, it was discovered by the Moodera group [5-7] that using V instead of Cr appears to lead to more reproducible samples with a more robust magnetic and quantum anomalous Hall state. Using this material system, the authors were able to reproduce both precise quantization of the Quantum Hall state [5], and unequivocal evidence of edge state transport [6]. While the described quantum anomalous Hall phenomenology is now well established, its microscopic origin remains much less clear. The proposed mechanism for the QAHE is the breaking of time reversal symmetry by a perpendicular to plane internal magnetic field which leads to the reversal of the band inversion of one of the two spin species in a ferromagnetic two dimensional topological insulator [1,8]. The origin of the ferromagnetic state in the original paper [1] w...

We report the discovery of a very large tunneling anisotropic magnetoresistance in an epitaxially grown (Ga,Mn)As/GaAs/(Ga,Mn)As structure. The key novel spintronics features of this effect are as follows: (i) both normal and inverted spin-valve-like signals; (ii) a large nonhysteretic magnetoresistance for magnetic fields perpendicular to the interfaces; (iii) magnetization orientations for extremal resistance are, in general, not aligned with the magnetic easy and hard axis; (iv) enormous amplification of the effect at low bias and temperatures.

We report on magneto-optical studies of Bi2Se3, a representative member of the 3D topological insulator family. Its electronic states in bulk are shown to be well described by a simple Diractype Hamiltonian for massive particles with only two parameters: the fundamental bandgap and the band velocity. In a magnetic field, this model implies a unique property -spin splitting equal to twice the cyclotron energy: Es = 2Ec. This explains the extensive magneto-transport studies concluding a fortuitous degeneracy of the spin and orbital split Landau levels in this material. The Es = 2Ec match differentiates the massive Dirac electrons in bulk Bi2Se3 from those in quantum electrodynamics, for which Es = Ec always holds.PACS numbers: 71.70. Di, 76.40.+b, Inspiring analogies to relativistic systems have largely helped to elucidate the electronic properties of twodimensional graphene [1,2], surface states of topological insulators (TIs) [3][4][5][6], novel three-dimensional (3D) semimetals [7][8][9] as well as certain narrow gap semiconductors [10]. Here, we report on magneto-optical studies of bulk Bi 2 Se 3 , which imply the approximate applicability of the Dirac Hamiltonian for massive relativistic particles to approach the band structure of this popular representative of the TI family.The dispersion relations of genuine massive Dirac fermions in quantum electrodynamics are defined by two parameters: the energy gap 2∆ between particles and antiparticles and velocity parameter v D . At low energies, i.e., in the non-relativistic limit, these dispersions become parabolic and characterized by the same effective mass m D = ∆/v 2 D (rest Dirac mass). Such dispersions resemble the cartoon sketch of a direct gap semiconductor, which may be conventionally described using Schrödinger equation, completed by extra Pauli terms in order to include the spin degree of freedom. In contrast, no additional terms are needed when Dirac equation is employed, since it inherently accounts for spin-related effects. For instance, when the magnetic field B is applied, Dirac equation describes both cyclotron (E c ) as well as spin (E s ) splitting of the electronic states and implies that these two splitting energies are the same and linear with B in the non-relativistic approximation:For free electrons, this condition is equivalent to the effective g factor of 2 in E s = gµ B B (Bohr magneton µ B = e /2m 0 ) [11].In this Letter, we demonstrate experimentally that the conduction and valence bands of Bi 2 Se 3 are both, with a good precision, parabolic (perpendicular to the c-axis) and characterized by approximately the same effective mass. This crucial observation implies a great simplification of the multi-parameter Dirac Hamiltonian [5,12] commonly used to describe the bands of this material.The resulting simplified Dirac Hamiltonian differs from that of the genuine quantum electrodynamics system only by relevant (additional) diagonal dispersive terms, and importantly, it remains to be defined by two parameters only: by the bandgap energy 2∆ an...

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The microstructure of Bi 2 Se 3 topological-insulator thin films grown by molecular beam epitaxy on InP(111)A and InP(111)B substrates that have different surface roughnesses has been studied in detail using X-ray diffraction, X-ray reflectivity, atomic force microscopy and probe-corrected scanning transmission electron microscopy. The use of a rough Fe-doped InP(111)B substrate results in complete suppression of twin formation in the Bi 2 Se 3 thin films and a perfect interface between the films and their substrates. The only type of structural defects that persist in the "twinfree" films is an antiphase domain boundary, which is associated with variations in substrate height.It is also shown that the substrate surface termination determines which family of twin domains dominates.

Epitaxial layers of the topological insulator Bi 2 Se 3 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 Bi 2 Se 3 crystal quality compared to layers deposited on other substrates. The measured full width at half maximum of the rocking curve is ∆ω = 13 arcsec, and the (ω − 2θ) 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.