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Cr and Mn monolayers on a triangular lattice are prototypical examples of frustrated spin systems in two dimensions. Collinear and noncollinear magnetic structures of these monolayers on Cu(111) substrate are investigated on the basis of first-principles total-energy calculations using the full-potential linearized augmented plane-wave method extended by the vector spin-density description for the interstitial and vacuum region. The search for the magnetic minimum-energy configurations included unit cells with one, two, and three atoms. For Cr the minimal energy was found for a 120° spin configuration in a (&sqrt3;×&sqrt3;)R30° unit cell, which is in agreement with the classical nearest-neighbor Heisenberg model with antiferromagnetic exchange interaction. The same behavior is expected for Mn, but a surprising result was found: the minimal energy was found for a collinear row-wise antiferromagnetic structure. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Exchange and Rashba spin-orbit interactions are expected to couple at ferromagnetic-metal/heavy-metal interfaces and influence their electronic structure. We examine these largely unexplored effects in Co(0001) films on W(110) by angle-resolved photoemission spectroscopy. We find breaking of inversion symmetry in the band dispersion of magnetic quantum well states along structurally equivalent directions of the Cofilms. Ab initio calculations ascribe this unusual band behavior to the relative orientation of coexisting exchange and Rashba fields. These findings elucidate fundamental interface electronic effects, which are associated to spin-orbit phenomena at ferromagnetic-metal/heavy-metal junctions. [ABSTRACT FROM AUTHOR]

Using spin density functional theory (SDFT) calculations, we have studied the magnetic states, including collinear and noncollinear magnetic interlayer coupling, of Fe1-xCox ultrathin films sandwiching Rh(001) layers. We found very large values for the interlayer exchange coupling (IEC) in Co/Rhn/Co or (FeCo)m/Rhn/Co structures as compared to, e.g., Ag or Au spacer layers. The IEC oscillates with the Rh spacer thickness showing a transition between strong antiferromagnetic and ferromagnetic coupling between five- and seven-layer thickness of the Rh film. Moreover, depending on the thickness of the FeCo film, a reorientation transition between in-plane and out-of-plane easy axis was found when spin-orbit coupling is considered in the calculations. This result suggests that, for specific arrangements such as (FeCo)2/Rh5/Co structures, a competition between IEC and magnetic anisotropy of coupled films may result in noncollinear ordering. This possibility was studied with constrained, noncollinear SDFT calculations and the results were mapped onto a classical spin model to explore the richness of spin structures that can arise in these multilayer systems. [ABSTRACT FROM AUTHOR]

We report on a combined low-temperature scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) investigation of the R30�Pb/Ag(111) surface alloy which provides a giant Rashba-type spin splitting. With STS we observed spectroscopic features that are assigned to two hole-like Rashba-split bands in the unoccupied energy range. By means of STS and quantum interference mapping we determine the band onsets, splitting strengths, and dispersions for both bands. The unambiguous assignment of scattering vectors is achieved by comparison to ARPES measurements. While intra-band scattering is found for both Rashba bands, inter-band scattering is only observed in the occupied energy range. Spin- and orbitally-resolved band structures were obtained by DFT calculations. Considering the scattering between states of different spin- and orbital character, the apparent deviation between experimentally observed scattering events and the theoretically predicted spin polarization could be resolved. [ABSTRACT FROM AUTHOR]

The Ir(111) surface is known to host a surface state with a giant spin–orbit splitting due to the Rashba effect. This surface state is stable even in air when Ir is protected with an epitaxial graphene overlayer. In the present paper, we reveal an effect allowing one to tune the binding energy of this spin-split surface state up and down and demonstrate the practical application of this effect by two different approaches. The first approach is related to a decoration of the moiré pattern of single-layer graphene on Ir(111) by self-assembled nanoclusters of different compositions. The clusters locally pin graphene to the Ir substrate and enhance the amplitude of its structural corrugation, which, in turn, leads to an increase in the surface state binding energy. The second approach is related to the synthesis of few-layer graphene on Ir(111) by segregation of carbon. Additional graphene layers induce a shift of the Ir surface state towards lower binding energies and bring it almost to the Fermi level. Based on density functional calculations performed for the graphene/Ir(111) system, we show that in both cases the effect causing the binding energy shifts is intimately related to the distance between graphene and the Ir surface, which is subject to change due to deposition of clusters or by increasing the amount of graphene overlayers. In contrast, the observed spin–orbit splitting of the Ir(111) surface state remains remarkably robust and constant in all cases. Our theoretical analysis reveals that such stability can be explained by the localization properties of the Ir surface state that is a deep surface resonance. [ABSTRACT FROM AUTHOR]

Using high-resolution spin- and angle-resolved photoemission spectroscopy, we map the electronic structure and spin texture of the surface states of the topological insulator Sb2Te3. In combination with density functional calculations (DFT), we directly show that Sb2Te3 exhibits a partially occupied, single spin-Dirac cone around the Fermi energy EF, which is topologically protected. DFT obtains a spin polarization of the occupied Dirac cone states of 80-90%, which is in reasonable agreement with the experimental data after careful background subtraction. Furthermore, we observe a strongly spin-orbit split surface band at lower energy. This state is found , at E - EF ≃ -0.8 eV at the T point, disperses upward, and disappears at about E - EF = -0.4 eV into two different bulk bands. Along the ̅̅̅Γ-K direction, the band is located within a spin-orbit gap. According to an argument given by Pendry and Gurman in 1975, such a gap must contain a surface state, if it is located away from the high-symmetry points of the Brillouin zone. Thus, the novel spin-split state is protected by symmetry, too. [ABSTRACT FROM AUTHOR]

The objective of this work is to employ spin-polarized density functional theory (sDFT) calculations for the exploration of ultrathin magnetic films with large magnetic moments and a strong perpendicular anisotropy. Monolayer films of Fe1-xCox (with x = 0, 0.25, 0.5, 0.75, and 1) on Rh(001) were addressed to study their magnetic properties using the all-electron full-potential linearized augmented plane wave (FLAPW) method in film geometry. We studied the magnetic order of these films including structural relaxations of the topmost layers. Fe1-xCox monolayer films were found to be ferromagnetic (FM) in a broad range of Co content x with a maximum magnetic moment of 2.8μB and of an out-of-plane magneto-crystalline anisotropy of 0.25 meV per magnetic atom at x = 0.5. The sDFT results were mapped onto a classical Heisenberg model, demonstrating FM Fe-Co and Co-Co couplings, while the Fe-Fe interaction is antiferromagnetic on Rh(001). The ordering temperature of the FeCo film was estimated to be well above room temperature (482 K). [ABSTRACT FROM AUTHOR]

The polar-to-nonpolar interface of DySc03 and SrTi03 was studied using density functional theory. Due to the polar discontinuity arising from nominally charged DyO or ScO2 layers, sharp interfaces induce a strong ferroelectriclike polarization in the SrTi03, while in chemically mixed interfaces this discontinuity is avoided and no such polarization can be found. In both scenarios the interface remains insulating with only a small reduction of the band gap. Our calculations show that mixed interfaces are energetically more favorable than sharp ones, in agreement with recent experimental results that confirmed intermixing at these interfaces. [ABSTRACT FROM AUTHOR]

We analyze the crystal and electronic structures of Tl-based strong topological insulators TlSbTe2, TlSbSe2, TlBiTe2, and TlBiSe2 by using first-principles calculation results. The topological nature of these materials is characterized by a single Dirac cone at the Γ point. Aside from the latter robust surface state (SS), we find trivial SSs at (around) the Fermi level for large momenta as well as deep trivial SSs at (around) Γ. The calculated energy cuts show an isotropic shape of the Dirac cone and a simple spin structure of the cone. The strong dependence of electronic structure on both optimization of the chalcogenide atom position in bulk and surface relaxations, as well as the slow convergence of the Dirac cone with respect to the film thickness, are discussed. The situation in the thallides is contrasted with results for isostructural indium compounds InBiTe2 and InSbTe2, the latter not being topological insulators. [ABSTRACT FROM AUTHOR]

We employ the full-potential linearized augmented plane-wave method to report a systematic density-functional theory study of the magnetic properties of the 3d transition-metal (V, Cr, Mn, Fe, Co, and Ni) monolayers deposited on the Rh(100) substrate. We find that all monolayer films are magnetic. The size of the local magnetic moments across the transition-metal series follows Hund's rule with a maximum magnetic moment of 3.77μB for Mn. The largest induced magnetic moment of about 0.46μB was found for Rh atoms adjacent to the Co film. When relaxations are included, we predict a ferromagnetic (FM) ground state for V, Co, and Ni, while Cr, Mn, and Fe favor a c(2 × 2)antiferromagnetic (AFM) state, a checkerboard arrangement of up and down magnetic moments. The magnetic anisotropy energies of these ultrathin magnetic films are calculated for the FM and AFM states. With the exception of Cr, the easy axis of the magnetization is predicted to be in the film plane. Rough estimates of the ordering temperatures are given. To gain an understanding of the c(2 × 2) AFM state of Fe/Rh(100), we analyze this result with respect to the trends of the magnetic order of 3d monolayers on other 4d substrates, such as Pd(100) and Ag(100). [ABSTRACT FROM AUTHOR]

Abstract: In magnetic systems lacking spatial inversion symmetry it is observed that the Dzyaloshinskii–Moriya interaction can entail long-ranged spin spirals with a unique sense of rotation. Here, we present a computationally efficient scheme to calculate these large magnetic structures within density functional theory and extract the interaction parameters using perturbative approaches of different order. We analyze the accuracy of these methods by investigating thin Fe films on W(110) and Mo(110) substrates. [Copyright &y& Elsevier]

Abstract: We report on ab initio calculations of Pt(111) B-type step-edges decorated with Co chains of different thickness. As found experimentally, these Co n-wires show for n =1 (monowire) a large magnetic anisotropy that decreases for larger n in a non-monotonous way. Also the easy axis shows a oscillatory behavior with increasing n that can be traced back to competing effects arising from different strands of the thicker wires. [Copyright &y& Elsevier]

Abstract: Surface states, that show a k-dependent splitting resulting from spin–orbit coupling, show wide similarities to a two-dimensional electron gas in semiconductor heterostructures, where the Rashba-effect lifts the spin-degeneracy of the bands and allows spin-manipulation by an electric field. We discuss the conditions for such a Rashba-effect at metallic surfaces by comparing ab initio calculations for Au(111), Ag(111), and Lu(0001) and investigate the influence of electric and magnetic (exchange) fields on these surface states. [Copyright &y& Elsevier]

Abstract: The calculation of interatomic magnetic exchange interactions entering the Heisenberg model is outlined from the standpoint of the density functional theory (DFT) for two Fe-based molecular magnets: a trinuclear complex with a Schiff base ligand, which is an antiferromagnetically coupled frustrated system, and a model bipyrimidine-connected planar network of Fe ions. First-principles electronic structure calculations are performed using the real-space method Siesta and the full-potential linearized augmented plane wave FLAPW method FLEUR, correspondingly. We discuss the application of fixed spin moment technique for preparing the system in a given magnetic configuration, and the effect of intraatomic Coulomb correlation, approximated by the LDA+ U technique, on the values of interaction parameters. [Copyright &y& Elsevier]

Abstract: The intermetallic ternary compounds of the type RMnX (R=Ca, La, Ba, Y and X=Si, Ge), crystallizing in the ThCrSi structure, show a large variety of collinear and noncollinear magnetic ground states (GS) depending on R and X and thus are good candidates for studying the dependence of the band structure contribution to the electronic transport on the different magnetic configurations. In this contribution we focus our analysis on LaMnGe. A qualitative understanding of the change in the conductivities with the magnetic structure in this material is provided on the basis of its coherent electronic structure. [Copyright &y& Elsevier]

We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells. [ABSTRACT FROM AUTHOR]

Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin. Phase change alloys are used as components for optical data storage like DVD and Blue-Ray disks but many of their conductive properties are only known phenomenologically. This paper reports detailed band structure mapping of the prototype alloy GST-225 using photoemission spectroscopy, and the observation of a Dirac-like surface state above the Fermi level. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy (STM) and spectroscopy (STS) in combination with density functional theory (DFT) calculations are employed to study the surface and subsurface properties of the metastable phase of the phase-change material Ge2 Sb2 Te5 as grown by molecular beam epitaxy. The (111) surface is covered by an intact Te layer, which nevertheless permits the detection of the more disordered subsurface layer made of Ge and Sb atoms. Centrally, we find that the subsurface layer is significantly more ordered than expected for metastable Ge2 Sb2 Te5. First, we show that vacancies are nearly absent within the subsurface layer. Secondly, the potential fluctuation, tracked by the spatial variation of the valence band onset, is significantly less than expected for a random distribution of atoms and vacancies in the subsurface layer. The strength of the fluctuation is compatible with the potential distribution of charged acceptors without being influenced by other types of defects. Thirdly, DFT calculations predict a partially tetrahedral Ge bonding within a disordered subsurface layer, exhibiting a clear fingerprint in the local density of states as a peak close to the conduction band onset. This peak is absent in the STS data implying the absence of tetrahedral Ge, which is likely due to the missing vacancies required for structural relaxation around the shorter tetrahedral Ge bonds. Finally, isolated defect configurations with a low density of 10-4 nm-2 are identified by comparison of STM and DFT data, which corroborates the significantly improved order in the epitaxial films driven by the buildup of vacancy layers. [ABSTRACT FROM AUTHOR]

Using relativistic first-principles calculations, we show that the chemical trend of the Dzyaloshinskii-Moriya interaction (DMI) in 3d-5d ultrathin films follows Hund's first rule with a tendency similar to their magnetic moments in either the unsupported 3d monolayers or 3d-5d interfaces. We demonstrate that, besides the spin-orbit coupling (SOC) effect in inversion asymmetric noncollinear magnetic systems, the driving force is the 3d orbital occupations and their spin-flip mixing processes with the spin-orbit active 5d states control directly the sign and magnitude of the DMI. The magnetic chirality changes are discussed in the light of the interplay between SOC, Hund's first rule, and the crystal-field splitting of d orbitals. [ABSTRACT FROM AUTHOR]

Based on the full-potential linearized augmented plane-wave method combined with the generalized gradient approximation, we determine the ground-state spin configurations and the total energies of 3d transition- metal monolayer and bilayer films on Fe(001) within the c(2×2) unit cell. We find by energy analysis that V, Cr, and Mn layers prefer the layered antiferromagnetic coupling, and Fe, Co, and Ni layers favor the ferromagnetic coupling to Fe(001). One exception is the Mn monolayer, which favors the c(2×2) ferrimagnetic superstructure. We discuss the stability of the 3d transition-metal monolayer films on Fe(001) against the bilayer formation and find that, with the exception of Cr, all 3d monolayers on Fe(001) are stable against bilayer formation. We have confirmed that the interlayer relaxations do not change the overall features of the present results. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Four different polymorphs of GdScO 3 are assessed theoretically and experimentally with respect to their suitability as a dielectric. The calculations carried out by density functional theory reveal lattice constants, band gaps and the energies of formation of three crystal phases. Experimentally all three crystal phases and the amorphous phase can be realized as thin films by pulsed laser deposition using various growth templates. Their respective crystal structures are confirmed by X-ray diffraction and transmission electron microscopy reflecting the calculated lattice constants. X-ray photoelectron spectroscopy unveils the band gaps of the different polymorphs of GdScO 3 which are above 5 eV for all films demonstrating good insulating properties. From capacitance voltage measurements, high permittivities of up to 27 for hexagonal GdScO 3 are deduced. [ABSTRACT FROM AUTHOR]

Heavy metal surface alloys represent model systems to study the correlation between electron scattering, spin-orbit interaction, and atomic structure. Here, we investigate the electron scattering from the atomic steps of monolayer BiAg2 on Ag(111) using quasiparticle interference measurements and density functional theory. We find that intraband transitions between states of opposite spin projection can occur via a spin-flip backward scattering mechanism driven by the spin-orbit interaction. The spin-flip scattering amplitude depends on the chemical composition of the steps, leading to total confinement for pure Bi step edges, and considerable leakage for mixed Bi-Ag step edges. Additionally, the different localization of the occupied and unoccupied surface bands at Ag and Bi sites leads to a spatial shift of the scattering potential barrier at pure Bi step edges. [ABSTRACT FROM AUTHOR]

The band structure and intra- and interband scattering processes of the electrons at the surface of a bismuth bilayer on Bi2Se3 have been experimentally investigated by low-temperature Fourier-transform scanning tunneling spectroscopy. The observed complex quasiparticle interference patterns are compared to a simulation based on the spin-dependent joint density of states approach using the surface-localized spectral function calculated from first principles as the only input. Thereby, the origin of the quasiparticle interferences can be traced back to intraband scattering in the bismuth-bilayer valence band and Bi2Se3 conduction band and to interband scattering between the two-dimensional topological state and the bismuth-bilayer valence band. The investigation reveals that the bilayer band gap, which is predicted to host one-dimensional topological states at the edges of the bilayer, is pushed several hundred meV above the Fermi level. This result is rationalized by an electron transfer from the bilayer to Bi2Se3 which also leads to a two-dimensional electron state in the Bi2Se3 conduction band with a strong Rashba spin splitting, coexisting with the topological state and bilayer valence band. [ABSTRACT FROM AUTHOR]

At ferroelectric longitudinal domain walls there is an uncompensated charge, which could form a two-dimensional electron gas in the insulator. However, the uncompensated charges can be accommodated by, e.g., defects or localized states that split off from the conduction band. We carried out density functional theory calculations to study these scenarios in PbTiO3 with and without consideration of strong correlation effects simulated via inclusion of a Hubbard parameter U. The optimized structure and electronic structure depend on the choice of this parameter: For vanishing U, a broad, conducting domain wall is obtained, while increasing U leads to localized Ti 3d states and an insulating, sharp domain wall. We also investigated the effects of varying the ferroelectric polarization on the electronic structure of these domain walls. [ABSTRACT FROM AUTHOR]

We investigate Co nanostructures on Bi2Se3 by means of scanning tunneling microscopy and spectroscopy (STM/STS), x-ray absorption spectroscopy, x-ray magnetic dichroism (XMCD) and calculations using the density functional theory (DFT). In the single adatom regime we find two different adsorption sites by STM. Our calculations reveal these to be the fcc and hcp hollow sites of the substrate. STS shows a pronounced peak for only one species of the Co adatoms indicating different electronic properties of both types. These are explained on the basis of our DFT calculations by different hybridizations with the substrate. Using XMCD we find a coverage dependent spin reorientation transition from easy-plane toward out-of-plane. We suggest clustering to be the predominant cause for this observation. [ABSTRACT FROM AUTHOR]

The understanding of the resistive switching mechanisms in perovskites is of particular importance for the development of novel non-volatile memories. Nanoscale investigations recently revealed that in the model material SrTiO3 a filamentary type of switching is present. In this paper, we show that upon donor doping with Nb the switching type changes fundamentally. We report on the observation of conducting clusters that can be switched independently between a high resistance and a low resistance state when applying a voltage. Furthermore, we show that the resistive switching takes place in a semiconducting surface layer on top of the metallic bulk of SrTiO3:Nb single crystals, which can change its properties easily under external gradients. Based on various measurements, we postulate that ionic movements leading to the creation of secondary phases as nano-filaments between the clusters have to be taken into account in modelling the resistive switching. [ABSTRACT FROM AUTHOR]

Abstract: We present ab initio calculations of an electronic structure and a topological invariant ν 0 of the Ge2Sb2Te5 compound. We have found that in the case of equiatomic composition of Ge/Sb layers the Ge2Sb2Te5 compound is the topological insulator. The ν 0 invariant does not depend on a specific location of Ge and Sb atoms in the mixed layers, and depends only on their concentration. The variation of the concentration in the Ge/Sb layers leads to a change of the topological invariant of the compound. [Copyright &y& Elsevier]

The magnitude of electron spin polarization in topologically protected surface states is an important parameter with respect to spintronics applications. In order to analyze the warped spin texture in Bi2Te3 thin films, we combine angle- and spin-resolved photoemission experiments with theoretical ab initio calculations. We find an in-plane spin polarization of up to ∼45% in the topologically protected Dirac cone states near the Fermi level. The Fermi surface of the Dirac cone state is warped and shows an out-of-plane spin polarization of ∼15%. These findings are in quantitative agreement with dedicated simulations which find electron density of the Dirac cone delocalized over the first quintuple layer with spin reversal occurring in the surface atomic layer. [ABSTRACT FROM AUTHOR]

We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼170 meV centered ∼65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of the time-reversal symmetry protected topological surface states in Sb2Te3 in the presence of impurities and defects. [ABSTRACT FROM AUTHOR]

The binary chalcogenides Bi2Te3 and Bi2Se3 are the most widely studied topological insulators. Although the quantum anomalous Hall effect has recently been observed in magnetically doped Sb2Te3, this compound has been studied to a much lesser extent. Here, by using energy-resolved quasiparticle interference mapping, we investigate the scattering events of pristine Sb2Te3 surfaces. We find that, in addition to the Dirac fermions, another strongly spin-polarized surface resonance emerges at higher energies in the unoccupied electronic states. Although the two surface states are of different origin, i.e., topologically protected and trivial, respectively, both show strongly directional scattering properties and absence of backscattering. A comparison with ab initio calculations demonstrates that this is a direct consequence of their spin-momentum-locked spin texture which is found to exhibit an opposite rotational sense for the trivial state and the Dirac state. [ABSTRACT FROM AUTHOR]

The interface of graphene with ferromagnets is highly relevant for spintronics, because graphene on Co(0001) shows a largely intact Dirac cone and strong hybridization with Co 3d states breaking the sublattice symmetry that had been considered mutually exclusive. Here we show by spin- and angle-resolved photoemission that the Dirac cone and Dirac point are also highly spin polarized (~-25%), which reinforces the puzzling issue of a strong graphene-substrate interaction. The problem is solved by our ab initio calculations which show that (i) the upper and lower halves of the Dirac cone belong to different sublattices and (ii) one half is spin polarized by spin-dependent hybridization because it is situated at the edge of a minority-spin band gap of the Co substrate. [ABSTRACT FROM AUTHOR]

We report on the epitaxial fabrication and electronic properties of a topological phase in strained a-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants. [ABSTRACT FROM AUTHOR]

The spin-orbit splitting of graphene p states can be strongly enhanced by external influences such as corrugation or proximity to heavier atoms. Here we investigate experimentally and theoretically whether such strong enhancement is possible for graphene on SiC(0001). By spin- and angle-resolved photoemission we found for two independently grown samples no resolvable spin-orbit splitting with an upper limit of our analysis of 20 meV. Our ab initio calculations predict a low spin-orbit splitting of 0.05 meV with small anisotropy but a local tenfold enhancement where hybridization with SiC substrate bands occurs. [ABSTRACT FROM AUTHOR]

We reveal a giant Rashba effect (&agr;R ≈ 1.3 eV Å) on a surface state of Ir(111) by angle-resolved photoemission and by density functional theory. It is demonstrated that the existence of the surface state, its spin polarization, and the size of its Rashba-type spin-orbit splitting remain unaffected when Ir is covered with graphene. The graphene protection is, in turn, sufficient for the spin-split surface state to survive in ambient atmosphere. We discuss this result along with indications for a topological protection of the surface state. [ABSTRACT FROM AUTHOR]

The electronic structure of ternary compounds PbSbTe, PbBiTe, and PbBiSe, which have a layered structure that consists of nine-layer atomic blocks separated by van der Waals gaps, has been theoretically studied. It has been shown that all studied compounds are three-dimensional topological insulators. The possibility of the existence of a two-dimensional topological insulator has been found in ultrathin (0001) PbSbTe and PbBiTe films. Oscillations of the ℤ topological invariant with an increase in the film thickness have been observed in the latter compound. [ABSTRACT FROM AUTHOR]

We theoretically study the generic behavior of the penetration depth of the edge states in two-dimensional quantum spin Hall systems. We found that the momentum-space width of the edge-state dispersion scales with the inverse of the penetration depth. As an example of well-localized edge states, we take the Bi(111) ultrathin film. Its edge states are found to extend almost over the whole Brillouin zone. Correspondingly, the bismuth (111) 1-bilayer system is proposed to have well-localized edge states in contrast to the HgTe quantum well. [ABSTRACT FROM AUTHOR]

We report high-resolution spin-resolved photoemission spectroscopy (spin-ARPES) measurements on the parent compound Sb of the recently discovered three-dimensional topological insulator Bi1-xSbx (Hsieh et al 2008 Nature 452 970, Hsieh et al 2009 Science 323 919). By modulating the incident photon energy, we are able to map both the bulk and the (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin-orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a k-splitting of its spin-polarized electronic channels. These results complement our previously published works on this class of materials and re-confirm our discovery of topological insulator states in the Bi1-xSbx series. [ABSTRACT FROM AUTHOR]

A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry-based classification of condensed matter. Exotic spin-transport phenomena, such as the dissipationless quantum spin Hall effect, have been speculated to originate from a topological order whose identification requires a spin-sensitive measurement, which does not exist to this date in any system. Using Mott polarimetry, we probed the spin degrees of freedom and demonstrated that topological quantum numbers are completely determined from spin texture-imaging measurements. Applying this method to Sb and Bi1-xSbx, we identified the origin of its topological order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a topological quantum Berry's phase and definite spin chirality, which are the key electronic properties component for realizing topological quantum computing bits with intrinsic spin Hall-like topological phenomena. [ABSTRACT FROM AUTHOR]

Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physics (in the charge-parity violation of electroweak interactions). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii–Moriya interaction, which arises from spin–orbit scattering of electrons in an inversion-asymmetric crystal field. This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur. Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii–Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure, causing a variety of excitations or manipulations of the magnetization and giving rise to microwave emission, magnetization switching, or magnetic motors. [ABSTRACT FROM AUTHOR]

The metastable impact electron spectra (MIES) and photoelectron spectra (UPS(HeI)) of undoped SrTiO3(0 0 1) single crystals are presented. The work function is 2.6 eV; the top of the valence band is located 3.1 eV below the Fermi level which coincides with the bottom of the conduction band. An interpretation of the spectra is made on the basis of first-principles electronic structure results, also presented in this paper. We find that the HeI spectra can be well described by the bulk density of states of the SrTiO3(0 0 1) valence band. The MIES spectra are best understood when assuming that the He 2s electron localizes at surface cations after its resonant transfer to the surface. This He-induced “dynamical” cation reduction greatly affects the He-surface interaction. The MIES spectra are similar to Auger deexcitation spectra, but are shifted by about 1.2 eV towards smaller kinetic energies. The shape of the spectra is best described by the density of states at about 4 a.u. in front of the surface. [Copyright &y& Elsevier]

In this paper we present a density functional theory investigation of complex magnetic structures in ultra-thin films. The focus is on magnetically frustrated antiferromagnetic Cr and Mn monolayers deposited on a triangular lattice provided by a Ag (111) substrate. This involves non-collinear magnetic structures, which we treat by first-principles calculations on the basis of the vector spin-density formulation of the density functional theory. We find for Cr/Ag (111) a coplanar non-collinear periodic 120° Néel structure, for Mn/Ag (111) a row-wise antiferromagnetic structure, and for Fe/Ag (111) a ferromagnetic structure as magnetic ground states. The spin-polarized scanning tunneling microscope (SP–STM) operated in the constant-current mode is proposed as a powerful tool to investigate complex atomic-scale magnetic structures of otherwise chemically equivalent atoms. We discuss a recent application of this operation mode of the SP–STM on Mn/W (110), which led to the first observation of a two-dimensional antiferromagnet on a non-magnetic metal. The future potential of this approach is demonstrated by calculating SP–STM images for different magnetic structures of Cr/Ag (111). The results show that the predicted non-collinear magnetic ground state structure can clearly be discriminated from competing magnetic structures. A general discussion of the application of different operation modes of the SP–STM is presented on the basis of the model of Tersoff and Hamann. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy was applied to study the lateral variation of the local density of electronic states on the Bi(111) surface in the vicinity of a point defect. At an energy close to the Fermi level a characteristic pattern with a threefold symmetry is found. The pattern can be attributed to the scattering between two electronic surface states which are split by spin orbit coupling. The observation is well described by the superposition of three monochromatic waves. The phase of the waves relative to the center of the defect leads to a reduction to a threefold symmetry. [ABSTRACT FROM AUTHOR]

The variation in the surface potential as a function of the ferroelectric polarization of micron scale domains in a thin epitaxial film of Pb(Zr0.48Ti0.52)O3 is measured using mirror electron microscopy. Domains were written using piezoforce microscopy. The surface potential for each polarization was deduced from the mirror to low energy electron microscopy transition in the local reflectivity curve. The effect of extrinsic screening of the fixed polarization charge at the ferroelectric surface is demonstrated. The results are compared with density functional theory calculations. [ABSTRACT FROM AUTHOR]

The structural and electronic properties of MnxGe1-x alloys (x≤=0.15) fabricated by ion implantation are investigated by means of x-ray diffraction and synchrotron radiation photoemission spectroscopy. The diffraction patterns point to the presence of ferromagnetic Mn5Ge3 nanoparticles; however, valence band spectra, interpreted by means of accurate ab initio calculations including Hubbard-like correlations, show clear fingerprints of an effective substitutional Mn dilution in the Ge semiconducting host [ABSTRACT FROM AUTHOR]

Topologically nontrivial states reveal themselves in strongly spin-orbit coupled systems by Dirac cones. However, their appearance is not a sufficient criterion for a topological phase. In topological insulators, where these states protect surface metallicity, they are straightforwardly assigned based on bulk-boundary correspondence. On metals, where these states are suspected to have tremendous impact as well, e.g., in catalysis, their topological protection is difficult to assess due to the lacking band gap and the frequent assignment to topological properties appears unjustified. Here, we discover by angle-resolved photoemission a state with the dispersion of a Dirac cone at a low-symmetry point of W(110). Our ab initio calculations predict this feature with a linear band crossing and high spin polarization. However, instead of being born by topology, the states arise from Rashba split bands and do not fundamentally depend on the opening of a spin-orbit gap. On the other hand, we find that the [001] mirror plane protects the band crossing point and renormalizes the dispersion towards a Dirac-cone shape. In this sense, the discovered state is the metal counterpart of the surface state of a topological crystalline insulator. The Dirac cone is tilted due to its origin in an accidental band crossing away from high symmetry points. Tilted Dirac cones have recently been predicted for two- and three-dimensional materials and were observed in three-dimensional Weyl semimetals. Accordingly, the protection and renormalization by mirror symmetry uncovered here are a potentially much wider spread phenomenon which does not require topological properties. Our results also indicate why the massive gapless crossing predicted for topological crystalline insulators has never been observed. [ABSTRACT FROM AUTHOR]

We explore the emergence of chiral magnetism in one-dimensional monatomic Mn, Fe, and Co chains deposited at the Pt(664) step edge carrying out an ab initio study based on density functional theory (DFT). The results are analyzed employing several models: (i) a micromagnetic model, which takes into account the Dzyaloshinskii-Moriya interaction (DMI) besides the spin stiffness and the magnetic anisotropy energy, and (ii) the Fert-Levy model of the DMI for diluted magnetic impurities in metals. Due to the step-edge geometry, the direction of the Dzyaloshinskii vector (D vector) is not predetermined by symmetry and points in an off-symmetry direction. For the Mn chain we predict a long-period cycloidal spin-spiral ground state of unique rotational sense on top of an otherwise atomic-scale antiferromagnetic phase. The spins rotate in a plane that is tilted relative to the Pt surface by 62¬ towards the upper step of the surface. The Fe and Co chains show a ferromagnetic ground state since the DMI is too weak to overcome their respective magnetic anisotropy barriers. An analysis of domain walls within the latter two systems reveals a preference for a Bloch wall for the Fe chain and a Néel wall of unique rotational sense for the Co chain in a plane tilted by 29° towards the lower step. Although the atomic structure is the same for all three systems, not only the size but also the direction of their effective D vectors differ from system to system. The latter is in contradiction to the Fert-Levy model. Due to the considered step-edge structure, this work provides also insight into the effect of roughness on DMI at surfaces and interfaces of magnets. Beyond the discussion of the monatomic chains we provide general expressions relating ab initio results to realistic model parameters that occur in a spin-lattice or in a micromagnetic model. We prove that a planar homogeneous spiral of classical spins with a given wave vector rotating in a plane whose normal is parallel to the D vector is an exact stationary state solution of a spin-lattice model for a periodic solid that includes Heisenberg exchange and DMI. In the vicinity of a collinear magnetic state, assuming that the DMI is much smaller than the exchange interaction, the curvature and slope of the stationary energy curve of the spiral as a function of the wave vector provide directly the values of the spin stiffness and the spiralization required in micromagnetic models. The validity of the Fert-Levy model for the evaluation of micromagnetic DMI parameters and for the analysis of ab initio calculations is explored for chains. The results suggest that some care has to be taken when applying the model to infinite periodic one-dimensional systems. [ABSTRACT FROM AUTHOR]

The joint effect of exchange and Rashba spin-orbit interactions is examined on the surface and quantum well states of Ag2Bi-terminated Ag films grown on ferromagnetic Fe(110). The system displays a particular combination of time-reversal and translational symmetry breaking that strongly influences its electronic structure. Angle-resolved photoemission reveals asymmetric band-gap openings, due to spin-selective hybridization between Rashba-split surface states and exchange-split quantum well states. This results in an unequal number of states along positive and negative reciprocal space directions. We suggest that the peculiar asymmetry of the discovered electronic structure can have significant influence on spin-polarized transport properties. [ABSTRACT FROM AUTHOR]

A correction to the article "Elemental Topological Insulator with Tunable Fermi Level: Strained α-Sn on InSb(001)," by A. Barfuss and colleagues that was published in the 2013 issue is presented.

Combining density-functional theory calculations with a classical Monte Carlo method, we show that for B2-type FeCo compounds, tetragonal distortion gives rise to a strong reduction of the Curie temperature TC. The TC monotonically decreases from 1575 K (for c/a=1) to 940 K (for c/a=2). We find that the nearest neighbor Fe-Co exchange interaction is sufficient to explain the c/a behavior of the TC. Combination of high magnetocrystalline anisotropy energy with a moderate TC value suggests tetragonal FeCo grown on the Rh substrate with c/a=1.24 to be a promising material for heat-assisted magnetic recording applications. [ABSTRACT FROM AUTHOR]