Lead overlayer dynamics on Ni(111).

Ultrathin lead films on metallic and semiconductor substrates are technologically demanded and actively studied by different experimental and theoretical methods. The formation of these films gives rise to new adsorbate-induced electron states and vibrational modes. The dynamical properties of atoms on surfaces depend sensitively on their bonding environment and thus provide valuable insight into the local geometry and chemical bonding at the boundary of a solid. In this paper, the vibrational properties of the , (3 × 3) and (4 × 4) overlayers formed by the adsorption of 1/3, 4/9, and 9/16 Pb ML, respectively, on the Ni(111) surface were studied for the first time. The surface relaxations, phonon modes and the local density of states were analyzed. We revealed the role of substrate dynamics in the stability of the Pb/Ni(111) system at submonolayer Pb adsorption.

Plasmon Resonance in a System of Bi Nanoparticles Embedded into (Al,Ga)As Matrix.

We reveal the feasibility of the localized surface plasmon resonance in a system of Bi nanoparticles embedded into an AlxGa1-xAs semiconductor matrix. With an ab initio determined dielectric function for bismuth and well-known dielectric properties of AlxGa1-xAs solid solution, we performed calculations of the optical extinction spectra for such metamaterial using Mie's theory. The calculations demonstrate a strong band of the optical extinction using the localized surface plasmons near a photon energy of 2.5 eV. For the semiconducting matrices with a high aluminum content x>0.7, the extinction by plasmonic nanoparticles plays the dominant role in the optical properties of the medium near the resonance photon energy.

Unveiling Influence of Dielectric Losses on the Localized Surface Plasmon Resonance in (Al,Ga)As:Sb Metamaterials.

We perform numerical modeling of the optical absorption spectra of metamaterials composed of systems of semimetal antimony nanoparticles embedded into AlxGa1-xAs semiconductor matrices. We reveal a localized surface plasmon resonance (LSPR) in these metamaterials, which results in a strong optical extinction band below, near, or above the direct band gap of the semiconductor matrices, depending on the chemical composition of the solid solutions. We elucidate the role of dielectric losses in AlxGa1-xAs, which impact the LSPR and cause non-plasmonic optical absorption. It appears that even a dilute system of plasmonic Sb nanoinclusions can substantially change the optical absorption spectra of the medium.

Emergence of quasi-1D spin-polarized states in ultrathin Bi films on InAs(111)A for spintronics applications.

The discovery, characterization, and control of heavy-fermion low-dimensional materials are central to nanoscience since quantum phenomena acquire an exotic and highly tunable character. In this work, through a variety of comprehensive experimental and theoretical techniques, it was observed and predicted that the synthesis of ultrathin Bi films on the InAs(111)A surface produces quasi-one-dimensional spin-polarized states, providing a platform for the realization of a unique spin-transport regime in the system. Scanning tunneling microscopy and low-energy electron diffraction measurements revealed that the InAs(111)A substrate facilitates the formation of the Bi-dimer phase of 2√3 × 3 periodicity with an admixture of the Bi-bilayer phase under submonolayer Bi deposition. X-ray photoelectron spectroscopy (XPS) measurements have shown the chemical stability of the Bi-induced phases, while spin and angle resolved photoemission spectroscopy (SARPES) observations combined with state-of-the-art DFT calculations have revealed that the electronic spectrum of the Bi-dimer phase holds a quasi-1D hole-like spin-split state at the Fermi level with advanced spin texture, whereas the Bi-bilayer phase demonstrates metallic states with large Rashba spin-splitting. The band structure of the Bi/InAs(111)A interface is discovered to hold great potential as a high-performance spintronics material fabricated in the ultimate two-dimensional limit.

Localized Surface Plasmon Resonance in Metamaterials Composed of As1-zSbz Semimetal Nanoparticles in AlxGa1-xAs1-ySby Semiconductor Matrix.

We analyze the possibility to realize a localized surface plasmon resonance in metamaterials composed of As1-zSbz nanoparticles embedded in an AlxGa1-xAs1-ySby semiconductor matrix. To this end, we perform ab initio calculations of the dielectric function of the As1-zSbz materials. Changing the chemical composition z, we trace the evolution of the band structure, dielectric function, and loss function. In terms of the Mie theory, we calculate the polarizability and optical extinction of a system of As1-zSbz nanoparticles in an AlxGa1-xAs1-ySby environment. We show a possibility to provide localized surface plasmon resonance near the band gap of the AlxGa1-xAs1-ySby semiconductor matrix by a built-in system of As1-zSbz nanoparticles strongly enriched by Sb. The results of our calculations are supported by available experimental data.

A 2D heavy fermion CePb3 kagome material on silicon: emergence of unique spin polarized states for spintronics.

We report on the successful synthesis of a 2D atomically thin heavy-fermion CePb3 kagome compound on a Si(111) surface. Growth and morphology were controlled and characterized through scanning tunneling microscopy observations revealing the high crystalline quality of the sample. Angle-resolved photoelectron spectroscopy measurements revealed the giant highly-anisotropic Rashba-like spin splitting of the surface states and semi-metallic character of the spectrum. According to the DFT calculations, the occupied hole and unoccupied electron states with huge spin-orbit splitting and out-of-plane spin polarization reside at the M̄ points near the Fermi level EF, which is ≈100 meV above the experimental one. The out-of-plane FM magnetization was found to be preferred with Ce spin and orbital magnetic momenta values of 0.895µB and -0.840µB, respectively. The spin-split states near EF are primarily formed by Pb pxy orbitals with the admixing of Ce d and f electrons due to the Ce f-d hybridization acquired asymmetry with respect to the sign of k∥. The observed electronic structure of the CePb3/Si(111)√3 × âˆš3 system is rather unique and in the hole-doped state, like in our experiment, can be enabled in the tunable spin current regime, which makes it a prospective 2D material for spintronic applications.

Electronic Structures and Surface Reconstructions in Magnetic Superconductor RbEuFe4As4.

In pnictide RbEuFe4As4, superconductivity sets in at 36 K and coexists, below 15-19 K, with the long-range magnetic ordering of Eu 4f spins. Here we report scanning tunneling experiments performed on cold-cleaved single crystals of the compound. The data revealed the coexistence of large Rb-terminated and small Eu-terminated terraces, both manifesting 1 × 2 and 2×2 reconstructions. On 2×2 surfaces, a hidden electronic order with a period ∼5 nm was discovered. A superconducting gap of ∼7 meV was seen to be strongly filled with quasiparticle states. The tunneling spectra compared with density functional theory calculations confirmed that flat electronic bands due to Eu 4f orbitals are situated ∼1.8 eV below the Fermi level and thus do not contribute directly to Cooper pair formation.

Interplay of Topological States on TI/TCI Interfaces.

Based on first-principles calculations, we study electronic structure of interfaces between a Z2 topological insulator (TI) SnBi2Te4 and a topological crystalline insulator (TCI) SnTe. We consider two interface models characterized by the different atomic structure on the contact of the SnTe(111) and SnBi2Te4(0001) slabs: the model when two materials are connected without intermixing (abrupt type of interface) and the interface model predicted to be realized at epitaxial immersion growth on topological insulator substrates (smooth interface). We find that a strong potential gradient at the abrupt interface leads to the redistribution of the topological states deeper from the interface plane which prevents the annihilation of the Γ¯ Dirac states, predicted earlier. In contrast, a smooth interface is characterized by minor charge transfer, which promotes the strong interplay between TI and TCI Γ¯ Dirac cones leading to their complete annihilation.The M¯ topologically protected Dirac state of SnTe(111) survives irrespective of the interface structure.

New Universal Type of Interface in the Magnetic Insulator/Topological Insulator Heterostructures.

Magnetic proximity effect at the interface between magnetic and topological insulators (MIs and TIs) is considered to have great potential in spintronics as, in principle, it allows realizing the quantum anomalous Hall and topological magneto-electric effects (QAHE and TME). Although an out-of-plane magnetization induced in a TI by the proximity effect was successfully probed in experiments, first-principles calculations reveal that a strong electrostatic potential mismatch at abrupt MI/TI interfaces creates harmful trivial states rendering both the QAHE and TME unfeasible in practice. Here on the basis of recent progress in formation of planar self-assembled single layer MI/TI heterostructure (T. Hirahara et al. Nano Lett. 2017 , 17 , 3493 - 3500 ), we propose a conceptually new type of the MI/TI interfaces by means of density functional theory calculations. By considering MnSe/Bi2Se3, MnTe/Bi2Te3, and EuS/Bi2Se3 we demonstrate that, instead of a sharp MI/TI interface clearly separating the two subsystems, it is energetically far more favorable to form a built-in interface via insertion of the MI film inside the TI's surface quintuple layer (e.g., Se-Bi-Se-[MnSe]-Bi-Se) where it forms a bulk-like MI structure. This results in a smooth MI-to-TI connection that yields the interface electronic structure essentially free of trivial states. Our findings open a new direction in studies of the MI/TI interfaces and restore their potential for the QAHE and TME observation.

Two-Dimensional In-Sb Compound on Silicon as a Quantum Spin Hall Insulator.

Two-dimensional (2D) topological insulator is a promising quantum phase for achieving dissipationless transport due to the robustness of the gapless edge states resided in the insulating gap providing realization of the quantum spin Hall effect. Searching for two-dimensional realistic materials that are able to provide the quantum spin Hall effect and possessing the feasibility of their experimental preparation is a growing field. Here we report on the two-dimensional (In, Sb)2[Formula: see text]2[Formula: see text] compound synthesized on Si(111) substrate and its comprehensive experimental and theoretical investigations based on an atomic-scale characterization by using scanning tunneling microscopy and angle-resolved photoelectron spectroscopy as well as ab initio density functional theory calculations identifying the synthesized 2D compound as a suitable system for realization of the quantum spin Hall effect without additional functionalization like chemical adsorption, applying strain, or gating.

Large-Gap Magnetic Topological Heterostructure Formed by Subsurface Incorporation of a Ferromagnetic Layer.

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future "topotronics" devices.

Spin-texture inversion in the giant Rashba semiconductor BiTeI.

Semiconductors with strong spin-orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimentally challenging and yet essential for exploiting spin-orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors.

Multiple Coexisting Dirac Surface States in Three-Dimensional Topological Insulator PbBi6Te10.

By means of angle-resolved photoemission spectroscopy (ARPES) measurements, we unveil the electronic band structure of three-dimensional PbBi6Te10 topological insulator. ARPES investigations evidence multiple coexisting Dirac surface states at the zone-center of the reciprocal space, displaying distinct electronic band dispersion, different constant energy contours, and Dirac point energies. We also provide evidence of Rashba-like split states close to the Fermi level, and deeper M- and V-shaped bands coexisting with the topological surface states. The experimental findings are in agreement with scanning tunneling microscopy measurements revealing different surface terminations according to the crystal structure of PbBi6Te10. Our experimental results are supported by density functional theory calculations predicting multiple topological surface states according to different surface cleavage planes.

Synthesis of two-dimensional Tl(x)Bi(1-x) compounds and Archimedean encoding of their atomic structure.

Crystalline atomic layers on solid surfaces are composed of a single building block, unit cell, that is copied and stacked together to form the entire two-dimensional crystal structure. However, it appears that this is not an unique possibility. We report here on synthesis and characterization of the one-atomic-layer-thick Tl(x)Bi(1-x) compounds which display quite a different arrangement. It represents a quasi-periodic tiling structures that are built by a set of tiling elements as building blocks. Though the layer is lacking strict periodicity, it shows up as an ideally-packed tiling of basic elements without any skips or halting. The two-dimensional Tl(x)Bi(1-x) compounds were formed by depositing Bi onto the Tl-covered Si(111) surface where Bi atoms substitute appropriate amount of Tl atoms. Atomic structure of each tiling element as well as arrangement of Tl(x)Bi(1-x) compounds were established in a detail. Electronic properties and spin texture of the selected compounds having periodic structures were characterized. The shown example demonstrates possibility for the formation of the exotic low-dimensional materials via unusual growth mechanisms.

New generation of two-dimensional spintronic systems realized by coupling of Rashba and Dirac fermions.

Intriguing phenomena and novel physics predicted for two-dimensional (2D) systems formed by electrons in Dirac or Rashba states motivate an active search for new materials or combinations of the already revealed ones. Being very promising ingredients in themselves, interplaying Dirac and Rashba systems can provide a base for next generation of spintronics devices, to a considerable extent, by mixing their striking properties or by improving technically significant characteristics of each other. Here, we demonstrate that in BiTeI@PbSb2Te4 composed of a BiTeI trilayer on top of the topological insulator (TI) PbSb2Te4 weakly- and strongly-coupled Dirac-Rashba hybrid systems are realized. The coupling strength depends on both interface hexagonal stacking and trilayer-stacking order. The weakly-coupled system can serve as a prototype to examine, e.g., plasmonic excitations, frictional drag, spin-polarized transport, and charge-spin separation effect in multilayer helical metals. In the strongly-coupled regime, within ~100 meV energy interval of the bulk TI projected bandgap a helical state substituting for the TI surface state appears. This new state is characterized by a larger momentum, similar velocity, and strong localization within BiTeI. We anticipate that our findings pave the way for designing a new type of spintronics devices based on Rashba-Dirac coupled systems.

Electrically tunable in-plane anisotropic magnetoresistance in topological insulator BiSbTeSe2 nanodevices.

We report tunable in-plane anisotropic magnetoresistance (AMR) in nanodevices based on topological insulator BiSbTeSe2 (BSTS) nanoflakes by electric gating. The AMR can be changed continuously from negative to positive when the Fermi level is manipulated to cross the Dirac point by an applied gate electric field. We also discuss effects of the gate electric field, current density, and magnetic field on the in-plane AMR with a simple physical model, which is based on the in-plane magnetic field induced shift of the spin-momentum locked topological two surface states that are coupled through side surfaces and bulk weak antilocalization (WAL). The large, tunable and bipolar in-plane AMR in BSTS devices provides the possibility of fabricating more sensitive logic and magnetic random access memory AMR devices.

A strategy to create spin-split metallic bands on silicon using a dense alloy layer.

To exploit Rashba effect in a 2D electron gas on silicon surface for spin transport, it is necessary to have surface reconstruction with spin-split metallic surface-state bands. However, metals with strong spin-orbit coupling (e.g., Bi, Tl, Sb, Pt) induce reconstructions on silicon with almost exclusively spin-split insulating bands. We propose a strategy to create spin-split metallic bands using a dense 2D alloy layer containing a metal with strong spin-orbit coupling and another metal to modify the surface reconstruction. Here we report two examples, i.e., alloying reconstruction with Na and Tl/Si(111)1 × 1 reconstruction with Pb. The strategy provides a new paradigm for creating metallic surface state bands with various spin textures on silicon and therefore enhances the possibility to integrate fascinating and promising capabilities of spintronics with current semiconductor technology.

Spin texture of Bi2Se3 thin films in the quantum tunneling limit.

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 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.

Experimental evidence of hidden topological surface states in PbBi4Te7.

A topological surface state that is protected physically under the Bi2Te3-like five-layer block has been revealed on the Pb-based topological insulator (TI) PbBi4Te7 by bulk sensitive angle-resolved photoelectron spectroscopy (ARPES). Furthermore, conservation of the spin polarization of the hidden topological surface states is directly confirmed by bulk-sensitive spin ARPES observation. This finding paves the way to realize the real spintronics devices by TIs that are operable in the real environment.

Disentanglement of surface and bulk Rashba spin splittings in noncentrosymmetric BiTeI.

BiTeI has a layered and noncentrosymmetric structure where strong spin-orbit interaction leads to a giant Rashba spin splitting in the bulk bands. We present direct measurements of the bulk band structure obtained with soft x-ray angle-resolved photoemission (ARPES), revealing the three-dimensional Fermi surface. The observed spindle torus shape bears the potential for a topological transition in the bulk by hole doping. Moreover, the bulk electronic structure is clearly disentangled from the two-dimensional surface electronic structure by means of high-resolution and spin-resolved ARPES measurements in the ultraviolet regime. All findings are supported by ab initio calculations.