We demonstrate that topological invariants of band insulators can be derived efficiently from the eigenvalues of the local-orbital (LO) based embedding potential, called also the contact (lead) self-energy. The LO based embedding potential is a bulk quantity. Given the tight-binding Hamiltonian describing the bulk valence and conduction bands, it is constructed straightforwardly from the bulk wave functions satisfying the generalized Bloch condition. When the one-electron energy epsilon is located within a projected bulk band gap at a given planar wave vector k, the embedding potential becomes Hermitian. Its real eigenvalues exhibit distinctly different behavior depending on the topological properties of the valence bands, thus enabling unambiguous identification of bulk topological invariants. We consider the Bernevig-Hughes-Zhang model as an example of a time-reversal invariant topological insulator and tin telluride (SnTe) crystallized in a rock-salt structure as an example of a topological crystalline insulator.

While the quantum spin Hall (QSH) effect and antiferromagnetic order constitute two of the most promising phenomena for embedding basic spintronic concepts into future technologies, almost all of the QSH insulators known to date are non-magnetic. Here, based on tight-binding arguments and first-principles theory, we predict two-dimensional antiferromagnets with honeycomb lattice structure to exhibit the QSH effect due to the combined symmetry of time reversal and spatial inversion. We identify functionalized Sn films as experimentally feasible examples which reveal large band gaps rendering these systems ideal for energy efficient spintronics applications. Remarkably, we discover that tensile strain can tune the magnetic order in these materials, accompanied by a topological phase transition from the QSH to the quantum anomalous Hall phase.

The embedding potential defined on the boundary surface of a semi-infinite crystal relates the value and normal derivative of generalized Bloch states propagating or decaying toward the interior of the crystal. It becomes Hermitian when the electron energy epsilon is located in a projected bulk band gap at a given wave vector k in the surface Brillouin zone (SBZ). If one plots the real eigenvalues of the embedding potential for a time-reversal invariant insulator in the projected bulk band gap along a path epsilon = epsilon(0)(k) passing between two time-reversal invariant momentum (TRIM) points in the SBZ, then, they form Kramers doublets at both end points. We will demonstrate that the Z(2) topological invariant, nu, which is either 0 or 1, depending on the product of time-reversal polarizations at the two TRIM points, can be determined from the two different ways these eigenvalues are connected between the two TRIM points. Furthermore, we will reveal a relation, nu = P mod 2, where P denotes the number of poles that the embedding potential exhibits along the path. We also discuss why gapless surface states crossing the bulk band gap inevitably occur on the surface of topological band insulators from the view point of the embedding theory.

Received 27 May 2014DOI:https://doi.org/10.1103/PhysRevLett.112.239903© 2014 American Physical Society

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.

We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-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.

In this paper, processing of carbon-fiber reinforced plastics (CFRPs) using ps pulsed laser radiation with an average power of up to 80 W is investigated. The influence of average power, scanning speed, and repetition rate on the ablation rate and the width of heat affected zone is evaluated and discussed. Processing with a heat affected zone smaller than 5 μm and ablation rates of almost 100 mm3/min are achieved. Finally, precise cutting of 2 mm thick CFRP samples by repetitive ablation of a 350 μm groove with negligible heat affected zone is demonstrated.

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 moire 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.

We examined by two-dimensional photoemission band mapping the electronic structure of Au films epitaxially grown on an Ag(111) substrate. The very similar structural and electronic properties of the two metals make this system extremely unfavorable for the occurrence and observation of electron confinement effects. At variance with previous spectroscopic studies, we show that the electron reflectivity at the interface sustains the formation of well-defined $sp$-derived quantum well states (QWS) and weak quantum well resonance states (QWRS) in the Au layers. The character and degree of confinement of these states are analyzed and quantitatively related to the Au/Ag interface reflectivity on the basis of density functional theory (DFT) band structure calculations.

Quantum-dot states in graphene nanoribbons (GNR) were calculated using density-functional theory, considering the effect of the electric field of gate electrodes. The field is parallel to the GNR plane and was generated by an inhomogeneous charge sheet placed atop the ribbon. Varying the electric field allowed to observe the development of the GNR states and the formation of localized, quantum-dot-like states in the band gap. The calculation has been performed for armchair GNRs and for armchair ribbons with a zigzag section. For the armchair GNR a static dielectric constant of {\epsilon} approx. 4 could be determined.

This chapter focuses on magnetic and transport properties of deposited on graphene heavy transition-metal adatoms. It shows that most of the 5d transition-metal adatoms display strong magnetism and gigantic values of the magnetocrystalline anisotropy energy. The chapter examines the instabilities of interacting electrons on the honeycomb bilayer by means of the functional renormalization group for a range of interactions up to the third-nearest neighbor. In bulk semi-metallic bismuth, the Fermi surface includes three cigar-shaped electron valleys lying almost perpendicular to the high-symmetry axis known as the trigonal axis. In the last years, the edge states of two-dimensional topological insulators (2D-TIs) attracted considerable interest as they support dissipationless spin-currents. The chapter also examines the effect of spin-orbit coupling on the atomic structure of zigzag Bi(111) and Sb(111) nanoribbons. The protected states localized at surfaces and interfaces of topological insulators are a consequence of the electronic structure of the bulk. Controlled Vocabulary Terms bismuth; fermi surface; honeycomb structures; spin-orbit interactions; transition metal compounds

We present a laser induced ablation process to fabricate ultrathin graphitic flakes. By varying the fluence of the ablating pulsed fs-laser radiation, we identify distinct values for "thermal" evaporation and so-called "non-thermal" ablation of graphitic flakes. The presence of the non-thermal ablation is a direct consequence of the strong asymmetry of the bonding strength in normal and in-plane direction in layered materials, such as graphite. The experimentally extracted non-thermal ablation threshold for graphite of 250 mJ/cm2 agrees well with theoretical predictions. Finally, we deposited ultrathin graphitic flakes of 50 μm2 in size, which we characterize by Raman spectroscopy and scanning force microscopy.

We report on our efforts in design and construction of a compact Extreme Ultraviolet (EUV)-pump-probe microscope. The goal is the observation of formation of nanostructures, induced by a femtosecond (fs)-laser pulse. The unique interaction processes of fs-laser radiation with matter open up new markets in laser material processing and, therefore, are actively investigated in the last decade. The resulting "sub 100 nm"-structures offer vast potential benefits in photonics, biotechnology, tribological surface design, plasmonic applications and production of nanoparticles.Focused fs-laser radiation causes a local modification resulting in nanostructures of high precision and reproducibility. However the formation dynamics is not well understood. Research in this field requires high temporal and spatial resolution. A combination of fs-laser and EUV-microscope provides a tool for "in situ"-observation of the formation dynamics. As exemplary structures to be investigated, we use nanojets on thin gold films and periodic surface structures (ripples) on dielectrics. In the future, the EUV-pump-probe microscope can become a versatile tool to observe physical or biological processes.Microscopy using EUV-light is capable of detecting structures on a scale down to several tens of nanometers. For detailed investigations a compact EUV-microscope has been realized utilizing O-VI Balmer-alpha radiation at 17.3 nm coming from a discharge produced oxygen plasma. As optical elements a grazing incidence elliptical collector and a zone plate with a width of outermost zone of 50 nm and a spectral filter to avoid chromatic aberrations are used. The detector is a fast gated microchannel plate with a pore size of 2 microns contacted by a low impedance transmission line. The expected spatial resolution of the setup is better than 100 nm and the time resolution is better than 1 ns. The newly developed EUV-microscope is a powerful tool for a wide field of investigations that need high time and spatial resolutions simultaneously.

Electronic tunneling through ferroelectric insulators is considered to be a key ingredient of future oxide electronics. We investigate the role of the electronic band structure of the decaying electronic states in the band gap by first discussing the expected behavior of tunneling in the effective mass model. We demonstrate that, even for the simple prototype ferroelectric oxides in the perovskite structures PbTiO3 and BaTiO3, the basic assumption of the effective mass model is not appropriate, and that the correct interpretation of tunneling in these materials requires a material-specific description of the evanescent states as provided by the complex band structure.

We monitored the sp-quantum-well states of Ag films on Pt(111) by angle-resolved photoemission in order to examine the electron transmission across the Ag/Pt interface. For thin layers up to 3.5 nm, the Ag states are characterized by broad quasiparticle peaks and a reversal of the parabolic curvature near the center of the surface Brillouin zone. Remarkable departures from the expected nearly-free-electronlike band dispersion persist in films of more than 14 nm thickness. First-principles calculations and symmetry analysis demonstrate that the observed anomalies in the spectroscopic data can be straightforwardly linked to variations in the Ag/Pt transmission coefficient in the energy-momentum space.

The material modification in the volume of transparent dielectrics using tightly focused fs-laser radiation is an important topic for many research groups all over the world. A wide range of applications like the writing of waveguides, micro-structuring by material modification and subsequent etching, or the micro-welding of glass is based on the localized melting and quenching in a different state.Time-resolved white-light interferometry is adopted for the measurement of the optical phase changes in processed materials. A modified Mach-Zehnder interferometer setup combined with microscope objectives is used. The white light is generated by focusing ultrafast laser radiation (t(p)=80 fs) in a sapphire crystal using a micro-lens array to minimize temporal and spatial fluctuations in the white-light continuum. Lateral and coaxial pump-probe measurements of the phase changes during material processing are performed using one or two coupled ultrafast laser sources at different repetition rates (f(rep)=1kHz-1MHz) or by adopting single pulses. The temporal delay between the pump and the probe can be adjusted in the range tau <= 1.8 mu s in dependence on the repetition rate of the pump radiation.The optical phase shift and therefore the refractive index of the material is calculated from the interference images. The knowledge of the refractive index during the modification process with a temporal resolution in the ps-range and a spatial resolution of several microns leads to a better understanding of the initial processes for the permanent material modifications.

By focusing fs-laser radiation in the volume of a transparent material the refractive index can be changed locally, leading to 3-dimensional waveguiding structures. Waveguides are written in phosphate glass (IOG from Schott) at a depth of 100 μm below the surface. The pulse energy and the scan velocity are varied. For the first time the optical path difference caused by the waveguides and therefore the refractive index distribution of the waveguides and their cross sections are determined using interference microscopy. The optical path difference measured in the written structures and their cross sections is analyzed by a phase-shift algorithm. Thus, the refractive index distribution both along a line perpendicular to the waveguide and in the plane of a cross section is determined. The results are visualized as 2-dimensional graphics. Several regions of opposite sign of the refractive index change are observed in the cross sections of waveguides generated by femtosecond laser pulses. The number and the size of these regions are increasing with increasing pulse energy and decreasing scan velocity.