Properties Of Surface States Research Articles

Optical evidence of surface state suppression in Bi-based topological insulators

A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi$_2$Te$_2$Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon mediated coupling between bulk and surface states suppresses surface conductance as temperature rises.

PH Dependent Behavior and Effects of Photoinduced Surface States during Water Photooxidation at TiO2/Solution Interface: Studied by Capacitance Measurements

By performing photocurrent and capacitance measurements in pH 6–14 solutions with using a nanotubular TiO2 photoanode, we have studied the pH-dependent behavior and properties of photoinduced surface states. Two types of surface states (i.e. the short- and long-lived surface states) were in situ detected at TiO2 surface. It was demonstrated that the density and response potential of these surface states are two essential factors influencing the recombination process and hence determining the overall photoconversion efficiencies for water splitting. With increasing pH, significant decrease in the estimated surface-state densities was observed at pH around 11–12, which was found well inline with the variation with pH both for the measured apparent recombination probabilities and for the maximum photoconversion efficiencies. For effectively inhibiting the occurrence of surface recombination process, and for obtaining maximum photo-conversion efficiencies, the most suitable Schottky barrier heights were revealed to be 0.9 V at pH <11–12 and 0.7 V at pH > 11. Based on the results of this work, some mechanistic aspects for the water photooxidation at TiO2 surface were revealed and discussed.

Nonlocal transport properties of topological insulator F/I/SC/I/F junction with perpendicular magnetization

We investigate theoretically the nonlocal transport properties in a ferromagnet/insulator/superconductor/insulator/ferromagnet (F/I/SC/I/F) junction with perpendicular magnetization formed on a topological insulator. The nonlocal conductance through the junction depends strongly on whether the perpendicular magnetizations of the two ferromagnets are in a parallel or an antiparallel alignment. This stems from the fact that on the surface of three dimensional topological insulator the exchange field acts as vector potential and from spin-momentum locking property of the topological insulator surface states. It is found that the nonlocal conductance as a function of barrier strength of the I regions exhibits a quantum switch on-off property.

Impact of Donor Traps on the 2DEG and Electrical Behavior of AlGaN/GaN MISFETs

As an important step in understanding trap-related mechanisms in AlGaN/GaN transistors, the physical properties of surface states have been analyzed through the study of the transfer characteristics of a MISFET. This letter focused initially on the relationship between donor parameters (concentration and energy level) and electron density in the channel in AlGaN/GaN heterostructures. This analysis was then correlated to dc and pulsed measurements of the transfer characteristics of a MISFET, where the gate bias was found to modulate either the channel density or the donor states. Traps-free and traps-frozen TCAD simulations were performed on an equivalent device to capture the donor behavior. A donor concentration of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$1.14\times 10^{13}~{\rm cm}^{-2}$</tex></formula> with an energy level located 0.2 eV below the conduction band edge gave the best fit to measurements. With the approach described here, we were able to analyze the region of the MISFET that corresponds to the drift region of a conventional HEMT.

Effect of structural modulation and thickness of a graphene overlayer on the binding energy of the Rashba-type surface state of Ir(111)

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.

Observation of Huge Surface Hole Mobility in the Topological InsulatorBi0.91Sb0.09(111)

We report the first direct measurement of transport properties of surface states in the topological insulator Bi(0.91)Sb(0.09) (111) from the weak-field Hall effect and Shubnikov-de Haas oscillations. We find that the holelike surface band displays an unexpectedly high mobility 23,000-85,000 cm(2)/V s, which is the highest mobility so far reported in bismuth-based topological insulators. This result provides the first quantitative assessment of the effect of alloy disorder on the mobility of surface states in topological insulators. We show that the 9% alloy disorder decreases the mobility of surface states by a factor of less than 2.3.

Nonreciprocal photonic surface states in periodic structures of magnetized plasma nanospheres

We report on the occurrence and properties of photonic surface states in periodic structures of magnetized plasma nanospheres by means of rigorous calculations using the full-electrodynamic layer-multiple-scattering method, properly extended to treat gyrotropic spheres with arbitrarily oriented gyration vector. More specifically, dispersion diagrams of Tamm states at the (001) surface of a semi-infinite fcc crystal of plasma nanospheres and of guided modes of a square array of such spheres supported by a quartz substrate, without and under the action of an in-plane static uniform magnetic field, are analyzed and nonreciprocal optical response, which emerges as a result of the simultaneous lack of space-inversion and time-reversal symmetries, is demonstrated in the Voigt geometry.

Interaction and disorder effects in three-dimensional topological insulator thin films

A theory of combined interference and interaction effects on the diffusive transport properties of 3D topological insulator surface states is developed. We focus on a slab geometry (characteristic for most experiments) and show that interactions between the top and bottom surfaces are important at not too high temperatures. We treat the general case of different surfaces (different carrier densities, uncorrelated disorder, arbitrary dielectric environment, etc.). In order to access the low-energy behavior of the system we renormalize the interacting diffusive sigma model in the one loop approximation. It is shown that intersurface interaction is relevant in the renormalization group (RG) sense and the case of decoupled surfaces is therefore unstable. An analysis of the emerging RG flow yields a rather rich behavior. We discuss realistic experimental scenarios and predict a characteristic non-monotonic temperature dependence of the conductivity. In the infrared (low-temperature) limit, the systems flows into a metallic fixed point. At this point, even initially different surfaces have the same transport properties. Investigating topological effects, we present a local expression of the $\mathbb Z_2$ theta term in the sigma model by first deriving the Wess-Zumino-Witten theory for class DIII by means of non-abelian bosonization and then breaking the symmetry down to AII. This allows us to study a response of the system to an external electromagnetic field. Further, we discuss the difference between the system of Dirac fermions on the top and bottom surfaces of a topological insulator slab and its non-topological counterpart in a double-well structure with strong spin-orbit interaction.

The Kondo effect in three-dimensional topological insulators

We investigate the role of magnetic impurities in the transport properties of surface states on a three-dimensional topological insulator. First, we use second-order perturbation theory and the Boltzmann transport equation to calculate the magnetically induced resistivity in a topological insulator. Our result shows a non-perturbative behavior when conducting electrons and magnetic impurities’ spins are antiferromagnetically coupled. The surface resistivity is found to display an oscillatory rather than isotropic behavior compared to the conventional Kondo effect. Both the variational method and renormalization group (RG) analysis are employed to compute the Kondo temperature, through which the non-perturbative behavior is confirmed. We further study the RG flows and demonstrate that the RG trajectories flow into a strong coupling regime if coupling is antiferromagnetic. Our work is motivated by the recent transport experiments with surface currents on topological insulators. Our calculation is qualitatively consistent with the low temperature dip observed in the experimental R–T curve and might be one of the possible origins of the dip.

Helical Fermi arcs and surface states in time-reversal invariant Weyl semimetals

Weyl semimetals are gapless three-dimensional topological materials where two bands touch at an even number of points in the Brillouin zone. In this work we study a zinc-blende lattice model realizing a time-reversal invariant Weyl semimetal. The bulk dynamics is described by 12 helical Weyl nodes. Surface states form a peculiar quasi-two-dimensional helical metal fundamentally different from the Dirac form typical for topological insulators. The allowed direction of velocity and spin of low-energy surface excitations are locked to the cubic symmetry axes. The studied system illustrates the general properties of surface states in systems with common crystal symmetries.

Topological Dangling Bonds with Large Spin Splitting and Enhanced Spin Polarization on the Surfaces of Bi2Se3

We investigate the topological surface state properties at various surface cleaves in the topological insulator Bi2Se3, via first principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS). While the typical surface termination occurs between two quintuple layers, we report the existence of a surface termination within a single quintuple layer where dangling bonds form with giant spin splitting owing to strong spin-orbit coupling. Unlike Rashba split states in a 2D electron gas, these states are constrained by the band topology of the host insulator with topological properties similar to the typical topological surface state, and thereby offer an alternative candidate for spintronics usage. We name these new states "topological dangling-bond states". The degree of the spin polarization of these states is greatly enhanced. Since dangling bonds are more chemically reactive, the observed topological dangling-bond states provide a new avenue for manipulating band dispersions and spin-textures by adsorbed atoms or molecules.

The importance of accurate visibility parameterization during atmospheric correction: impact on boreal forest classification

Observation of the Earth's surface from spaceborne platforms is complicated by the various layers of the Earth's atmosphere that reflect, scatter, and attenuate electromagnetic radiation passing through them, thus influencing (upward or downward) the signal strength recorded at the sensor relative to the true quantity of radiance reflected from the observed surfaces. The magnitude and spatial distribution of atmospheric effects is non-stationary and will vary due to numerous factors. While the effect of these factors cannot be eliminated completely, the understanding of radiative transfer physics, atmospheric states, and electromagnetic wave propagation permits much of these effects to be appropriately modelled and minimized. Such corrections for atmospheric effects permit the extraction of more accurate physical properties of surface materials and states from imagery than if atmospheric effects were ignored. Modelling of atmospheric effects with radiative transfer models, however, requires appropriate parameterization. We explore the sensitivity of the important visibility parameter of the popular Atmospheric and Topographic Correction (ATCOR) model for atmospheric correction over boreal forest land cover. Further, we provide a methodology for estimating reasonable values for the visibility parameter in the event that this information is not readily available. Our sensitivity analyses, performed on Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery from northern Québec and Ontario, rely on both incremental adjustments to the visibility parameter to assess the degree of atmospheric effect removal and the cascading effect on land-cover classification. We build confidence around our measures using a spatial bootstrapping analysis within each of the two images we analyse. Our analysis demonstrates that exceeding a magnitude of error of approximately 2 km in estimating a visibility parameter values can decrease classification accuracy by nearly 10%. Our assessments of the spatial structure of the mitigated atmospheric component within our scenes, testing for complete spatial randomness, clustering of like values, or evenness in value distributions are inconclusive, but hint towards more clustered results with greater magnitudes of parameterization error.

Surface states of InAlN film grown by MOCVD

The surface state properties (such as surface state density, time constant and level position related to the bottom of InAlN conduction band) of Ni/Au/-InAlN Schottky barrier diodes with nearly lattice matched (InN=18%) and tensilely (15%) or compressively (21%) strained InAlN barrier layer were evaluated, by using current-voltage (I-V), frequency-dependent capacitance-voltage (C-V) measurements and atomic force microscope (AFM) images. It was found that, with increasing content of In the surface state density increased and the barrier height of the Schottky contacts decreased, respectively. The C-V curves shifted toward the positive bias values with reducing measured frequencies, which became more apparent with increasing In content. It may be due to the hole emission from the surface states of Ni/Au/-InAlN Schottcky contacts. Atomic force microscope (AFM) images indicated that the InAlN surface became rougher with increasing In content, which may be the main reason for the increased surface state densities.

Mode of Growth of Ultrathin Topological Insulator Bi2Te3 Films on Si (111) Substrates

Layered materials such as graphene, bi-, and multilayer graphene as well as various compounds of topological insulators are currently in the focus of interest due to their extraordinary physical properties related to Dirac surface states. The ability to grow thin films of these complex layered materials is the key to explore their fundamental phenomena giving insights into modern solid-state physics. However, complex materials composed of layers only weakly bonded via van der Waals forces offer unmatched challenges for the deposition of thin epitaxial films. Here, we report on the growth of Bi2Te3 ultrathin films on Si (111) substrates using molecular beam epitaxy. Special emphasis is put on the nucleation phenomena and growth dynamics studied in detail by in situ scanning tunnelling microscopy and high-resolution scanning transmission electron microscopy. The morphology of the Bi2Te3 surface and the structure of the Si(111)/Bi2Te3 interface as well as the formation of threading dislocations and crystal domains are studied at the atomic level. Our data indicate that the film is formed via the nucleation of islands, which float on the substrate; thus, the islands are only weakly bonded to the substrate and rather mobile. Apparently, these floating islands are able to arrange themselves by moving in the x–y direction to perfectly coalesce and form a continuous film. The results present a crucial step toward understanding growth and defect formation in this class of materials and thus pave the avenue to a higher control over both their structural and electronic properties, in order to study the electronic properties of the Dirac surface states.

Diffusion at the surface of topological insulators

We consider the dc transport properties of topological insulator surface states in the presence of uncorrelated point-like disorder, both in the classical and quantum regimes. The dc conductivity of those two-dimensional surface states depends strongly on the amplitude of the hexagonal warping of their Fermi surface. A perturbative analysis of the warping is shown to fail to describe the transport in Bi2Se3 over a broad range of experimentally available Fermi energies, and in Bi2Te3 for the higher Fermi energies. Hence we develop a fully non-perturbative description of these effects. In particular, we find that the dependence of the warping amplitude on the Fermi energy manifests itself in a strong dependence of the diffusion constant on this Fermi energy, leading to several important experimental consequences. Moreover, the combination of a strong warping with an in-plane Zeeman effect leads to an attenuation of conductance fluctuations in contrast to the situation of unwarped Dirac surface states.

Effects of surface modification on the properties of topological surface states in Bi2Se3

The effects of surface modification on the topological surface states in Bi2Se3 have been studied using first-principles calculations. Although surface modification cannot change the topological order, however, the spin polarization, dispersion of topological surface bands and the position of Dirac points are altered. All the studied surface modifications can push Dirac cone upwards effectively. Bi-capped surface can attract the quantum well states into bulk band gap, forming new Dirac cone at M¯ point. Both Bi-capped and Se-removed surface can slightly enhance spin polarization of topological surface states, while flatten the energy dispersion of the topological surface bands above Dirac point.

Photonic surface states in plasmonic crystals of metallic nanoshells

We report on the occurrence and properties of photonic surface states in fcc crystals of metallic nanoshells, by means of full-electrodynamic calculations using the layer-multiple-scattering method, properly extended. Detailed dispersion diagrams of the surface states associated with the $(001)$ and $(111)$ surfaces are calculated for such semi-infinite crystals and corresponding finite slabs, and convergence by increasing the slab thickness is discussed. It is shown that these states can be tuned over a broad frequency range by varying the shell thickness and can be characterized, along high-symmetry directions, according to their symmetry. Absorption in the metallic material limits the propagation length which can, however, be as long as several tens of lattice constants for low-loss metals and relatively broad bands.

Studies on the electronic structures of three-dimensional topological insulators by angle resolved photoemission spectroscopy

Three-dimensional (3D) topological insulators represent a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. The unusual surface states of topological insulators rise from the nontrivial topology of their electronic structures as a result of strong spin-orbital coupling. In this review, we will briefly introduce the concept of topological insulators and the experimental method that can directly probe their unique electronic structure: angle resolved photoemission spectroscopy (ARPES). A few examples are then presented to demonstrate the unique band structures of different families of topological insulators and the unusual properties of the topological surface states. Finally, we will briefly discuss the future development of topological quantum materials.

Laser-Induced Modification of the Surface State and Optical Properties of CdTe Crystals

Optical properties and surface state of semiconductor CdTe crystals subjected to irradiation with nanosecond laser pulses were studied. Ellipsometric parameters Δ and Ψ were measured on two opposite surfaces of (111) oriented CdTe wafers before. The samples were subjected to polishing chemical etching and laser irradiation with energy densities lower and higher than the melting threshold of CdTe. The morphology and structure of CdTe crystals were monitored before and after treatments. Irradiation formed a thin modified surface layer on the both sides of CdTe crystals (Cd- and Te-terminated) with the similar optical constants n and k, respectively. However, the effective thicknesses of such modified layers differed and depended on the chemical and laser treatment conditions and also on the faces of CdTe(111) crystals.

Deconvoluted Si 2p Photoelectron Spectra of Ultra thin SiO2 film with FitXPS method

The main impetus for our research is provided by the growing interest worldwide in ultra thin silicon dioxide on silicon based nano devices. The obvious need for better knowledge in the ultra thin gate silicon dioxides, is motivated both by interests in fundamental research and phenomenology as well as by interests in possible applications, which can be found with better fitting of experimental spectra. The up – and down- spin roles are considered for studying the nano structural properties of bulk, interface and surface states of ultra thin film, down to 2 nm and also appealing to the field of surface science. The obtained results show the above states can be determined and distinguished with spin orientations in FitXPS method.