Surface Local Density Of States Research Articles

Topological superconductivity in helical crystals

We study superconductivity and surface Andreev bound states in helical crystals. We consider the interlayer pairings along the helical hopping and investigate the surface local density of states on the (001) and zigzag surfaces for all the possible irreducible representations. There are three and four irreducible representations exhibiting the zero-energy peaks in the local density of states at the (001) and zigzag surfaces of helical lattices, respectively. By calculating the one-dimensional winging number, we show that these appearances of the zero-energy peaks stem from the surface Andreev bound states.

Topological effects of phonons in GaN and AlGaN: A potential perspective for tuning phonon transport

Tuning thermal transport in semiconductor nanostructures is of great significance for thermal management in information and power electronics. With excellent transport properties, such as ballistic transport, immunity to point defects and disorders, and forbidden backscattering, topological phonon surface states show remarkable potential in addressing this issue. Herein, topological phonon analyses are performed on hexagonal wurtzite gallium nitride (GaN) to check the topological characteristics of phonons. And other nitrides of the same family, i.e., AlN and AlGaN crystals, are also calculated from a topological phonon phase transition perspective. With the aid of first-principle calculations and topological phonon theory, Weyl phonon states, which host surfaces states without backscattering, are investigated for all these materials. The results show that there is no nontrivial topological phonon state in GaN. However, by introducing Al atoms, i.e., in wurtzite type AlN and AlGaN, more than one Weyl phonon point is found, confirmed by obvious topological characteristics, including non-zero integer topological charges, source/sink in Berry curvature distributions, surface local density of states, and surface arcs. As AlN and AlGaN are typical materials in AlGaN/GaN heterostructure based electronics, the existence of topological phonon states in them will benefit thermal management by facilitating the design of one-way interfacial phonon transport without backscattering.

Interface Effects on Epilayer Surface Density of States by Scanning Tunneling Spectroscopy and Density Functional Theory

AbstractSub‐nanometric Ni‐silicide films are self‐assembled by solid phase epitaxy of Ni on a vicinal Si(111) surface. The combination of in situ scanning tunneling microscopy and surface electron diffraction with ex situ X‐ray photoelectron spectroscopy allows us to identify θ‐Ni2Si as the major constituent crystal phase at the overgrown surface. Then scanning tunneling spectroscopy is employed with ab initio electronic structure calculations, to identify the origins of the surface local density of states and use these as a fingerprint for in situ phase identification. The above approach enhances the chemical contrast capability of scanning tunneling microscopy, and offers a useful local probe with atomic resolution for in situ crystal phase identification of ultra‐thin surface structures.

Empirical pseudopotential method with nonspherical passivants for the atomistic study of silicon nanostructures

Atomistic calculations of passivated nanostructures remain difficult due to the computational demands related to the high number of atoms to be typically considered. The empirical pseudopotential method (EPM) offers a good alternative in this sense, but finding trustable pseudopotentials for passivants in this method is still elusive. Following the idea of extracting nonspherically symmetric potentials from density functional theory (DFT) calculations, hydrogen pseudopotentials for silicon passivation are derived here and used to calculate the electronic states of low-dimensional structures within the EPM scheme. The single-particle Schr\odinger equation is solved with the ensuing pseudopotentials for slabs with surfaces on the (111), (110), and (100) planes, as well as for passivated quantum dots and wires of different size. In all cases, the band gap is traced as a function of the sample size, showing good convergence towards the bulk value. For the slabs, the surface local density of states is also calculated and compared successfully to experiments. The derivation of the nonspherical pseudopotentials is based on an analytic formulation of the crystal potential and its connection to a series of DFT calculations, resulting in reliable, highly transferable and first-principles based passivant pseudopotentials to be used with the EPM.

Nematic superconductivity in CuxBi2Se3 : Surface Andreev bound states

We study theoretically the topological surface states (TSSs) and the possible surface Andreev bound states (SABSs) of Cu$_{x}$Bi$_{2}$Se$_{3}$ which is known to be a topological insulator at $x=0$. The superconductivity (SC) pairing of this compound is assumed to have the broken spin-rotation symmetry, similar to that of the A-phase of $^{3}$He as suggested by recent nuclear-magnetic resonance experiments. For both spheroidal and corrugated cylindrical Fermi surfaces with the hexagonal warping terms, we show that the bulk SC gap is rather anisotropic; the minimum of the gap is negligibly small as comparing to the maximum of the gap. This would make the fully-gapped pairing effectively nodal. For a clean system, our results indicate the bulk of this compound to be a topological superconductor with the SABSs appearing inside the bulk SC gap. The zero-energy SABSs which are Majorana fermions, together with the TSSs not gapped by the pairing, produce a zero-energy peak in the surface density of states (SDOS). The SABSs are expected to be stable against short-range nonmagnetic impurities, and the local SDOS is calculated around a nonmagnetic impurity. The relevance of our results to experiments is discussed.

Annealing-Induced Bi Bilayer on Bi2Te3 Investigated via Quasi-Particle-Interference Mapping.

Topological insulators (TIs) are renowned for their exotic topological surface states (TSSs) that reside in the top atomic layers, and hence, detailed knowledge of the surface top atomic layers is of utmost importance. Here we present the remarkable morphology changes of Bi2Te3 surfaces, which have been freshly cleaved in air, upon subsequent systematic annealing in ultrahigh vacuum and the resulting effects on the local and area-averaging electronic properties of the surface states, which are investigated by combining scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and Auger electron spectroscopy (AES) experiments with density functional theory (DFT) calculations. Our findings demonstrate that the annealing induces the formation of a Bi bilayer atop the Bi2Te3 surface. The adlayer results in n-type doping, and the atomic defects act as scattering centers of the TSS electrons. We also investigated the annealing-induced Bi bilayer surface on Bi2Te3 via voltage-dependent quasi-particle-interference (QPI) mapping of the surface local density of states and via comparison with the calculated constant-energy contours and QPI patterns. We observed closed hexagonal patterns in the Fourier transform of real-space QPI maps with secondary outer spikes. DFT calculations attribute these complex QPI patterns to the appearance of a "second" cone due to the surface charge transfer between the Bi bilayer and the Bi2Te3. Annealing in ultrahigh vacuum offers a facile route for tuning of the topological properties and may yield similar results for other topological materials.

Tunneling spectroscopy of a phosphorus impurity atom on the Ge(111)-(2 × 1) surface

We numerically model the Ge(111)-(2 × 1) surface electronic properties in the vicinity of a P donor impurity atom located near the surface. We find a notable increase in the surface local density of states (LDOS) around the surface dopant near the bottom of the empty surface state band π*, which we call a split state due to its limited spatial extent and energetic position inside the band gap. We show that despite the well-established bulk donor impurity energy level position at the very bottom of the conduction band, a surface donor impurity on the Ge(111)-(2 × 1) surface might produce an energy level below the Fermi energy, depending on the impurity atom local environment. It is demonstrated that the impurity located in subsurface atomic layers is visible in a scanning tunneling microscope (STM) experiment on the Ge(111)-(2 × 1) surface. The quasi-1D character of the impurity image, observed in STM experiments, is confirmed by our computer simulations with a note that a few π-bonded dimer rows may be affected by the presence of the impurity atom. We elaborate a model that allows classifying atoms on the experimental low-temperature STM image. We show the presence of spatial oscillations of the LDOS by the density-functional theory method.

Multifractal nature of the surface local density of states in three-dimensional topological insulators with magnetic and nonmagnetic disorder

We compute the multifractal spectra associated to local density of states (LDOS) fluctuations due to weak quenched disorder for a single Dirac fermion in two spatial dimensions. Our results are relevant to the surfaces of ${\mathbb{Z}}_{2}$ topological insulators such as ${\text{Bi}}_{2}{\text{Se}}_{3}$ and ${\text{Bi}}_{2}{\text{Te}}_{3}$, where LDOS modulations can be directly probed via scanning tunneling microscopy. We find a qualitative difference in spectra obtained for magnetic versus nonmagnetic disorder. Randomly polarized magnetic impurities induce quadratic multifractality at first order in the impurity density; by contrast, no operator exhibits multifractal scaling at this order for a nonmagnetic impurity profile. For the time-reversal invariant case, we compute the first nontrivial multifractal correction, which appears at two loops (impurity density squared). We discuss spectral enhancement approaching the Dirac point due to renormalization, and we survey known results for the opposite limit of strong disorder.

Surface edge state and half-quantized Hall conductance in topological insulators

We study the surface local density of states and the transport properties of a three-dimensional (3D) topological insulator (TI) in the presence of a uniform spin-splitting Zeeman field. We find chiral edge states exist on the gapped surfaces of the 3D TI, which can be considered as interface states between domains of massive and massless Dirac fermions. Effectively these states are the result of splitting of a perfect interface conducting channel. This picture is confirmed by the Landauer-B\"uttiker calculations in four-terminal Hall bars made of 3D TIs. It is demonstrated that the difference between the clockwise and counterclockwise transmission coefficients of the two neighboring terminals is approximately one-half, which suggests that the half-quantized Hall conductance can be manifested in an appropriate experimental setup. We also predict that the quantized anomalous Hall effect exists in thin films of TIs where such effective Zeeman felds are present.

Quasi-stationary states of an electron confined in a rectangular quantum corral and STMimages

We elucidated quasi-stationary states of an electron confined in a two-dimensional quantumcorral (QC) built up with double barriers having a finite width. It is shown that theHamiltonian of the QC is not a Hermitian operator for an electron confined in the QC,which results in a complex eigenenergy and the inverse of the imaginary part yields thelifetime of the electron. Considering that the electron confined in a rectangular QCstays in a quasi-stationary state, we obtained expressions for STM images as anexplicit function of both the two-dimensional position and the bias voltage. Fromthose expressions, we clarified relations among the surface local density of states(LDOS), the probability density, topographical images and differential conductance (dI/dV) images. We pointed out the existence of a filtering function that connects thedI/dV image with the LDOS image for the first time. Our results reproducedall experimental STM images observed in topographical anddI/dV modes for a rectangular QC. Moreover, we specified the main components of electron statesthat characterize those STM images.

Model for tunneling-mediated impurity resonances in bilayer cuprate superconductors

We have studied tunneling-mediated local density of states (LDOS) of the surface layer of a bilayer cuprate, where a Zn impurity is located on the second Cu-O layer. When the tunneling strength between two Cu-O layers is larger than a critical value, the LDOS on the site just above the Zn impurity first exhibits a resonant peak near the Fermi surface. The larger the tunneling strength, the stronger the resonant peak. It is also shown that the height of the resonant peak oscillates decreasingly with the distance from the site just above the Zn impurity. The location of the resonant peak in the surface LDOS depends on doping, energy gap, and the tunneling strength and has an opposite bias voltage to that on its nearest-neighboring sites. The results could be tested by the scanning tunnel microscopy experiments and be used to further understand the electronic properties of high-temperature superconductors.

First Principles Study of Cu-Embedded Ni(110) Surfaces

The atomic geometries and electronic band structures of Cu-embedded Ni(110) surfaces were studied theoretically. First principles calculation with spin-resolved local density functional theory was applied to the Ni(110) surface, and one to four Cu-embedded Ni(110) surfaces in the 2×2 surface unit cell. The optimized structures for the Ni(110) surface showed that interlayer spacing between the first and the second layer shrunk 11.1% of the bulk distance, but the second and third layer spacing expanded by 2.9% in agreement with previous studies. For the Cu-embedded surfaces, embedded Cu atoms are 11-14 pm higher than the top Ni atoms, which is about four times larger than the difference of the atomic radii. The detailed band structures and corresponding local density of states (LDOS) were calculated. At the embedded Cu atom sites, the surface LDOS near the Fermi level was reduced in agreement with previous scanning tunneling microscopy observation. In addition, by increasing the number of Cu atoms in the top layer, magnetic moments were decreased near the surface regions. [DOI: 10.1380/ejssnt.2009.681]

Neutralization of Li + ions scattered by the Cu (1 0 0) and (1 1 1) surfaces: A localized picture of the atom-surface interaction

Large and face dependent neutral fractions have been found recently in the scattering of Li + by Cu(1 0 0) and Cu(1 1 1) surfaces. These results for high work function surfaces are unexpected within the ‘traditional’ picture of a Li + ion departing from a jellium surface model. In the present work the Li +/Cu(1 0 0) and Li +/Cu(1 1 1) interacting systems are described by a previously developed bond-pair model based on the localized interactions between the projectile ion and the atoms of the surface, and on the extended features of the electronic band structure through the surface local density of states. By only including the resonant neutralization to the Li atom ground state we explained the face and energy dependences of the measured neutral fractions for large outgoing energy values. We found that the downward shift of the Li ionization level below the Fermi level caused by the short range chemical interactions, is the main responsible of a high neutralization by the resonant mechanism. The remaining differences between theory and experiment values can be explained in terms of the energy gaps and image potential states appearing in these surfaces. The calculated distance behaviours of the energy levels corresponding to the first excited (Li-1s 22p) and the negative (Li-1s 22s 2) atomic configurations indicate that they can also participate in the ion-surface charge exchange process.

Mixed layer formation on the copper-deposited Ni(110) surface

We studied initial Cu overlayer formation on the Ni(110) surface by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Initially, deposited Cu displaces the top Ni layer, forming two-dimensional Cu-Ni alloy on the substrate. The Cu atom embedded into the top layer was depressed in the STM image. The depression due to the lack of $\text{Ni}\text{ }3d$-derived surface local density of states was confirmed by the first-principles calculation. Ni atoms squeezed out from the top substrate layer agglomerated in the anisotropic Ni island along the close-packed $[1\overline{1}0]$ direction. Further Cu deposition resulted in a Cu-Ni mixed island. Quantitative measurement of the Cu fraction on the substrate showed that $0.71\ifmmode\pm\else\textpm\fi{}0.04$ of deposited Cu was embedded in the top Ni layer, whereas significant Cu enrichment was seen on the island.

Analysis of Scanning Tunneling Microscopy Images and Scanning Tunneling Spectrum of Electrons Confined in Equilateral Triangular Quantum Corrals

We determined the stationary states of an electron confined in an equilateral triangular quantum corral by the superposition of electron waves undergoing successive reflections at three barrier walls. By using wave functions and eigenenergies, we analyzed scanning tunneling microscopy (STM) images and scanning tunneling spectra for the quantum corral. Our results are consistent with experimental STM images observed in the differential conductance mode (d I /d V ) and scanning tunneling spectra. It is pointed out that the d I /d V image is not simply proportional to the surface local density of states but is greatly dependent on a filtering function.

Theory of surface chemical reactivity

Fundamental reactivity concepts of relevance to the reactivity of transition metal surfaces are reviewed using elementary quantum-chemical concepts. The Newns-Anderson tight binding model of chemisorption is presented and subsequently used to outline the electronic structure characteristics of weak versus strong chemisorption. Fundamental concepts such as electron localization and surface complex embedding energies are defined and used to help explain surface reactivity. The emphasis here is on establishing an understanding of the surface chemical bond as a function of adatom coordination number, degree of coordinative unsaturation of the surface atoms and electron occupation of the d-type valence electron band. We derive from formal chemisorption theory the important relationships that exist between measured chemisorption properties and the average position of the d-valence electron band. The Newns-Anderson model is also used to show the relationship that exists between the d-band center and the coordinative unsaturation of the metal surface atoms. The general conclusion is that for Group VIII metals the shift of the average energy of the surface local density of states correlates with the strength of the interaction of the surface atoms with the metal atoms next to the surface layer. The same model is then used to analyze the Shustorovich bond order conservation model. The BOC or its modern version UBI-QEP is found to be consistent with a surface interaction potential comprised of a two-body repulsive term along with a constant attractive interaction independent of the number of coordinating atoms. The concepts of weak and strong chemisorption provide a very good basis for the subsequent analysis of the Bronsted-Eyring-Polanyi (BEP) relation. The extreme values of the BEP proportionality constant are related to the concept of loose and tight transition states. This proportionality constant between transition state energy and reaction energy can be expressed in parameters from the Newns-Anderson model by identifying loose transition states with intermediates in which the bond to be activated has not yet been broken, whereas in tight transition states this bond can be considered to be broken. We conclude the paper with an analysis of surface reconstruction. The power of the surface-molecule complex view of chemisorption will be quite apparent. The paper has an extensive introductory section to relate the topics of the four sections that follow with important classical catalytic notions.

Scanning Tunneling Spectroscopy on Pr$_{0.68}$Pb$_{0.32}$MnO$_{3}$ Single Crystals

Scanning tunneling microscopy/spectroscopy studies were carried out on single crystals of colossal magnetoresistive manganite Pr0.68Pb0.32MnO3 at different temperatures in order to probe their spatial homogeneity across the metal-insulator transition temperature TM-I(~255K). A metallic behavior of the local conductance was observed for temperatures T<TM-I. Zero bias conductance (dI/dV)V=0, which is directly proportional to the local surface density of states at the Fermi level, shows a single distribution at temperatures T< 200 K suggesting a homogeneous electronic phase at low temperatures. In a narrow temperature window of 200 K <T<TM-I, however, an inhomogeneous distribution of (dI/dV)V=0 has been observed. This result gives evidence for phase separation in the transition region in this compound

Local density of states of atoms interacting with finite-bandwidth surfaces: Spin-statistics effects

The density of states on atoms interacting with solid surfaces is a required physical quantity for the understanding of processes related to scanning tunneling and photoemission spectroscopy, single-atom conductance, and emission and scattering of atoms from surfaces. In this work, we present a model calculation that allows including the localized aspects of atomic interactions and the extended features of the surface, together with alternative treatments of the Coulomb repulsion terms in the atom site. The effects of the spin fluctuation statistics treated up to a second order in the atom-surface coupling term are especially explored in this case. This approximation is comparatively analyzed with the exact results available in a model system of four levels and then used in the description of hydrogen interacting with an Al surface. Effects due to finite bandwidth and energy dependence of the local surface density of states are discussed.

Structure and composition of the segregated Cu in V 2O 5/Cu system

We have investigated segregation of copper at the surface of V 2O 5 films deposited onto Cu substrate by employing surface analysis techniques. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) confirmed that the Cu is segregated at the surface and its chemical state is Cu 2O. According to secondary ion mass spectroscopy (SIMS) and glow discharge spectroscopy (GDS), the Cu concentration inside the deposited V 2O 5 layer is low. Ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS) revealed the segregation alters the surface local density of states. Surface analysis of deposited samples in ultra high vacuum (UHV) condition verified that the segregation occurs during the deposition. We have extended kinetic tight binding Ising model (KTBIM) to explain the surface segregation during the deposition. Simulation data approve the possibility of surface segregation during room temperature deposition. These results point out that on pure Cu substrate, oxidation occurs during the segregation and low surface energy of Cu 2O is the original cause of the segregation.

Barrier height imaging of Si(1 1 1)3 × 1–Ag reconstructed surfaces

We investigated the bias voltage polarity dependence of atomically resolved barrier height (BH) images on Si(111)3×1–Ag surfaces. The BH images were very similar to scanning tunneling microscopy (STM) images in both the empty and filled states. This similarity strongly supports the interpretation that the BH image reflects the vertical decay rate of the surface local density of states (LDOS). Differences in contrast and protrusion shapes between BH and STM images were observed. We attributed these differences to the geometric contribution to the STM image and to the improved spatial resolution of the BH image due to the lock-in technique.