Direct observation of standing wave formation at surface steps using scanning tunneling spectroscopy

Quantifying many-body effects by high-resolution Fourier transform scanning tunneling spectroscopy

High-resolution Fourier transform scanning tunneling spectroscopy (FT-STS) is used to study many-body effects on the surface state of Ag(111). Our results reveal a kink in the otherwise parabolic band dispersion of the surface electrons and an increase in the quasiparticle lifetime near the Fermi energy Ef. The experimental data are accurately modeled with the T-matrix formalism for scattering from a single impurity, assuming that the surface electrons are dressed by the electron-electron and electron-phonon interactions. We confirm the latter as the interaction responsible for the deviations from bare dispersion. We further show how FT-STS can be used to simultaneously extract real and imaginary parts of the self-energy for both occupied and unoccupied states with a momentum and energy resolution competitive with angle-resolved photoemission spectroscopy. From our quantitative analysis of the data we extract a Debye energy of ℏΩD=14±1  meV and an electron-phonon coupling strength of λ=0.13±0.02, consistent with previous results. This proof-of-principle measurement advances FT-STS as a method for probing many body effects, which give rise to a rich array of material properties.

The k-space origins of scattering in Bi2Sr2CaCu2O8+x

We demonstrate a general, computer automated procedure that inverts the reciprocal space scattering data (q-space) that are measured by spectroscopic imaging scanning tunnelling microscopy (SI-STM) in order to determine the momentum space (k-space) scattering structure. This allows a detailed examination of the k-space origins of the quasiparticle interference (QPI) pattern in Bi2Sr2CaCu2O8+x within the theoretical constraints of the joint density of states (JDOS). Our new method allows measurement of the differences between the positive and negative energy dispersions, the gap structure and an energy dependent scattering length scale. Furthermore, it resolves the transition between the dispersive QPI and the checkerboard ([Formula: see text] excitation). We have measured the k-space scattering structure over a wide range of doping (p ∼ 0.22-0.08), including regions where the octet model is not applicable. Our technique allows the complete mapping of the k-space scattering origins of the spatial excitations in Bi2Sr2CaCu2O8+x, which allows for better comparisons between SI-STM and other experimental probes of the band structure. By applying our new technique to such a heavily studied compound, we can validate our new general approach for determining the k-space scattering origins from SI-STM data.

From the bottom up: dimensional control and characterization in molecular monolayers

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.

Atomic-scale evidence for potential barriers and strong carrier scattering at graphene grain boundaries: a scanning tunneling microscopy study

We use scanning tunneling microscopy and spectroscopy to examine the electronic nature of grain boundaries (GBs) in polycrystalline graphene grown by chemical vapor deposition (CVD) on Cu foil and transferred to SiO(2) substrates. We find no preferential orientation angle between grains, and the GBs are continuous across graphene wrinkles and SiO(2) topography. Scanning tunneling spectroscopy shows enhanced empty states tunneling conductance for most of the GBs and a shift toward more n-type behavior compared to the bulk of the graphene. We also observe standing wave patterns adjacent to GBs propagating in a zigzag direction with a decay length of ~1 nm. Fourier analysis of these patterns indicates that backscattering and intervalley scattering are the dominant mechanisms responsible for the mobility reduction in the presence of GBs in CVD-grown graphene.

Boron nitride on Cu(111): an electronically corrugated monolayer

Ultrathin films of boron nitride (BN) have recently attracted considerable interest given their successful incorporation in graphene nanodevices and their use as spacer layers to electronically decouple and order functional adsorbates. Here, we introduce a BN monolayer grown by chemical vapor deposition of borazine on a single crystal Cu support, representing a model system for an electronically patterned but topographically smooth substrate. Scanning tunneling microscopy and spectroscopy experiments evidence a weak bonding of the single BN sheet to Cu, preserving the insulating character of bulk hexagonal boron nitride, combined with a periodic lateral variation of the local work function and the surface potential. Complementary density functional theory calculations reveal a varying registry of the BN relative to the Cu lattice as origin of this electronic Moiré-like superstructure.

Potential steps at C₆₀-TiOPc-Ag(111) interfaces: ultrahigh-vacuum-noncontact scanning probe metrology

Nanoscale structure-electric potential relations in films of the organic molecular semiconductors C(60) and titanyl phthalocyanine (TiOPc) on Ag(111) have been measured under UHV conditions. Noncontact force methods were utilized to image domain structures and boundaries with molecular resolution, while simultaneously quantifying the local surface electric potential. Sensitivity and spatial resolution for the local potential measurement were first established on Ag(111) through direct observation of the electrical dipole and potential step, φ(step) = 10 ± 3 mV, of monatomic crystallographic steps. A local surface potential increase of 27 ± 11 mV occurs upon crossing the boundary between the neat Ag(111) surface and C(60) islands. Potential steps in binary C(60)-TiOPc films, nanophase-separated into crystalline C(60) and TiOPc domains, were then mapped quantitatively. The 207 ± 66 mV potential step across the C(60)-to-TiOPc domain boundary exhibits a 3.6 nm width that reflects the spatial resolution for electric potential across a material interface. The absence of potential asymmetry across this lateral interface sets the upper bound for the C(60)-TiOPc interface dipole moment as 0.012 e nm.

STM imaging of impurity resonances on Bi2Se3

In this Letter we present detailed study of the density of states near defects in Bi2Se3. In particular, we present data on the commonly found triangular defects in this system. While we do not find any measurable quasiparticle scattering interference effects, we do find localized resonances, which can be well fitted by theory [R. R. Biswas and A. V. Balatsky, Phys. Rev. B 81, 233405(R) (2010)] once the potential is taken to be extended to properly account for the observed defects. The data together with the fits confirm that while the local density of states around the Dirac point of the electronic spectrum at the surface is significantly disrupted near the impurity by the creation of low-energy resonance state, the Dirac point is not locally destroyed. We discuss our results in terms of the expected protected surface state of topological insulators.

Quasiparticle scattering off phase boundaries in epitaxial graphene

We investigate the electronic structure of terraces of single layer graphene (SLG) by scanning tunnelling microscopy (STM) on samples grown by thermal decomposition of 6H-SiC(0001) crystals in ultra-high vacuum. We focus on the perturbations of the local density of states (LDOS) in the vicinity of edges of SLG terraces. Armchair edges are found to favour intervalley quasiparticle scattering, leading to the (√3 x √3)R30° LDOS superstructure already reported for graphite edges and more recently for SLG on SiC(0001). Using the Fourier transform of LDOS images, we demonstrate that the intrinsic doping of SLG is responsible for a LDOS pattern at the Fermi energy which is more complex than for neutral graphene or graphite, since it combines local (√3 x √3)R30° superstructure and long range beating modulation. Although these features have already been reported by Yang et al (2010 Nano Lett. 10 943-7) we propose here an alternative interpretation based on simple arguments classically used to describe standing wave patterns in standard two-dimensional systems. Finally, we discuss the absence of intervalley scattering off other typical boundaries: zig-zag edges and SLG/bilayer graphene junctions.

Long-wavelength local density of states oscillations near graphene step edges

Using scanning tunneling microscopy and spectroscopy, we have studied the local density of states (LDOS) of graphene over step edges in boron nitride. Long-wavelength oscillations in the LDOS are observed with maxima parallel to the step edge. Their wavelength and amplitude are controlled by the energy of the quasiparticles allowing a direct probe of the graphene dispersion relation. We also observe a faster decay of the LDOS oscillations away from the step edge than in conventional metals. This is due to the chiral nature of the Dirac fermions in graphene.

Formation of unconventional standing waves at graphene edges by valley mixing and pseudospin rotation

We investigate the roles of the pseudospin and the valley degeneracy in electron scattering at graphene edges. It is found that they are strongly correlated with charge density modulations of short-wavelength oscillations and slowly decaying beat patterns in the electronic density profile. Theoretical analyses using nearest-neighbor tight-binding methods and first-principles density-functional theory calculations agree well with our experimental data from scanning tunneling microscopy. The armchair edge shows almost perfect intervalley scattering with pseudospin invariance regardless of the presence of the hydrogen atom at the edge, whereas the zigzag edge only allows for intravalley scattering with the change in the pseudospin orientation. The effect of structural defects at the graphene edges is also discussed.

Do two-dimensional "noble gas atoms" produce molecular honeycombs at a metal surface?

Anthraquinone self-assembles on Cu(111) into a giant honeycomb network with exactly three molecules on each side. Here we propose that the exceptional degree of order achieved in this system can be explained as a consequence of the confinement of substrate electrons in the pores, with the pore size tailored so that the confined electrons can adopt a noble-gas-like two-dimensional quasi-atom configuration with two filled shells. Formation of identical pores in a related adsorption system (at different overall periodicity due to the different molecule size) corroborates this concept. A combination of photoemission spectroscopy with density functional theory computations (including van der Waals interactions) of adsorbate-substrate interactions allows quantum mechanical modeling of the spectra of the resultant quasi-atoms and their energetics.

Electron states confined within nano-steps on metal surfaces

To elucidate electron states confined within steps on metal surfaces, we demonstrated a new point of view that a linear step on a noble metal surface can be treated as a dipole potential composed of delta functions. For an electron confined by two pairs of dipole potentials, we derived quasi-stationary eigenstates whose eigenenergies are complex numbers which lead to the lifetime of the electron. To derive the local density of states (LDOS), scanning tunneling microscopy (STM) images and scanning tunneling spectra (STS) for stepped surfaces, we incorporated the lifetime effect on them and clarified the relation between the LDOS and the STM current by applying the expression for the STM current derived by Selloni et al (1985 Phys. Rev. B 31 2602). Although, in previous studies, the Fabry-Pérot interference mechanism has been used to explain electron states confined within two steps, it requires four fitting parameters, in contrast our method requires one fitting parameter which specifies the height of the delta functions. Our results for LDOS images, topographical images and STS are consistent with experimental ones for both the cases where electrons stay on a terrace confined by two steps and on a wide terrace outside the step, which confirms the validity of our model.

Electron standing waves on the GaN(0001)-pseudo (1 × 1) surface: a FT-STM study at room temperature

We report the direct imaging of standing waves on a GaN(0001)-pseudo (1 × 1) metallic surface, which consists of two atomic Ga layers with the top layer incommensurate. Two types of periodic oscillation are observed by scanning tunneling microscopy at room temperature. The longer wavelength standing waves are due to electron scattering by dislocation-induced steps and two-dimensional InN islands. The localized shorter wavelength waves are attributed to a structural transition of the incommensurate Ga bilayer to a tetrahedral Ga bilayer after the growth of the InN islands.

A versatile high resolution scanning tunneling potentiometry implementation

We have developed a new scanning tunneling potentiometry technique which can-with only minor changes of the electronic setup-be easily added to any standard scanning tunneling microscope (STM). This extension can be combined with common STM techniques such as constant current imaging or scanning tunneling spectroscopy. It is capable of performing measurements of the electrochemical potential with microvolt resolution. Two examples demonstrate the versatile application. First of all, we have determined local variations of the electrochemical potential due to charge transport of biased samples down to angstrom length scales. Second, with tip and sample at different temperatures we investigated the locally varying thermovoltage occurring at the tunneling junction. Aside from its use in determining the chemical identity of substances at the sample surface our method provides a controlled way to eliminate the influence of laterally varying thermovoltages on low-bias constant current topographies.

Pinning of adsorbed cobalt atoms by defects embedded in the copper (111) surface

Using a low-temperature scanning tunneling microscope (STM), we observe that Co adatoms are unusually strongly bound to a particular type of pinning centers on the Cu(111) surface. Using density-functional-theory calculations, the pinning centers are identified as Ag substitutional atoms embedded in the topmost atomic layer of the surface. These impurities are hardly detectable in the STM images as they have low topographic height and produce no standing-wave patterns. They do not affect the exchange coupling of the Co adsorbate with the substrate electrons; thus, the Kondo resonances measured on pinned and free Co adatoms show no detectable differences. Whereas free Co adatoms undergo significant surface diffusion already above 8 K, Ag-stabilized Co adatoms remain pinned up to 12.7 K.

Quantum interference channeling at graphene edges

Electron scattering at graphene edges is expected to make a crucial contribution to the electron transport in graphene nanodevices by producing quantum interferences. Atomic-scale scanning tunneling microscopy (STM) topographies of different edge structures of monolayer graphene show that the localization of the electronic density of states along the C-C bonds, a property unique to monolayer graphene, results in quantum interference patterns along the graphene carbon bond network, whose shapes depend only on the edge structure and not on the electron energy.

STM imaging of electronic waves on the surface of Bi2Te3: topologically protected surface states and hexagonal warping effects

Scanning tunneling spectroscopy studies on high-quality Bi2Te3 crystals exhibit perfect correspondence to angle-resolved photoemission spectroscopy data, hence enabling identification of different regimes measured in the local density of states (LDOS). Oscillations of LDOS near a step are analyzed. Within the main part of the surface band oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes concave, oscillations appear, dispersing with a wave vector that may result from a hexagonal warping term.

Dislocation-induced local modulation of the surface states of Ag(111) thin films on Si(111) 7 x 7 substrates

Local modulation of the Shockley-type surface state was studied around threading dislocations at the surfaces of ultrathin Ag(111) epitaxial films on Si(111) substrates. Scanning tunneling microscope (STM) observations indicated that the wavelength of the surface state electron was shortened around the dislocations in the electron standing wave pattern. Scanning tunneling spectroscopy (STS) revealed that the bottom of the local surface state (E0) shifts downward around the dislocation. The shift in E0 and the lattice displacement Delta u(z) have a linear relation, which indicates that the shift of the surface state is caused by local relaxation of the misfit strain around the dislocation.

Experimental demonstration of topological surface states protected by time-reversal symmetry

We report direct imaging of standing waves of the nontrivial surface states of topological insulator Bi2Te3 using a scanning tunneling microscope. The interference fringes are caused by the scattering of the topological states off Ag impurities and step edges on the Bi2Te3(111) surface. By studying the voltage-dependent standing wave patterns, we determine the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. We further show that, very different from the conventional surface states, backscattering of the topological states by nonmagnetic impurities is completely suppressed. The absence of backscattering is a spectacular manifestation of the time-reversal symmetry, which offers a direct proof of the topological nature of the surface states.

Friedel oscillations in microwave billiards

Friedel oscillations of electron densities near step edges have an analog in microwave billiards. A random plane-wave model, normally only appropriate for the eigenfunctions of a purely chaotic system, can be applied and is tested for non-purely-chaotic dynamical systems with measurements on pseudointegrable and mixed dynamics geometries. It is found that the oscillations in the pseudointegrable microwave cavity match the random plane-wave modeling. Separating the chaotic from the regular states for the mixed system requires incorporating an appropriate phase-space projection into the modeling in multiple ways for good agreement with experiment.

Self-patterned molecular photoswitching in nanoscale surface assemblies

Photomechanical switching (photoisomerization) of molecules at a surface is found to strongly depend on molecule-molecule interactions and molecule-surface orientation. Scanning tunneling microscopy was used to image photoswitching behavior in the single-molecule limit of tetra-tert-butyl-azobenzene molecules adsorbed onto Au(111) at 30 K. Photoswitching behavior varied strongly with surface molecular island structure, and self-patterned stripes of switching and nonswitching regions were observed having approximately 10 nm pitch. These findings can be summarized into photoswitching selection rules that highlight the important role played by a molecule's nanoscale environment in determining its switching properties.

Direct evidence for the effect of quantum confinement of surface-state electrons on atomic diffusion

We report on the direct observations of the effect of quantum confinement of surface-state electrons on atomic diffusion. Confined electronic states induced by open nanoscale resonators [consisting of two parallel monatomic Cu chains on Cu(111)] are studied by means of scanning tunneling microscope measurements and first-principles calculations. Strongly anisotropic diffusion of adatoms around and inside resonators is revealed at low temperatures. The formation of diffusion channels and empty zones is demonstrated. We show that it is possible to engineer atomic diffusion by varying the distance between the resonator walls.

Quasiparticle chirality in epitaxial graphene probed at the nanometer scale

Graphene exhibits unconventional two-dimensional electronic properties resulting from the symmetry of its quasiparticles, which leads to the concepts of pseudospin and electronic chirality. Here, we report that scanning tunneling microscopy can be used to probe these unique symmetry properties at the nanometer scale. They are reflected in the quantum interference pattern resulting from elastic scattering off impurities, and they can be directly read from its fast Fourier transform. Our data, complemented by theoretical calculations, demonstrate that the pseudospin and the electronic chirality in epitaxial graphene on SiC(0001) correspond to the ones predicted for ideal graphene.

Room-temperature self-assembly of equilateral triangular clusters via Friedel oscillations

We report on the formation of equilateral triangular clusters hollow inside with 5-6 atoms per side, self-assembled on Ni adislands grown on Rh(111). The observation of standing wave patterns on the Ni adislands and the Rh(111) indicates that the self-assembly is mediated by Friedel oscillations. In this context, we propose a model based on the energy of interaction between adsorbates, which explains the formation of the clusters as a result of the assembly of rows of 5-6 adatoms.

Imaging confined electrons with plasmonic light

Variations of the spectra of plasmonic light emitted from the junction of a scanning tunneling microscope have been observed for different lateral positions of the scanning tunneling microscope tip on a Au(111) surface. Subnanometer spatial variations of the light emission intensity over a triangular island and in the vicinity of surface step edges have been recorded at different photon energies. They reveal surface standing wave patterns characteristic for two-dimensional confined electrons.

Formation of dispersive hybrid bands at an organic-metal interface

An electronic band with quasi-one-dimensional dispersion is found at the interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a Au(111) surface. Combined local spectroscopy and numerical calculations show that the band results from a complex mixing of metal and molecular states. The molecular layer folds the underlying metal states and mixes with them selectively, through the TTF component, giving rise to anisotropic hybrid bands. Our results suggest that, by tuning the components of such molecular layers, the dimensionality and dispersion of organic-metal interface states can be engineered.

Functional and spectroscopic measurements with scanning tunneling microscopy

Invented as a surface analytical technique capable of imaging individual atoms and molecules in real space, scanning tunneling microscopy (STM) has developed and advanced into a technique able to measure a variety of structural, functional, and spectroscopic properties and relationships at the single-molecule level. Here, we review basic STM operation and image interpretation, techniques developed to manipulate single atoms and molecules with the STM to measure functional properties of surfaces, local spectroscopies used to characterize atoms and molecules at the single-molecule level, and surface perturbations affecting surface coverage and surface reactions. Each section focuses on determining the identity and function of chemical species so as to elucidate information beyond topography with STM.

Standing Friedel waves: a quantum probe of electronic states in nanoscale devices

We report a theoretical study of the dynamic response of electrons in a metallic nanowire or a two-dimensional electron gas under a capacitively coupled "spot gate" driven by an ac voltage. A dynamic standing Friedel wave (SFW) is formed near the spot gate and near edges and boundaries, analogous to the static Friedel oscillations near defects. The SFW wavelength is controlled by the ac voltage frequency and the device's Fermi velocity, whereby the latter can be measured. In addition, the SFW amplitude exhibits resonant behavior at driving frequencies that are related to eigenenergy spacings in the device, allowing their direct measurement.

Nanopatterning the electronic properties of gold surfaces with self-organized superlattices of metallic nanostructures

The self-organized growth of nanostructures on surfaces could offer many advantages in the development of new catalysts, electronic devices and magnetic data-storage media. The local density of electronic states on the surface at the relevant energy scale strongly influences chemical reactivity, as does the shape of the nanoparticles. The electronic properties of surfaces also influence the growth and decay of nanostructures such as dimers, chains and superlattices of atoms or noble metal islands. Controlling these properties on length scales shorter than the diffusion lengths of the electrons and spins (some tens of nanometres for metals) is a major goal in electronics and spintronics. However, to date, there have been few studies of the electronic properties of self-organized nanostructures. Here we report the self-organized growth of macroscopic superlattices of Ag or Cu nanostructures on Au vicinal surfaces, and demonstrate that the electronic properties of these systems depend on the balance between the confinement and the perturbation of the surface states caused by the steps and the nanostructures' superlattice. We also show that the local density of states can be modified in a controlled way by adjusting simple parameters such as the type of metal deposited and the degree of coverage.

Electronic structure of epitaxial graphene layers on SiC: effect of the substrate

A strong substrate-graphite bond is found in the first all-carbon layer by density functional theory calculations and x-ray diffraction for few graphene layers grown epitaxially on SiC. This first layer is devoid of graphene electronic properties and acts as a buffer layer. The graphene nature of the film is recovered by the second carbon layer grown on both the (0001) and (0001[over]) 4H-SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. Hence the graphene is doped and a gap opens at the Dirac point after three Bernal stacked carbon layers are formed.

Observing spin polarization of individual magnetic adatoms

We have used spin-polarized scanning tunneling spectroscopy to observe the spin polarization state of individual Fe and Cr atoms adsorbed onto Co nanoislands. These magnetic adatoms exhibit stationary out-of-plane spin polarization, but have opposite sign of the exchange coupling between electron states of the adatom and the Co island surface state: Fe adatoms exhibit parallel spin polarization to the Co surface state while Cr adatoms exhibit antiparallel spin polarization. First-principles calculations predict ferromagnetic and antiferromagnetic alignment of the spin moment for individual Fe and Cr adatoms on a Co film, respectively, implying negative spin polarization for Fe and Cr adatoms over the energy range of the Co surface state.

Magnon coherent conductance via atomic nanocontacts

A calculation for the coherent scattering and conductance of magnons via atomic nanocontacts is presented. The model system is composed of two groups of semi-infinite magnetically ordered Heisenberg monatomic chains, joined together by the magnetic nanocontact, and the system is supported on a non-magnetic substrate and considered otherwise free from magnetic interactions. The coherent transmission and reflection coefficients are derived as elements of a Landauer-type scattering matrix. Transmission and reflection scattering cross sections are calculated specifically for three distinct symmetric and asymmetric geometric configurations of the nanocontact. Three cases of local magnetic exchange on the nanocontact domain are analysed for each configuration to investigate the influence of softening and hardening of the magnetic boundary conditions. In analogy with coherent electronic transport, we calculate the magnon coherent transport. The numerical results show the interference effects between the incident scattered magnons and the localized spin states on the nanocontact, with characteristic Fano resonances. The numerical results yield an understanding of the relationship between the coherent magnon conductance and the architecture of the embedded magnetic nanocontact.

Spin-resolved electronic structure of nanoscale cobalt islands on Cu(111)

Using spin-polarized scanning tunneling spectroscopy, we reveal how the standing wave patterns of confined surface state electrons on top of nanometer-scale ferromagnetic Co islands on Cu(111) are affected by the spin character of the responsible state, thus experimentally confirming a very recent theoretical result. Furthermore, at the rim of the islands a spin-polarized state is found giving rise to enhanced zero bias conductance. Its polarization is opposite to that of the islands. The experimental findings are in accordance with ab initio spin-density calculations.

Bulk electronic structure of metals resolved with scanning tunneling microscopy

We demonstrate that bulk band structure can have a strong influence in scanning tunneling microscopy measurements by resolving electronic interference patterns associated with scattering phenomena of bulk states at a metal surface and reconstructing the bulk band topology. Our data reveal that bulk information can be detected because states at the edge of the surface-projected bulk band have a predominant role on the scattering patterns. With the aid of density functional calculations, we associate this effect with an intrinsic increase in the projected density of states of edge states. This enhancement is characteristic of the three-dimensional bulk band curvature, a phenomenon analog to a van Hove singularity.

Electrostatic potential screened by a two-dimensional electron system: a real-space observation by scanning-tunneling spectroscopy

Scanning-tunneling spectroscopy at 5 K was used to investigate the electrostatic potential profile on the Si(111)-square root of 3 x square root of 3 Ag surface at subnanometer spatial resolution. The potential was measured from an energy-level shift of electronic states on the surface. The potential images obtained reveal that the potential drops around the steps and Ag adsorbates, upon which positive charges are presumably accumulated. The profiles of the reduced potentials are explained with the screening of potential due to the charges by two-dimensional electron gas (2DEG) existing on the surface. The Friedel oscillation, which results from the screening and has a period of the half Fermi wavelength of the 2DEG, was also observed in the potential images.

Evidence of hole-electron quasiparticle interference in ErSi2 semimetal by Fourier-transform scanning tunneling spectroscopy

The semimetallic ErSi2 layer grown on Si(111) substrates provides an ideally confined 2D electron and hole gas that reflects in complex standing wave pattern at 77 K. The quasiparticles exist in a wide energy range from -800 to 300 meV without mixing with silicon bulk excitations. By comparing high resolution Fourier transform of dI/dV maps, with joint density of states calculations, we are able to determine the 2D band structure. We also clearly demonstrate that hole-hole and hole-electron quantum interferences dominate over electron-electron ones.

Probing Surface Short Range Order and Inter-Adsorbate Interactions through IR Vibrational Spectroscopy: CO on Cu(100)

It is demonstrated that surface vibrational spectroscopy can be used to probe local ordering of adsorbates and the nature of inter-adsorbate interactions through dynamical dipole coupling among neighboring adsorbates. Our studies show that for CO adsorbed on Cu(100), first and second nearest neighbor interactions are repulsive. This repulsion prevents the formation of long range ordered structures at low and medium coverages and can be best understood as an adsorbate induced modification in the local electron density, i.e., Friedel oscillation.

Sexithiophene Adlayer Growth on Vicinal Gold Surfaces

We have investigated the initial stages of vacuum-deposited sexithiophene (alpha-6T) adlayer formation on Au(111) vicinal surfaces at room temperature. The in situ scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) reveal a step edge-driven growth of alpha-6T on the Au(111) vicinal surfaces that first leads to the formation of an ordered monolayer, comprising two phases with the molecular major axes aligned along the step edges. The monolayer formation is then followed by the appearance of a single-phase 2D superstructure at a two-monolayer coverage. The results highlight the potential of using vicinal metal surfaces as templates for generating organized organic nanostructures over macroscopic areas for applications in organic electronics and moletronics.