Spin-polarized surface states on Fe-deposited Au(111) surface: A theoretical study

We have studied electronic structure of Fe-deposited Au(111) by performing ab initio density functional theory calculations. We find that the magnetic moment on the deposited Fe layer is enhanced as compared to that in bulk iron. We observe a large number of new states on the Fe-deposited surface — one of which is in the majority spin channel having similar dispersion to that on the clean surface, and others in the minority spin channel. The effective mass of electrons in surface states near the Fermi level increases on Fe deposition. The electronic properties are found to be insensitive to the stacking of near-surface layers. We need to use very thick slabs in our calculations to avoid splitting of surface states due to spurious interactions between the two surfaces of the slab. Using the local density of states profiles for different surface states, we conclude that in scanning tunneling microscope experiments one can detect two of the surface states — one in the majority channel below the Fermi level, and another in the minority channel appearing just above the Fermi energy. We compare our results to those from scanning tunneling spectroscopy experiments.     

Observation of electron localization in rough gold nanoclusters on the graphite surface

The results of scanning tunneling spectroscopy of the electronic states of Au nanoclusters on the graphite surface are presented. The tunneling current is found to be different at different points of a rough-surface nanocluster. The measured differential current-voltage curve of the clusters is nonmonotonic near the Fermi energy, and the tunneling conductance decreases by almost a factor of two as the cluster volume changes from 1 to 0.1 nm³. This decrease can be associated with the change in the density of the electronic states near the Fermi energy. The observed features are qualitatively described within the framework of the mechanism of electron localization in disordered systems.     

Valence surface electronic states on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.     

Electronic structure of a polydiacetylene nanowire fabricated on highly ordered pyrolytic graphite

A molecular wire candidate, the polydiacetylene chain, fabricated in a substantial support layer of monomers self-assembled on a highly ordered pyrolytic graphite surface, was studied using scanning tunneling microscopy and spectroscopy. The density of states of individual polymers and constituent monomers were observed on the same surface, and then compared with the calculated results. The spectrum delineating the density of states of the polydiacetylene wire clearly reveals the theoretically predicted pi-band and band edge singularities of the one-dimensional polymer.     

Spin-density wave near the vortex cores in the high-temperature superconductor Bi2Sr2CaCu2O8+y

Competition with magnetism is at the heart of high-temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe based upon NMR spin-lattice-relaxation spectroscopy. With this approach we have found a spin-density wave associated with the vortex core in Bi(2)Sr(2)CaCu(2)O(8+y), similar to checkerboard patterns in the local density of electronic states reported from scanning tunneling microscope experiments. We have determined both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30 T.     

Electronic states on the surface of graphite

Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip-sample distance beyond which the effect of sample-tip interaction is negligible.     

Folding and cracking of graphene oxide sheets upon deposition

Graphene Oxide (GO) sheets, suspended in an aqueous solution, were deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG) and studied using Raman spectroscopy, atomic force microscopy (AFM) and scanning tunneling microscopy (STM). AFM phase imaging shows a distinct contrast between GO and the underlying HOPG substrate. Raman spectroscopy clearly showed the presence of GO sheets on the top of HOPG substrate. The AFM and STM images also reveal wrinkling, folding, and tearing of individual GO sheets after depositing onto an HOPG substrate. We have also observed a distinct cracking of a GO sheet after folding. We attribute this new cracking phenomenon to a weakening of C–C bonds during the oxidation of a graphene sheet.     

Probing semiconductor gap states with resonant tunneling

Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.     

Imaging the wave-function amplitudes in cleaved semiconductor quantum boxes

We have investigated the electronic structure of the conduction band states in InAs quantum boxes embedded in GaAs. Using cross-sectional scanning tunneling microscopy and spectroscopy, we report the direct observation of standing wave patterns in the boxes at room temperature. Electronic structure calculation of similar cleaved boxes allows the identification of the standing waves pattern as the probability density of the ground and first excited states. Their spatial distribution in the (001) plane is significantly affected by the strain relaxation due to the cleavage of the boxes.     

Spatially mapping the spectral density of a single C60 molecule

We have used scanning tunneling spectroscopy to spatially map the energy-resolved local density of states of individual C60 molecules on the Ag(100) surface. Spectral maps were obtained for molecular states derived from the C60 HOMO, LUMO, and LUMO+1 orbitals, revealing new details of the spatially inhomogeneous C60 local electronic structure. Spatial inhomogeneities are explained using ab initio pseudopotential density functional calculations. These calculations emphasize the need for explicitly including the C60-Ag interaction and STM tip trajectory to understand the observed C60 local electronic structure.     

Real-space imaging of alternate localization and extension of quasi-two-dimensional electronic States at graphite surfaces in magnetic fields

We measured the local density of states (LDOS) of a quasi-two-dimensional (2D) electron system near point defects on a surface of highly oriented pyrolytic graphite with scanning tunneling microscopy and spectroscopy. Differential tunnel conductance images taken at very low temperatures and in high magnetic fields show a clear contrast between localized and extended spatial distributions of the LDOS at the valley and peak energies of the Landau level spectrum, respectively. The localized electronic state has a single circular distribution around the defects with a radius comparable to the magnetic length. The localized LDOS is in good agreement with a spatial distribution of a calculated wave function for a single electron in 2D in a Coulomb potential in magnetic fields.     

Characterization of free-standing GaN substrate grown through hydride vapor phase epitaxy with a TiN interlayer

Large-scale GaN free-standing substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The bottom surface N-face and top surface Ga-face showed great difference in anti-etching and optical properties. The variation of optical and structure characteristics were also microscopically identified using spatially resolved cathodoluminescence and micro-Raman spectroscopy in cross-section of the GaN substrate. Three different regions were separated according to luminescent intensity along the film growth orientation. Some tapered inversion domains with high free carrier concentration of 5 × 10¹⁹ cm⁻³ protruded up to the surface forming the hexagonal pits. The dark region of upper layer showed good crystalline quality with narrow donor bound exciton peak and low free carrier concentration. Unlike the exponential dependence of the strain distribution, the free-standing GaN substrate revealed a gradual increase of the strain mainly within the near N-polar side region with a thickness of about 50 μm, then almost kept constant to the top surface.     

STS observations of Landau levels at graphite surfaces

Scanning tunneling spectroscopy (STS) measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra associated with Landau quantization of the quasi-two-dimensional electrons and holes. A comparison with the calculated local density of states at the surface layers allows us to identify Kish graphite as bulk graphite and HOPG as graphite with a finite thickness of 40 layers. This explains the qualitative difference between the two graphites reported in the recent transport measurements which suggested the quantum-Hall effect in HOPG. This work demonstrates how powerful the combined approach between the high quality STS measurement and the first-principles calculation is in material science.     

Several large-scale superperiodicities on highly oriented pyrolytic graphite observed by scanning tunneling microscopy

On freshly cleaved highly oriented pyrolytic graphite we observed large-scale superperiodicities by a scanning tunneling microscope at room temperature in air. Several hexagonal superstructures with periods of 30 nm, 4.2 nm, 2.4 nm, and 2.0 nm, respectively, and a strip-like superstructure with a period of 1 nm were obtained. With exception of the largest hexagonal superperiodicity (30 nm spacing), all other superstructures are superimposed on the atomic corrugation of graphite. The origin of these superstructures is not clear yet. We assume that they arise from crystal defects in graphite. The hexagonal superstructure may be caused by the Moiré effect due to the rotational misorientation of the two top layers or of two successive layers near the surface.     

Confined Shockley surface states on the (111) facets of gold clusters

Combining low-temperature scanning tunneling microscopy and spectroscopy with high-resolution ultraviolet photoemission, we have revealed a confined Shockley surface state on the (111) facets of gold clusters with about N=10(4) atoms grown in nanopits on highly oriented graphite. With tunneling spectroscopy, we observed energy dependent nodal patterns in the dI/dV maps, which are in quantitative agreement with the two-dimensional confinement of the surface state within the hexagonal facet area. The results indicate that the lattice of the ionic cores influences the electronic properties of the clusters significantly.     

Surface scattering via bulk continuum states in the 3D topological insulator Bi2Se3

We have performed scanning tunneling microscopy and differential tunneling conductance (dI/dV) mapping for the surface of the three-dimensional topological insulator Bi(2)Se(3). The fast Fourier transformation applied to the dI/dV image shows an electron interference pattern near Dirac node despite the general belief that the backscattering is well suppressed in the bulk energy gap region. The comparison of the present experimental result with theoretical surface and bulk band structures shows that the electron interference occurs through the scattering between the surface states near the Dirac node and the bulk continuum states.     

Donor and acceptor-like electronic states in a one-dimensional semiconductor

Donor and acceptor-like electronic states were observed near the conduction and valence band, respectively, in a semiconducting single-wall carbon nanotube using scanning tunneling microscopy and spectroscopy. They are observed to be, spatially, at the same location and spread within a few nm. Their physical origin is suggested as locally formed excessive and deficient number of electrons per a closed carbon network.     

Spatial imaging of valence band electronic structures in a GaSb/InAs quantum well

We measure local density of states (LDOS) for GaSb/InAs heterostructures with quantum wells in the valence band by scanning tunneling spectroscopy (STS) on the cleaved surface. Clear standingwave patterns of LDOS corresponding to the holes confined in the quantum wells are observed.     

自旋极化扫描隧道显微术

自旋极化扫描隧道显微术是一种新兴的表面自旋分辨技术，文章主要介绍了自旋极化的扫描隧道显微镜和扫描隧道谱实现表面自旋分辨的原理以及在各种磁性表面研究中的应用，采用自旋极化技术的扫描隧道显微镜可以测量表面磁结构，其空间分辨可以达到原子尺度，分辨率超过其他磁显微技术，而自旋极化扫描隧道谱不但可以分辨空间精细磁畴结构，而且能研究表面态的交换劈裂，文章作者还进一步提出了利用自旋极化扫描隧道显微镜实现自旋注入的设想。     

Cross-sectional scanning tunneling microscopy and spectroscopy of strain in buried heterostructure lasers

Cross-sectional scanning tunneling microscopy and spectroscopy have been used to probe the unreconstructed (1 1 0) surface of a commercially available buried heterostructure laser in ultra high vacuum. Complex re-growth above the non-linear blocking layers is shown to induce tensile strain in the device. Spectroscopic measurements show an increase in both the density of filled valence band states and empty conduction band states as a result of the strain, with a particularly large increase at −3.1 V. Current imaging tunneling spectroscopy measurements show an increase in the tunneling current in to and out of the strained regions at both gap voltage polarities, consistent with the spectroscopy. Moving towards tensile strain, InP is known to maintain much the same bandgap, with the split-off level and lower lying states being drawn up towards the valence band edge, consistent with the data.