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Showing papers on "Scanning tunneling spectroscopy published in 1988"


Journal ArticleDOI
TL;DR: The eAect of on-site Coulomb repulsion on the process of resonant tunneling is studied and it is found that the tunneling peak results from a crossover from the high-temperature Kondo phase to the lowtemperature mixed-valence phase of the system when the chemical potential is varied across the on- site localized-state energy.
Abstract: We study the eAect of on-site Coulomb repulsion on the process of resonant tunneling. We find that the tunneling peak results from a crossover from the high-temperature Kondo phase to the lowtemperature mixed-valence phase of the system when the chemical potential is varied across the on-site localized-state energy. Consequently, the line shape is non-Lorentzian, with rather unusual temperature dependence. Moreover, a magnetic field does not split the tunneling peak, but the line shape is modified. The eAect of coupling between localized states is also discussed, PACS numbers: 71.55.Jv, 73.40.QV In this Letter we discuss the effect of intra-atomic Coulomb interaction on the resonant site in the process of resonant tunneling. Resonant tunneling is thought to be the dominating mechanism for conduction at very low temperature through small systems with localized states. ' In the noninteracting case, this mechanism has been discussed by a number of authors and the phenomenon can be described by a 1D model Hamiltonian Hp, r

744 citations


Journal ArticleDOI
TL;DR: Estimated tunneling rates of the magnetization in a single-domain particle through an energy barrier between easy directions prove to be large enough for observation of the effect with the use of existing experimental techniques.
Abstract: The probability of tunneling of the magnetization in a single-domain particle through an energy barrier between easy directions is calculated for several forms of magnetic anisotropy. Estimated tunneling rates prove to be large enough for observation of the effect with the use of existing experimental techniques.

420 citations




Journal ArticleDOI
TL;DR: By placing a photon detector near the tip-sample region of a scanning tunneling microscope, the authors measured isochromat photon-emission spectra of polycrystalline tantalum and Si(111)7×7 at photon energies of 9.5 eV.
Abstract: By placing a photon detector near the tip-sample region of a scanning tunneling microscope, we have measured isochromat photon-emission spectra of polycrystalline tantalum and Si(111)7×7 at photon energies of 9.5 eV. Such spectra contain electronic-structure information comparable to inverse photoemission spectroscopy, but with high lateral/spatial resolution. The implications of this new observation are discussed.

216 citations



Journal ArticleDOI
TL;DR: In this article, it was shown that at sufficiently small separations, ∼1−2 A, the tip and flat surfaces in the scanning tunneling microscope or atomic force microscope (AFM) will jump together, irrespective of apparatus construction.
Abstract: It is shown that at sufficiently small separations, ∼1–2 A, the tip and flat surfaces in the scanning tunneling microscope or atomic force microscope (AFM) will jump together, irrespective of apparatus construction. Both continuum and atomistic calculations are presented. We discuss the consequences of the resulting forbidden separations for the behavior of the vacuum barrier as it is quenched, and for the resolution of the AFM.

148 citations


Book ChapterDOI
Randall M. Feenstra1, P. Mårtensson1
TL;DR: Antimony overlayers on the GaAs(110) surface have been studied by scanning tunneling microscopy and spectroscopy and a characteristic spectrum of surface states is observed.
Abstract: Antimony overlayers on the GaAs(110) surface have been studied by scanning tunneling microscopy and spectroscopy. On ordered Sb terraces, a characteristic spectrum of surface states is observed. Near the edges of Sb islands, additional states are found within the band-gap region. These band-gap states comprise two separate bands of filled and empty states, with the surface Fermi level pinned between the two bands.

140 citations


Journal ArticleDOI
23 Dec 1988-Science
TL;DR: Real-space images with atomic resolution of the BiO plane of Bi2Sr2CaCu2O8+δ were obtained with a scanning tunneling microscope, revealing the structural nature of the superlattice.
Abstract: Real-space images with atomic resolution of the BiO plane of Bi2Sr2CaCu2O8+δ were obtained with a scanning tunneling microscope. Single-crystal samples were cleaved and imaged under ultrahigh vacuum conditions at room temperature. The images clearly show the one-dimensional incommensurate superstructure along the b -axis that is common to this phase. High-resolution images show the position of the Bi atoms, revealing the structural nature of the superlattice. A missing row of Bi atoms occurs either every nine or ten atomic sites in both (110) directions, accounting for the measured incommensurate periodicity of the superstructure. A model is proposed that includes missing rows of atoms, as well as displacements of the atomic positions along both the a - and c -axis directions.

114 citations


Journal ArticleDOI
TL;DR: In this paper, a new three-dimensional tunneling theory is introduced for interpreting scanning tunneling spectroscopy (STS) images, which can be applied to interpret tunneling microscopy and STS images immediately.
Abstract: A new three‐dimensional tunneling theory is introduced for interpreting scanning tunneling spectroscopy (STS) images. By expanding the asymptotic wave function of the acting atom in terms of complete sets of eigenfunctions in spherical coordinates and parabolic coordinates, a derivative rule is derived, which can be applied to interpret scanning tunneling microscopy and STS images immediately. The relation between the observed dynamic conductance and the density of states of the sample is shown in conjunction with a linear bias‐distortion correction.

99 citations


Journal ArticleDOI
TL;DR: The tunneling characteristics show clear evidence of single-electron tunneling, induced by a Coulomb blockade, and are in quantitative agreement with theoretical predictions.
Abstract: We have studied the low-temperature tunneling behavior of point-contact junctions with adjustable capacitances in the ${10}^{\ensuremath{-}18}$-F range. The tunneling characteristics show clear evidence of single-electron tunneling, induced by a Coulomb blockade, and are in quantitative agreement with theoretical predictions.

Journal ArticleDOI
TL;DR: In this paper, the authors calculated interatomic forces and charge densities including the tip (atom) as an integral part of the system rather than treating it perturbatively, and revealed that the tip at close proximity to the surface disturbs the states near the Fermi level, and induces localized states which in turn influence the STM images.
Abstract: The graphite surface, due to its unusual electronic structure, offers challenges and opportunities for scanning tunneling microscopy (STM) and atomic force microscopy (AFM). To draw a measurable current at small voltages (1 nA at 50 mV) the tip has to approach close to the surface; estimated tip‐to‐surface distance is ∼2 A. At such distances repulsive interatomic forces between the tip and surface set in (the basis of AFM) and it is important to consider the tip–surface interaction. Therefore, we have calculated interatomic forces and charge densities including the tip (atom) as an integral part of the system rather than treating it perturbatively. Our calculations reveal that the tip at close proximity to the surface disturbs the states near the Fermi level, and induces localized states which in turn influence the STM images. The tunneling barrier appears to collapse at small tip‐to‐surface distances. Some experimental evidence for this effect is cited. The repulsive force on the tip has a value in the r...

Journal ArticleDOI
TL;DR: In this paper, a voltage-dependent scanning tunneling microscopy (STM) data from various semiconductor surfaces is presented, and different techniques for acquiring the data are demonstrated, and the interpretation of spectroscopic data is discussed.
Abstract: We present voltage-dependent scanning tunneling microscopy (STM) data from various semiconductor surfaces. Different techniques for acquiring the data are demonstrated, and the interpretation of spectroscopic data is discussed. For the Si(111)2 × 1, GaAs (110), and O/ GaAs(110) surfaces, we find that the voltage dependence of constant-current contours provides the most direct method for understanding the electronic and geometric structure of the surface. For the case of oxygen on n-type GaAs, we observe a reversal in the constant-current contours when the polarity of the bias voltage is reversed, which is interpreted in terms of band bending due to a negatively charged adsorbate. Theoretical calculations for a screened Coulomb potential are shown to agree with the observed reversal in the STM contours of the adsorbed oxygen.

Journal ArticleDOI
Robert J. Hamers1, J. E. Demuth1
TL;DR: Scanning tunneling microscopy is used to determine the atomic structure, charge state, and electronic energy spectrum of isolated Si dangling-bond defects at the Al/Si(111) surface.
Abstract: Scanning tunneling microscopy is used to determine the atomic structure, charge state, and electronic energy spectrum of isolated Si dangling-bond defects at the Al/Si(111) surface. Si adatoms substituting for Al in the first atomic layer give rise to a dangling-bond defect state near —0.4 eV which is strongly localized in space. Tunneling spectra and local band-bending measurements are inconsistent with simple one-electron band theory and demonstrate the importance of many-electron effects.

Journal ArticleDOI
B. N. J. Persson1, A. Baratoff1
TL;DR: In this paper, a generalized WKB expression for the tunneling exponent is derived for tunneling into or out of a conductor (e.g., in field emission), as well as between two semi-infinite polarizable electrodes separated by a potential barrier.
Abstract: We discuss the self-consistent determination of the dynamic polarization or ``image'' potential in tunneling within linear-response theory. A generalized WKB expression for the tunneling exponent is derived for tunneling into or out of a conductor (e.g., in field emission), as well as between two semi-infinite polarizable electrodes separated by a potential barrier. Numerical results are presented for the tunneling exponent, the barrier traversal time, and effective thickness for simple models assuming coupling to dispersionless surface plasmons of frequency ${\ensuremath{\omega}}_{s}$. These results demonstrate a systematic reduction of the image-potential contribution to the tunneling exponent if the barrier traversal time becomes shorter than ${\ensuremath{\omega}}_{s}^{\mathrm{\ensuremath{-}}1}$. We estimate that, in tunneling from a metal tip to a metal surface, the static image potential overestimates the polarization contribution to the tunneling exponent by 20--30 % in a typical scanning-tunneling-microscope measurement.

Journal ArticleDOI
TL;DR: In this article, the tunneling process of photogenerated carriers in AlAs/GaAs double quantum well structures in which two wells have different widths (60 and 80 A) was studied by steadystate and time-resolved photoluminescence spectra.
Abstract: The tunneling process of photogenerated carriers in AlAs/GaAs double quantum well structures in which two wells have different widths (60 and 80 A) was studied by steady‐state and time‐resolved photoluminescence spectra. The tunneling process was found to play an important role for a barrier thickness narrower than about 40 A. The tunneling rate was determined as 2×1010 s−1 for a 30‐A barrier. The quantum‐mechanical penetration depth of the wave function into an AlAs barrier was estimated as 6 A from the barrier width dependence of luminescence intensity ratio between the two wells. The tunneling rate and penetration depth are consistent with a simple envelope function approximation with no Γ‐X mixing.

Journal ArticleDOI
TL;DR: In this paper, the authors obtained atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact using point contact with a contamination layer, which provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy imaging.
Abstract: We have been successful in obtaining atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact. Direct contact between tip and sample or contact through a contamination layer provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy (STM) imaging. Current–voltage (I–V) spectra were obtained while scanning in the current imaging mode with the feedback circuit interrupted in order to study the graphite imaging mechanism. Multiple tunneling tips are probably responsible for images without the expected hexagonal or trigonal symmetry. Our observations indicate that the use of HOPG for testing and calibration of STM instrumentation may be misleading.


Journal ArticleDOI
TL;DR: In this paper, a unique surface preparation method is used to prepare a low surface state density Si surface, which enables the control of band bending with scanning tunneling microscopy (STM) to investigate semiconductor subsurface properties.
Abstract: Scanning tunneling microscopy (STM) methods are used to directly control the barrier height of a metal tunnel tip-semiconductor tunnel junction. Barrier behavior is measured by tunnel current-voltage spectroscopy and compared to theory. A unique surface preparation method is used to prepare a low surface state density Si surface. Control of band bending with this method enables STM investigation of semiconductor subsurface properties.

Journal ArticleDOI
TL;DR: In this paper, tunneling microscopy and spectroscopy are used to characterize defects in Al overlayers on Si(111) at low coverages where Al and Si adatoms are both present.
Abstract: Tunneling microscopy and spectroscopy are used to characterize defects in Al overlayers on Si(111). At low coverages where Al and Si adatoms are both present, voltage‐dependent scanning tunneling microscopy imaging allows them to be selectively imaged due to their different electronic structures. In (3)1/2‐Al, Si adatoms substituting for Al give rise to a defect state at −0.4 eV which is strongly localized in space. Band‐bending measurements indicate that the Si substitutional defects are electrically neutral. The results are interpreted in terms of a Mott–Hubbard model for a strongly correlated system.

Journal ArticleDOI
TL;DR: This model unifies reflection high-energy electron diffraction, x-ray standing-wave, and tunneling observations and suggests a graphitelike silicon-gallium top layer that is weakly bonded to the lattice below and stabilized by a periodic array of misfit dislocations.
Abstract: Real-space images of an incommensurate superlattice on a monolayer-gallium-covered silicon (111) surface have been obtained with the tunneling microscope. Large, internally ordered supercells which in turn form a lattice with discrete boundaries are observed. A graphitelike silicon-gallium top layer is suggested that is weakly bonded to the lattice below and stabilized by a periodic array of misfit dislocations. This model unifies reflection high-energy electron diffraction, x-ray standing-wave, and tunneling observations.


Journal ArticleDOI
TL;DR: In this article, a study of a semiconductor surface chemical reaction at the atomic level using a scanning tunneling microscope (STM) was performed, where the topographic structure and electronic density of states were measured before and after reacting Si(001)•(2×1) with NH3.
Abstract: We have performed a study of a semiconductor surface chemical reaction at the atomic level using a scanning tunneling microscope (STM). The ‘‘topographic’’ structure and electronic density of states are measured before and after reacting Si(001)‐(2×1) with NH3. While both clean and NH3‐dosed surfaces exhibit a (2×1) symmetry, STM images reveal changes in the spatial distribution of occupied electronic states which allow us to distinguish reacted and unreacted Si(001) dimers. Using tunneling spectroscopy, the occupied and unoccupied surface states of the clean and reacted surface are identified. The results are interpreted in terms of a Si(001)‐(2×1)H monohydride resulting from dissociative adsorption of NH3.

Journal ArticleDOI
TL;DR: In this article, a model describing the essentials of tunneling between the tip of a scanning tunneling microscope (STM) and an electronic resonance on an isolated adsorbed molecule is presented, with conditions for observing enhanced inelastic tunneling and its experimental manifestations in terms of the energy and width of the resonance and of its coupling to a particular molecular vibration.
Abstract: We present a model describing the essentials of tunneling between the tip of a scanning tunneling microscope (STM) and an electronic resonance on an isolated adsorbed molecule. Conditions for observing enhanced inelastic tunneling and its experimental manifestations are identified in terms of the energy and width of the resonance and of its coupling to a particular molecular vibration. For a broad resonance, this occurs if it is partially filled; we then predict a conductance decrease as large as 10% at the inelastic threshold for typical chemisorption systems. For a resonance weakly hybridized with the underlying electrode, strong multiphonon excitation is possible, although as a rule it will not be readily apparent. Relevant systems of interest for local spectroscopy with future STM’s are discussed.

Journal ArticleDOI
TL;DR: In this article, the authors considered inelastic vacuum tunneling from a metal tip to a metal surface with an adsorbed molecule and showed that the dynamical image potential in tunneling overestimates the contribution from the image potential by 20-30% in a typical case.
Abstract: I consider inelastic vacuum tunneling from a metal tip to a metal surface with an adsorbed molecule. Both dipole and resonance coupling is considered. Resonance coupling gives rise to characteristic changes in the tunneling conductance similar to those observed by Bayman et al. for CH3NC chemisorbed on small Rh-particles. The concept of traversal time in tunneling is discussed and it is shown that there actually exist a distribution of traversal times. I present results for the dynamical image potential in tunneling and I estimate that using the static image potential when calculating the tunneling exponent in tunneling from a metal tip to a metal surface, overestimates the contribution from the image potential by 20-30% in a typical case.

Journal ArticleDOI
TL;DR: Atomic resolution topographs and current images show the symmetry of the surface unit cell and clearly reveal two distinct atomic sites in agreement with the well-known x-ray crystal structure of 2H-MoS2.
Abstract: Molybdenum disulfide, a layered semiconductor, is an interesting material to study with the tunneling microscope because two structurally and electronically different atomic species may be probed at its surface. We report on a vacuum scanning tunneling microscopy study of 2H-MoS2. Atomic resolution topographs and current images show the symmetry of the surface unit cell and clearly reveal two distinct atomic sites in agreement with the well-known x-ray crystal structure.

Journal ArticleDOI
TL;DR: In this paper, the authors used a scanning tunneling microscopy (STM) to measure the tunneling current versus applied voltage for oxygen adsorbed on the GaAs(110) surface.
Abstract: Spatially resolved measurements of the tunneling current versus applied voltage are obtained for oxygen adsorbed on the GaAs(110) surface, using a scanning tunneling microscopy (STM) Effects with different length scales are observed, arising from two sources of charge in the system: negatively charged adsorbates and a positively charged space charge layer The space charge layer produces band bending on n‐type material, which is observed as a shift in the onsets for tunneling out of and into the valence and conduction bands, respectively We analyze these shifts using theoretical calculations of the tunneling current through surface space charge layers Directly above an oxygen adsorbate on n‐type material, an enhancement in tunneling is observed near the top of the valence band edge, while a corresponding decrease is observed near the bottom of the conduction band This local behavior is identified as arising from changes is the surface density‐of‐states produced by the Coulomb potential of the charged

Journal ArticleDOI
TL;DR: The temperature dependence of the tunneling spectrum of the 80 K phase of the recently discovered high-temperature superconductor Bi-Sr-Ca-Cu-O system is investigated in this paper.
Abstract: The temperature dependence of the tunneling spectrum of the 80 K phase of the recently discovered high-temperature superconductor Bi-Sr-Ca-Cu-O system is reported. The shape of the tunneling spectrum and its temperature dependence are discussed.

01 Jan 1988
TL;DR: In this article, the superconductive energy gap of YBa 2 Cu 3 O 7−δ was measured in anovel broken film edge junction in a cryogenic environment along with the SrTiO 3 substrate.
Abstract: Abstract Superconductive energy gap of YBa 2 Cu 3 O 7−δ are measured in anovel broken film edge junction. (001), (103) and (110) oriented films are broken in a cryogenic environment along the appropriate directions together with the SrTiO 3 substrate. Pb electrode is brought close in situ to the clean broken film edge. The normalized energy gap 2Δ(0)/k B T c measured in the direction along and perpendicular to the Cu-O plane are found to be 5.9±0.2 and 3.6±0.2 respectively. These values are independent of the variation in the values of T c within the examined range of 40K∽90K.

Journal ArticleDOI
TL;DR: In this article, the Langmuir-Blodgett films of cadmium-arachidate have been studied by scanning tunneling microscopy (STM), and the results indicate disturbances on the tunneling current caused by the films.
Abstract: Langmuir-Blodgett films of cadmium-arachidate have been studied by scanning tunneling microscopy (STM). Substrates of gold and graphite are used to make mono- and bilayer films. The image of the individual molecule cannot be obtained for the films on gold, but the data indicate disturbances on the tunneling current caused by the films. As for the films on graphite, the molecular images with a spacing of 0.4–0.7 nm are recognized in some frames of the variable tunneling current image recorded on videotape. Possible mechanisms of the imaging are discussed.