Showing papers on "Scanning tunneling spectroscopy published in 1994"

Tunneling spectroscopy of the (110) surface of direct-gap III-V semiconductors.

Abstract: Results of tunneling spectroscopy measurements of the (110) cleaved surface of GaAs, InP, GaSb, InAs, and InSb are presented These materials form the family of direct-gap III-V binary semiconductors Spectroscopic measurements are performed in ultrahigh vacuum, using a scanning tunneling microscope (STM) Techniques based on variable tip-sample separation are used to obtain high dynamic range (six orders of magnitude) in the measured current and conductance Detailed spectra are obtained for all the materials, revealing the conduction- and valence-band edges, onset of the higher lying conduction band at the L point in the Brillouin zone, and various features associated with surface states The precision and accuracy in determining energetic locations of spectral features are discussed In particular, limitations in the accuracy due to tip-induced band bending is considered

Wave-vector-dependent tunneling through magnetic barriers.

Abstract: New tunneling structures are proposed consisting of magnetic barriers that can be created by lithographic patterning of ferromagnetic or superconducting films. The form of the equivalent potential for such a barrier depends on the wave vector of the incident electron. This renders the transmission through such structures an inherently two-dimensional process since the tunneling probability depends not only on the electron's momentum perpendicular to the tunneling barrier but also on its momentum parallel to the barrier. Pronounced resonances are obtained for the tunneling probability and the conductance of a resonant tunneling device consisting of such magnetic barriers.

Electron conduction in molecular wires. II. Application to scanning tunneling microscopy

Abstract: We use scattering methods to calculate the conductance of molecular wires. We show that three kinds of wire length dependences of the conductance arise: the decay can be exponential, polynomial, or very slow, depending on whether the reservoir Fermi level lies far from, in, or at the edge of the molecular energy band. We use the formalism to discuss simple models of tip‐induced pressure and of imaging in scanning tunneling microscopy (STM), and point out a paradoxical situation in which the current can decrease with increased tip pressure. We also consider the connection of this formalism with the conventional theory of intramolecular, nonadiabatic electron transfer (ET).

Silicon oxide resonant tunneling diode structure

Abstract: A resonant tunneling diode (400) made of a silicon quantum well (406) with silicon oxide tunneling barriers (404, 408). The tunneling barriers have openings (430) of size smaller than the electron wave packet spread to insure crystal alignment through the diode without affecting the tunneling barrier height, and the openings (430) have an irregular (nonperiodic) shape.

Observation of Quantum-Size Effects at Room Temperature on Metal Surfaces With STM.

Abstract: Surface steps act as confining barriers for electrons in metal-surface states. Thus, narrow terraces and small single-atom—high metal islands act as low-dimensional, electron-confining structures. In sufficiently small structures, quantum-size effects are observable even at room temperature. Scanning tunneling spectroscopy is used to image the probability amplitude distributions and discrete spectra of the confined states. Examination of the electronic structure of the steps provides evidence for electron-density smoothing and the formation of step-edge states. Estimates of the electron-confining barriers are obtained.

Imaging of surface plasmon propagation and edge interaction using a photon scanning tunneling microscope

Abstract: We report the direct imaging of surface plasmon propagation on thin silver films using the photon scanning tunneling microscope. It is found that the surface plasmon remains tightly confined in the original launch direction with insignificant scattering to other momentum states. A propagation length of 13.2 \ensuremath{\mu}m is measured at \ensuremath{\lambda}=632.8 nm. We also present images showing the interaction of a surface plasmon with the edge of the metal film supporting it. The most remarkable feature is the absence of specularly reflected beam.

Photon-assisted tunneling through a quantum dot

Abstract: We study single-electron tunneling in a two-junction device in the presence of microwave radiation. We introduce a model for numerical simulations that extends the Tien-Gordon theory for photon-assisted tunneling to encompass correlated single-electron tunneling. We predict sharp current jumps which reflect the discrete photon energy hf, and a zero-bias current whose sign changes when an electron is added to the central island of the device. Measurements on split-gate quantum dots show microwave-induced features that are in good agreement with the model.

Possibility of coherent multiple excitation in atom transfer with a scanning tunneling microscope.

Abstract: We examine atom transfer resulting from coherent multiple excitation of the adsorbate-substrate bond caused by inelastic tunneling of a single electron (or hole) via a negative- (or positive-) ion resonance. At low biases and in particular for the transitions resulting in atom transfer, the rates of coherent multiple excitation are nonlinear and also highly asymmetrical with respect to the polarity of the bias. We establish a simple criterion for the regime in which this mechanism dominates over earlier proposed mechanisms for atom transfer resulting from vibrational heating by sequential (incoherent) inelastic resonance tunneling. In the case of the atomic switch, where a Xe atom is transferred between a Ni surface and a tip, the vibrational heating mechanism is found to dominate over the coherent mechanism. For other systems, such as Na adsorption on Cu or ${\mathrm{O}}_{2}$ adsorbed on Pt, the coherent mechanism is argued to play a role in bond breaking.

Fermi-edge singularity in resonant tunneling.

Abstract: We have observed a Fermi-edge singularity in the tunneling current between a two-dimensional electron gas (2DEG) and a zero-dimensional localized state. A sharp peak in the tunnel current is observed when the energy of the localized state matches the Fermi energy of the 2DEG. The peak grows and becomes sharper as the temperature is decreased to our lowest temperature of 70 mK. We attribute the singularity to the Coulomb interaction between the tunneling electron on the localized site and the Fermi sea.

Scanning tunneling spectroscopy of carbon nanotubes

Abstract: Calculations predict that carbon nanotubes may exist as either semimetals or semiconductors, depending on diameter and degree of helicity. This communication presents experimental evidence supporting the calculations. Scanning tunneling microscopy and spectroscopy (STM-S) data taken in air on nanotubes with outer diameters from 17 to 90 A show evidence of one-dimensional behavior; the current-voltage (I-V) characteristics are consistent with a density of states containing Van Hove type singularities for which the energies vary linearly with inverse nanotube diameter.

Scanning tunneling microscopy studies of formaldehyde synthesis on Cu(110).

Abstract: The synthesis of formaldehyde from methanol on oxygen-predosed Cu(110) surfaces has been studied using scanning tunneling microscopy (STM). Sequential STM images show the shrinking of the (2\ifmmode\times\else\texttimes\fi{}1) Cu-O islands in the [001] direction and the formation of a (5\ifmmode\times\else\texttimes\fi{}2) methoxy-induced reconstruction that incorporates the added-row Cu atoms of the (2\ifmmode\times\else\texttimes\fi{}1) reconstruction. The decomposition of methoxy to formaldehyde occurs upon desorption; STM images show methoxy islands shrinking in the [001] direction and the release of incorporated Cu atoms.

Field evaporation between a gold tip and a gold surface in the scanning tunneling microscope configuration.

Abstract: Mounds of gold atoms of horizontal size less than 200 \AA{} and height 20 \AA{}, and pits of width 50 \AA{} and depth 10 \AA{} can be produced on a gold surface with nearly equal probabilities by applying voltage pulses to a gold tip in the scanning tunneling microscope. We study the effects of the polarity and the height of the applied voltage pulses, and those of atmospheric environments. Our data show that field evaporation can occur both in positive and negative fields, but in ultrahigh vacuum, a tip can sustain itself only if it is in negative polarity.

Observation of Mott localization gap using low temperature scanning tunneling spectroscopy in commensurate 1T-TaS2.

Abstract: The site-specified tunneling spectroscopy on $1T\ensuremath{-}\mathrm{T}\mathrm{a}{\mathrm{Sa}}_{2}$ at 77 K and room temperature indicated a wide opening of energy gap at the Fermi level at 77 K. The gap at the Fermi level was attributed to the Mott localization in the commensurate charge density wave (CDW) phase. The determined gap size on both sides of the Fermi level was about 400-500 mV at 77 K. A significant charge transfer was observed from CDW trough to crest. This charge transfer was derived from the electron localization on the CDW crest induced by Mott localization.

New trends in magnetism, magnetic materials, and their applications

Abstract: Magnetic Clusters: Magnetic Properties of Small Transition Metal Clusters in a Molecular Beam (W.A. de Heer, I.M.L. Billas). Magnetic Properties of Transition Metal and Rareearth Metal Clusters (P.J. Jensen, K.H. Bennemann). One and Twodimensional Systems: The Onedimensional Hubbard Model with Attractive U as a Soluble Model for Exciton Bands and Electronhole Droplets (P. Schlottmann). Superlattices and Multilayers: Magnetization Patterns of Exchange Coupled Metallic Multilayers (D. Altbir, M. Kiwi). Surfaces and Interfaces: Spinpolarized Scanning Tunneling Spectroscopy on Fe and Ni (S.F. Alvarado). Alloys and Amorphous Materials: New Trends in Ferrimagnetism (T. Kaneyoshi). Special Techniques and Materials: Lessconventional Magnetic Domain Investigations (M. Schlenker). General Theoretical Developments: Slave Boson Approach to Local Moment Formation in the Hubbard Model (R.M. RibeiroTeixeira, M. Avignon). Concluding Remarks. 39 additional articles. Index.

First-principles theory of atom extraction by scanning tunneling microscopy.

Abstract: A new method to provide a self-consistent electronic structure, field, and current distribution for an atomistic bielectrode system with applied bias voltages is presented. In our method the scattering waves are calculated by a step-by-step recursion-matrix method and two different Fermi levels are assigned to each electrode in accord with a given applied bias voltage. The method is applied to the scanning tunneling microscope (STM) system around the contact region. The tip-surface chemical interaction induced by the electric fields is shown to be important for the extreme specificity of atom extraction by STM.

Tunneling Barriers in Electrochemical Scanning Tunneling Microscopy

Abstract: We have measured barriers for electron tunneling between a Pt-Ir tip and a gold substrate under potential control, obtaining values similar to those reported in ultrahigh vacuum. However, in contrast to vacuum tunneling, the data show a strong dependence on the bias applied between the tip and the substrate. They are only weakly dependent on the electrochemical potential of the substrate. The barrier changes with the direction of electron tunneling, an effect we attribute to permanent polarization in the gap. We observe a sharp dip near zero bias for tunneling in water. It is not observed for tunneling in a nonpolar solvent, and we attribute it to induced polarization in the tunnel gap

Single-electron tunneling effects in granular metal films.

Abstract: Cryogenic scanning tunneling microscopy is used to study local electrical-transport properties of thin granular Au/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ films in the vicinity of the percolation threshold. The current-voltage characteristics are found to vary dramatically from one tip position to another over distances of the order of a few nanometers. These characteristics often exhibit interesting Coulomb-staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple-barrier tunnel-junction model accounts quantitatively for the experimental results.

III-V nitride resonant tunneling

Abstract: A resonant tunneling diode ( 400 ) made of a quantum well ( 406 ) with tunneling barriers ( 404, 408 ) made of two different materials such as calcium fluoride ( 408 ) and silicon dioxide ( 404 ). The calcium fluoride provides lattice match between the emitter ( 410 ) and the quantum well ( 406 ). Further resonant tunneling diodes with silicon lattice match barriers may be made of III-V compounds containing nitrogen.

Change in the electronic states of graphite overlayers depending on thickness

Abstract: Electronic states of graphite overlayers formed on the TaC(111) surface have been investigated with the use of scanning tunneling microscopy and photoelectron spectroscopy. The graphite film grows on the substrate layer by layer. The thickness of the overlayer has been adjusted precisely to be either one or two monolayers. The physical properties of the monolayer graphite film are modified by chemical bonding at the interface. This interfacial bonding becomes weak upon the formation of the second layer of graphite, which makes the properties of the double-layer graphite film similar to those of bulk graphite.

Observation of a-b plane gap anisotropy in Bi2Sr2CaCu2O8 with a low temperature scanning tunneling microscope.

Abstract: Significant gap anisotropy within the a-b plane of Bi2Sr2CaCu2O8 has been observed with a low temperature scanning tunneling microscope. The energy gap is largest along the a-axis direction, with DELTA(p-p) = 36.6 meV and decreases gradually to a value of 23 meV along the direction 45-degrees from the a axis. The tunneling data have been fit well using a simple model which assumes a d-wave-pairing energy gap, and allows tunneling in all directions close to the normal of the surface.