Showing papers on "Scanning tunneling spectroscopy published in 1997"

Quantum tunneling of the magnetization in an iron cluster nanomagnet

Abstract: We have investigated the magnetic relaxation of clusters of eight iron ions characterized by a spin ground state of ten and an Ising anisotropy. Below 400 mK the relaxation rate is temperature independent suggesting that tunneling of the magnetic moment across its anisotropy energy barrier occurs. Using the anisotropy constants derived from EPR data, we can calculate both the crossover temperature ${T}_{c}$ and the expected tunneling frequency $1/\ensuremath{\tau}$. The field dependence of the relaxation shows evidence of resonant tunneling.

Probing the Local Effects of Magnetic Impurities on Superconductivity

Abstract: The local effects of isolated magnetic adatoms on the electronic properties of the surface of a superconductor were studied with a low-temperature scanning tunneling microscope. Tunneling spectra obtained near magnetic adsorbates reveal the presence of excitations within the superconductor's energy gap that can be detected over a few atomic diameters around the impurity at the surface. These excitations are locally asymmetric with respect to tunneling of electrons and holes. A model calculation based on the Bogoliubov-de Gennes equations can be used to understand the details of the local tunneling spectra.

Electronic structure and localized states at carbon nanotube tips

Abstract: Topology related changes in the local density of states near the ends of closed carbon nanotubes are investigated using spatially resolved scanning tunneling spectroscopy and tight binding calculations. Sharp resonant valence band states are observed in the experiment at the tube ends, dominating the valence band edge and filling the band gap. Calculations show that the strength and position of these states with respect to the Fermi level depend sensitively on the relative positions of pentagons and their degree of confinement at the tube ends.

Scanning tunneling microscopy studies of the TiO 2 ( 110 ) surface: Structure and the nucleation growth of Pd

Abstract: The surface structure of ${\mathrm{TiO}}_{2}(110)$ has been reexamined using scanning tunneling microscopy (STM), especially in terms of its temperature dependency. A dramatic topographic change was observed around 725 K, which is most likely due to the oxygen desorption into the gas phase and the simultaneous diffusion of Ti into the bulk. The atomically resolved STM images show a strong dependency on the tip composition and allow a double-bridging oxygen vacancy to be identified. The nucleation and growth of Pd on the ${\mathrm{TiO}}_{2}(110)$ surface have also been studied at the nucleation stage with atomic resolution. Both dimer and tetramer Pd clusters have been observed; however no single Pd atoms were detected. These results support the ``classical'' nucleation model which assumes that only monomers are mobile, whereas dimers are stable nuclei. A marked preferential nucleation and growth of Pd clusters at step edges have also been observed. In addition to topographic structures, the local electronic properties of the Pd clusters have been studied to relate cluster structure to electronic composition.

The physical and chemical properties of ultrathin oxide films.

Abstract: Thin oxide films (from one to tens of monolayers) of SiO2, MgO, NiO, Al2O3, FexOy, and TiO2 supported on refractory metal substrates have been prepared by depositing the oxide metal precursor in a background of oxygen (ca 1 x 10(-5) Torr). The thinness of these oxide samples facilitates investigation by an array of surface techniques, many of which are precluded when applied to the corresponding bulk oxide. Layered and mixed binary oxides have been prepared by sequential synthesis of dissimilar oxide layers or co-deposition of two different oxides. Recent work has shown that the underlying oxide substrate can markedly influence the electronic and chemical properties of the overlayer oxide. The structural, electronic, and chemical properties of these ultrathin oxide films have been probed using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (ELS), ion-scattering spectroscopy (ISS), high-resolution electron energy loss spectroscopy (HREELS), infrared reflectance absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS).

Tunneling spectroscopy of isolated C 60 molecules in the presence of charging effects

Abstract: The discrete molecular-level spectrum of an isolated ${\mathrm{C}}_{60}$ molecule is resolved and its interplay with single-electron charging effects is studied using room-temperature and cryogenic scanning tunneling spectroscopy. The tunneling current-voltage spectra of these molecules exhibit rich structures resulting from both resonant tunneling through the discrete levels and charging effects. In particular, we observe degeneracy lifting within the molecular orbitals, probably due to the Jahn-Teller effect and local electric fields. Theoretical fits account well for our experimental data.

Local density of states from spectroscopic scanning-tunneling-microscope images: Ag(111)

Abstract: Electron scattering from steps on the Ag(111) surface has been studied by scanning tunneling microscopy (STM) and spectroscopic STM images of the differential conductance $(dI/dV).$ The amplitudes and positions of maxima of $dI/dV$ deviate significantly from those of the local density of states (LDOS). These deviations are due to variations of the vertical tip position which exponentially affect the tunneling barrier transmission and, hence, $dI/dV.$ We show that the LDOS can be recovered from simultaneous topographic and spectroscopic measurements.

Validity of the Julliere model of spin-dependent tunneling

Abstract: We consider spin-dependent tunneling between two ferromagnets separated by a simple step barrier, and examine four models for the magnetoconductance ratio $\ensuremath{\Delta}G/G:$ A model due to Julliere which characterizes the magnetoconductance solely in terms of the tunneling spin polarization, a model due to Slonczewski which provides an approximate expression for the magnetoconductance of free electrons tunneling through a barrier, the exact expression for the magnetoconductance of free electrons tunneling through a barrier, and the numerical calculation of the magnetoconductance of band electrons in iron tunneling through a barrier. We find that the Julliere model does not accurately describe the magnetoconductance of free electrons tunneling through a barrier. Although Slonczewski's model provides a good approximation to the exact expression for free electrons in the limit of thick barriers, we find that the tunneling of band electrons shows features that are not described well by any free electron picture and which reflect the details of the band structure of iron at the Fermi energy.

Electron Fringes on a Quantum Wedge

Abstract: Using molecular beam epitaxy we have fabricated a quantum wedge: a nanoscale flat-top lead island on a stepped Si(111) surface. Imaging the top surface of the wedge with a scanning tunneling microscopy reveals the phenomenon of electron interference fringes: a discrete periodic spatial variation of the tunnel current originating from the quantization of electron states in the wedge.

Surface states on NiO (100) and the origin of the contrast reversal in atomically resolved scanning tunneling microscope images

Abstract: The local spin-density approximation+$U$ method has been applied to identify the origin of surface electronic states on the (100) surface of nickel monoxide. The results of ab initio calculations show a substantial reduction of the forbidden gap at the surface and point to the presence of two types of electronic states associated with the termination of the crystal lattice. The energy of one of the surface states corresponds to the top of the valence band and the wave function of this state is predominantly localized on the oxygen surface sites. The other state is situated at the bottom of the conduction band and its wave function is localized on the nickel sites. The presence of these surface states explains the origin of the contrast reversal of scanning tunneling microscope images of the (100) NiO surface observed experimentally by altering the sign of the bias applied to the sample.

Theory of atom transfer with a scanning tunneling microscope

Abstract: We present and discuss in detail a theory for atom transfer (or bond breaking) using the tip of a scanning tunneling microscope that was outlined by us [Solid State Commun. 84, 271 (1992)]. The theory is applied to an atomic switch [Nature 352, 600 (1991)]. In this theory the bond is broken by overcoming the associated potential barrier thanks to a gain in energy from the tunneling electrons. The barrier crossing is described by a truncated harmonic oscillator and the inelastic electron tunneling is modeled by a simple resonance model for the electronic structure. The rate of atom transfer is shown to be Arrhenius-like with a vibrational temperature set by the inelastic tunneling rate. Characteristic features of this mechanism include a crossover from current-driven to thermally activated bond breaking with decreasing applied voltage and a power-law dependence of the bond-breaking rate with the applied voltage, the latter in agreement with experimental findings. We have also identified a current-induced force in the resonance model for tunneling, which in some cases may give an important current-dependent contribution to the potential-energy surface. The general features of our theory should have relevance for many other electronically driven surface processes.

Characterization of metal clusters (pd and au) supported on various metal oxide surfaces (mgo and tio2)

Abstract: Au clusters supported on the TiO2(001)/Mo(100) surface and Pd clusters supported on the MgO(100)/Mo(100) surface have been studied using temperature programmed desorption, ion scattering spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Three dimensional growth has been observed at 300 K for both cases. The adsorption energy of Au on the TiO2(001) surface has been measured to be 50 kcal/mol using temperature programmed desorption, compared to the adhesive energy of 90 kcal/mol for the bulk Au metal. At an equivalent Au coverage of 1.8 ML, very homogenous, hemispherical Au particles with an average diameter of 25 A and a height of 10 A are present on a TiO2(001)/Mo(100) surface. Increasing the Au coverage to 3 equivalent monolayers produces ∼60 A diameter clusters. The scanning tunneling spectroscopy measurements show a gradual development of the metallic character of the Au clusters with increasing cluster size. Very similar results have been obtained for the Pd clusters su...

Exploring Scanning Probe Microscopy with MATHEMATICA

Abstract: 1 Introduction. 2 Uniform Cantilevers. 3 Cantilever Conversion Tables. 4 V-Shaped Cantilevers. 5 Tip Sample Adhesion. 6 Tip Sample Force Curve. 7 Free Vibrations. 8 Noncontact Mode. 9 Tapping Mode. 10 Metal-Insulator-Metal Tunneling. 11 Fowler-Nordheim Tunneling. 12 Scanning Tunneling Spectroscopy. 13 Coulomb Blockade. 14 Density of States. 15 Electrostatics. 16 Near-Field Optics. 17 Constriction and Boundary Resistence. 18 Scanning Thermal Conductivity Microscopy. 19 Kelvin Probe Force Microscopy. 20 Raman Scattering in Nanocrystals.

A low-temperature ultrahigh-vacuum scanning tunneling microscope with rotatable magnetic field

Abstract: We present a new design of a low-temperature ultrahigh-vacuum (UHV) scanning tunneling microscope setup with a combination of a solenoid and a split-pair magnet The scanning tunneling microscope can be operated at temperatures down to 8 K and in a rotatable magnetic field of up to 1 T Magnetic fields of up to 7 T perpendicular and 2 T parallel to the sample surface can be applied The UHV part of the system allows in situ preparation and low energy electron diffraction/Auger analysis of samples First topographic and spectroscopic measurements on p-InAs(110) are presented

Structure determination of two-dimensional adenine crystals on graphite

Abstract: Two-dimensional molecular-packing structure and monolayer preparation of adenine adsorbates on the graphite (0001) surface have been studied using scanning tunneling microscopy, low-energy electron diffraction, and thermal-desorption spectroscopy. By combining real-space images and diffraction data a close-packed hydrogen-bonded network of adenine dimers is proposed, containing two dimers in a unit cell with the symmetry group p2gg. The energy-minimized molecular arrangement could be determined by force field calculations. Adenine adsorbate layers were prepared by sublimation in UHV.

Tunneling Evidence of Half-Metallic Ferromagnetism in La(0.7)Ca(0.3)MnO(3)

Abstract: Direct experimental evidence of half-metallic density of states (DOS) is observed by scanning tunneling spectroscopy on ferromagnetic La(0.7)Ca(0.3)MnO(3) which exhibits colossal magnetoresistance (SMR).

Single electron tunneling and level spectroscopy of isolated C60 molecules

Abstract: The interplay between single electron tunneling effects and the discrete molecular levels of C60 molecules is studied using scanning tunneling microscopy. Isolated C60 molecules were deposited onto a gold substrate, covered by a thin insulating layer. The tunneling current–voltage characteristics of isolated molecules, both at room temperature and at 4.2 K, exhibit rich structures, resulting from the interplay between charging effects and the electronic spectrum. In particular, we observe degeneracy lifting within the C60 molecular orbitals, probably due to the Jahn–Teller effect and local electric fields.

System and method for determining near--surface lifetimes and the tunneling field of a dielectric in a semiconductor

Abstract: The present invention is directed to a system for, and method of, determining a non-contact, near-surface generation and recombination lifetimes and near surface doping of a semiconductor material. The system includes: (1) a radiation pulse source that biases a dielectric on top of the semiconductor material, (2) a voltage sensor to sense the surface voltage, and (3) a photon source to create carriers. For lifetime measurements both the excitation and measurement signals are time dependent and may be probed near the surface of the semiconductor to obtain various electrical properties. For high-field tunneling and leakage characteristics of a thin dielectric (<15 nm) on top of the semiconductor, a high bias charge density is used to induce tunneling, from which tunneling fields and charge-fluence to tunneling of the dielectric are determined.

Observation of atomic tunneling from an accelerating optical potential

Abstract: We observe quantum tunneling of cold sodium atoms from an accelerating one-dimensional standing wave of light. Atoms are trapped in a far-detuned standing wave that is accelerated for a controlled duration. For sufficiently large values of the acceleration, we observe an exponential decay in the number of atoms that remain trapped as a function of the interaction time. We show that this loss is due to quantum tunneling, and compare the decay rates with Landau-Zener theory. We also observe oscillations in the tunneling rate as a function of the acceleration which are due to quantum interference effects.

Controlled Atom by Atom Restructuring of a Metal Surface with the Scanning Tunneling Microscope

Abstract: We report the ability to completely restructure a metal surface by precision manipulation of individual atoms with the scanning tunneling microscope: Besides extracting atoms from kink sites on Cu(211) we are now also able to “dig out” atoms from the even more strongly bound intrinsic step sites and thus to create adatom-vacancy pairs. Together with the processes of moving adatoms along and across intrinsic step edges and the possibility of healing out adatom-vacancy pairs, we have a complete set of lateral manipulation processes at hand. The reliability of all these processes opens up exciting “engineering” possibilities for structuring of extended surface areas. [S0031-9007(97)02523-4]

Observation Of Atomic Tunneling From An Accelerating Optical Potential

Abstract: ~Received 30 October 1996! We observe quantum tunneling of cold sodium atoms from an accelerating one-dimensional standing wave of light. Atoms are trapped in a far-detuned standing wave that is accelerated for a controlled duration. For sufficiently large values of the acceleration, we observe an exponential decay in the number of atoms that remain trapped as a function of the interaction time. We show that this loss is due to quantum tunneling, and compare the decay rates with Landau-Zener theory. We also observe oscillations in the tunneling rate as a function of the acceleration which are due to quantum interference effects. @S1050-2947~97!50702-5# PACS number~s!: 03.75.Be, 32.80.Pj, 42.50.Vk Tunneling in the center-of mass motion of atoms should become an important process at the low temperatures that are now achieved with laser cooling. This can appear, for example, as a loss mechanism for atoms confined in an optical lattice constructed from interfering beams of light. While atomic motion in these structures has become an active area of research @1,2#, tunneling has been obscured by spontaneous scattering @3#. An alternative atom optics system where tunneling can be significant has appeared in our efforts to develop a cold atomic beam for interferometry. Our approach has been to launch laser-cooled atoms in the potential created by an accelerating, far-detuned, standing wave of light. This system was previously used to study Wannier-Stark ladders @4# and Bloch oscillations @5#. The results reported in this Rapid Communications provide an observation of tunneling in atom optics, and the high degree of experimental control has enabled a quantitative comparison with theory. We consider atomic motion in an accelerating standing wave of light. For sufficiently large detuning from atomic resonance, the atoms remain in their internal ground state and experience an effective one-dimensional potential given by V0cos@2kL(x2at 2 /2)#, where k L52p/l L is the wave number, and a is the acceleration of the standing wave @6#. The well depth V0 is the amplitude of the optical dipole potential. It is proportional to the laser intensity and inversely proportional to detuning from atomic resonance. We neglect variations of the potential in the two transverse directions, which is justified for beams that are sufficiently

Theory of the Quasiparticle Spectra around a Vortex in the Two-Dimensional t- J Model

Abstract: The microscopic structure of vortex cores in high- T c superconductors is studied using the two-dimensional t - J model under a uniformly applied magnetic field. The spatial dependence of the pair potential is calculated for various doping rates via the Gutzwiller approximation, in which the effect of strong correlation is taken into account. The results show that, in the wide range of doping rates, the pair potential has d -wave nature and the local density of states near the core center shows a zero-energy peak. However, in the low-doping region, an s -wave component is locally induced around the core and the zero-energy peak splits into two levels. This doping-dependence is characteristic of the t - J model. Our results give a possible explanation for the experimental observation by scanning tunneling spectroscopy [Phys. Rev. Lett. 75 (1995) 2200] which demonstrates the splitting of the local density of states.

Schottky-barrier formation at nanoscale metal-oxide interfaces

Abstract: Isolated, nanoscale (5.0--20.0 nm diameter) Cu clusters on a reduced ${\mathrm{TiO}}_{2}$ (110) surface exhibit the initiation of the Schottky effect. Apparent height changes of isolated clusters occur in scanning tunneling microscopy imaging as bias conditions are changed. This apparent height change is directly related to current flow through the cluster-oxide interface barrier. Further, depletion zones along the substrate surface adjacent to the clusters exhibit the same bias dependence indicating that changes are associated with local band-bending, analogous to that of macroscopic Schottky barriers. Barrier-height variations with cluster size and with applied voltage are quantified. When compared to models of edge effects in finite-sized systems a direct correlation between geometry and barrier formation is made.

Observation of Landau levels at the InAs(110) surface by scanning tunneling spectroscopy

Abstract: Low-temperature scanning tunneling spectroscopy measurements on clean InAs~110! surfaces demonstrate the possibility to investigate Landau quantization with subnanometer resolution. Separate Landau levels are resolved at magnetic fields of 2 T and larger. Experiments with different tips show a significant tip dependence. In zero field resonances are observed near the onset of the conduction band, which are attributed to tip-induced band bending. Although Landau quantization is only present parallel to the sample surface, the Landau levels give a large contribution to the total tunnel current. @S0163-1829~97!52424-6#

Kink defects and Fermi level pinning on (2 × 4) reconstructed molecular beam epitaxially grown surfaces of GaAs and InP studied by ultrahigh-vacuum scanning tunneling microscopy and x-ray photoelectron spectroscopy

Abstract: The relationship between kink defects and Fermi level pinning on molecular beam epitaxially grown GaAs and InP(001)-(2×4) surfaces is studied in detail by scanning tunneling microscopy and x-ray photoelectron spectroscopy. In Si-doped GaAs, the kink density increased with doping as previously found. However, actual density depended very much on the reconstruction phases and experimental conditions. At high Si doping levels, Fermi level was strongly pinned below mid-gap, but the measured kink density was found not to be large enough to explain pinning by the previous kink-deep-acceptor model assuming that each kink forms a single discrete level. In Si-doped InP, the kink density remained constant with the increase of Si doping, although the Fermi level was pinned above mid-gap. The result cannot be explained by the kink-deep-acceptor model either.