Spatial variations in submolecular vibronic spectroscopy on a thin insulating film

Scanning tunneling spectroscopy on single naphthalocyanine molecules adsorbed on an ultrathin aluminum oxide film exhibits electron-vibronic coupling that varies with the position of tunneling over the molecule. The spectra at different positions are composed of several series of equally spaced peaks, which are interpreted as progression of progressions of molecular vibrational modes. The spatial variations correlate with the molecular orbital structure, revealing spatially dependent electron-vibronic coupling and selective vibrational excitation.     

Tunneling current oscillations excited by DC voltage

Investigating a polycrystalline gold layer on glass by a scanning tunneling microscope in air, tunneling current oscillations were found, which are excited by the DC voltage across the gap. The oscillation amplitude is dependent on the place on the surface of the sample and correlates with its topography. The frequency spectra of these oscillations are influenced by resonances of the mechanical system (z-piezo/sample holder/transducer/sample). A piezoelectric transducer is able to detect alternating forces originating from the tunnel junction. The resonances in the spectrum of the AC tunneling current and the mechanical resonances of the STM system seem to be related. Trapping and subsequent delayed desorption of charge carriers at localized surface states could play a role in generating the observed time-dependent forces across the gap and thereby creating tunneling current oscillations.     

Heating of vibrational states of adatoms by the current of a scanning tunneling microscope

Effects associated with the vibrational heating of adatoms by the current of inelastically tunneling electrons were detected in spectroscopic experiments with a scanning tunneling microscope. A relation between the envelope curves of autoemission resonances and the vibrational distribution of particles located under the tip was ascertained.     

用扫描隧道显微术进行单原子操纵的机理研究

本文简要综述利用扫描隧道显微镜（ＳＴＭ）进行单原子操纵的物理机制。主要介绍了场增强的扩散、在表面上拖动（ｐｕｌｉｎｇ）推动（ｐｕｓｈｉｎｇ）原子、原子在针尖表面间接触和近接触转移、场致蒸发／脱附、隧道电子非弹性射激发和电子迁移的“电子风力”等过程。同时介绍了一些理论处理方法和对一些实验结果的解释。     

On the theory of scanning tunnel microscopy of nanosized clusters

Studies of nanosized clusters by scanning tunnel microscopy require not only the determination of the energy spectra and wave functions of the system but also information about the Green function and dynamics of intracluster transitions. For solving this problem, we suggest a modified variant of the theory of multiple scattering. This method allows analysis to be performed using physically visual concepts of tunneling electron transitions between centers that constitute a nanocluster. It is assumed that one-electron nanocluster states (or at least part of them) can be filled at an intermediate electron tunneling stage. Under these conditions, nanosystem energy levels are interpreted as resonances in the tunnel transition of an electron from the tip to the metal surface through centers situated on it. We reveal a peculiar behavior of such resonances in uniform chain structures situated parallel to the surface. If the tip is in the plane that is perpendicular to the surface and passes through the center of the chain, resonances related to antisymmetric states are completely absent in scanning tunnel microscopy spectra. The effects specified remain in force also in the presence of impurities (including spin-dependent impurities) in chains if these impurities do not distort their spatial symmetry.     

Tuning the transmission phase by the dot size

The phase shift of the electron's wave function after a tunneling event (i.e. the transmission phase) was at first measured for its fundamental or applicative relevance for quantum circuitry, but later the phase study self-motivated due to a number of unexpected results. One such result was the reduced increment of the phase on some resonances - with only fractions of π - in the few-electrons “mesoscopic” regime. In this paper we address such a regime for a rectangular quantum dot and compute the total phase increase on the first four resonances by means of accurate configuration–interaction method and a generalized Friedel sum rule as proposed by Rontani (2006) [17]. Our findings confirm that the electronic correlations reduce the on-resonance phase growth which is also found to decrease quasi–linearly with the dot size, the decrease being more pronounced as the number of electrons on the dot is raised. Sudden jumps (of small amplitude) of the phase are found to accompany ground states spin transitions.     

Tunneling effects,phase coherence and the large mass of the η or η′ meson in non-abelian gauge theory

The large mass of the η or η′ meson compared with the pion mass is explained within the framework of non-Abelian gauge theory. Effects of tunneling phenomena are analysed by introducing a “tunneling Hamiltonian”. It is found that there is order in the vacuum produced by “phase coherence”. The η or η′ meson is viewed as a wave due to disturbance of the order.     

Correlation between shape and electronic states in nanostructures

We show how the electronic states of quantum wires and quantum dots can be evaluated exactly starting from the profile of the nanostructure observed by transmission electron microscopy, scanning tunneling microscopy and atomic force microscopy. The calculated quantization energies reproduce the energy position of the luminescence resonances in the optical spectra of different samples, without fitting parameters.     

Vibrational transitions in experiments with a scanning tunneling microscope

A method of in-cavity scanning tunneling spectroscopy is proposed by which one can observe distinct spectra of electronic-vibrational field-emission resonances. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 306–310 (25 February 1997)     

Two-dimensional imaging of the current-density distribution in superconducting tunnel junctions

Using a scanning electron microscope equipped with a low-temperature stage, we show that under the electron irradiation a superconducting tunnel junction can operate regularly. The electron beam generates a voltage signal across the current-biased junction. By recording the small voltage change synchronously with the coordinate irradiated by the beam, a two-dimensional “voltage image” of the density distribution of the junction tunneling current can be obtained. The distributions are in agreement with the magnetic interference patterns of the dc-Josephson currents in the junctions.     

Dipolar broadening of double-quantum nuclear magnetic resonances in solids

Expressions are derived for the dipolar second moment of double-quantum nuclear magnetic resonances in solids. For like nuclei, double-quantum resonances are found to be narrower than single-quantum resonances because of a smaller “flip-flop” contribution.     

Enhanced photon emission by field emission resonances and local surface plasmon in tunneling junction

We investigated the photon emission spectra on Ag (111) surface excited by tunneling electrons using a low temperature scanning tunneling microscope in ultrahigh vacuum. Characteristic plasmon modes were illustrated as a function of the bias voltage. The one electron excitation process was revealed by the linear relationship between the luminescence intensity and the tunneling current. Luminescence enhancement is observed in the tunneling regime for the relatively high bias voltages, as well as at the field emission resonance with bias voltage increased up to 9 V. Presence of a silver (Ag) nanoparticle in the tunneling junction results in an abnormally strong photon emission at the high field emission resonances, which is explained by the further enhancement due to coupling between the localized surface plasmon and the vacuum. The results are of potential value for applications where ultimate enhancement of photon emission is desired.     

Quasi-distribution of tunneling time

Using real time Feynman histories, a quasi-distribution of tunneling time Q(τ) is introduced. For the tunneling time of resident time type, an explicit expression for Q is shown for square barriers. Q becomes oscillatory as the barrier becomes opaque. Some well-known tunneling times fall within the range of τ where Q takes non-negligible values. The formal “average” and the “variance” of the tunneling time are found to be related to known tunneling times. It is thus demonstrated that the quasi-distribution extracts the temporal information about tunneling from real time Feynman histories.     

Influence of a heterogeneous Al2O3 surface on the electronic properties of single Pd atoms

Electronic properties of single Pd atoms, deposited on Al(2)O(3)/NiAl(110), have been characterized by scanning tunneling spectroscopy at 12 K. The spectra reveal distinct conductivity resonances, assigned to discrete electronic levels in the atom. The energy position of the resonances reflects adsorption properties of Pd atoms on different sites of the oxide support. Mapping the spatial extent of conductivity channels in the Pd atoms yields the symmetry of the underlying electronic states. The results demonstrate the effect of a heterogeneous oxide surface on the electronic structure of adsorbed metal atoms.     

Nanoelectromechanics and single-charge tunneling

The coupling of electronic and mechanical degrees of freedom has important consequences in nanoscale systems, as emphasized in recent theoretical and experimental work. In particular, the electrical properties of composite nanosystems containing elements with quite different abilities to conduct electricity and with different mechanical properties have been found to be strongly affected. Here we briefly review some of our recent work on the nanoelectromechanics of “heteroconducting” and “heteroelastic” Coulomb blockade systems, where single charge tunneling is the dominant conduction mechanism. We examplify nanoelectromechanical effects both in normal and superconducting systems by discussing (i) a self-assembled single-electron tunneling device exhibiting a dynamical instability leading to “shuttling” of electrons by a movable Coulomb dot and (ii) shuttling of Cooper pairs by a movable single-Cooper-pair box.     

Plasmon resonance in sputtered gold films observed in scanning tunneling microscopy

Gold films grown on BaTiO₃ substrates, illuminated by a monochromatic beam in the visible range, were investigated using scanning tunneling microscopy (STM). Irregular variations in the tip displacements versus photon energy are observed at energies of the surface plasmon resonances (2.0 ± 0.1 and 2.7 ± 0.1 eV) of the rough gold film.     

Scanning tunneling microscopy of the nonequilibrium interaction of impurity states at semiconductor surfaces

Interaction of two localized impurity states of Si atoms at a GaAs surface was studied by scanning tunneling microscopy and spectroscopy. The effects of a twofold “switching” on and off of the states of each of the interacting atoms, the tunneling-interaction-induced mutual level pulling of these states, and the level stabilization near E _F were observed. These effects are explained in terms of the extended Anderson model.     

Theory of resonant tunneling through the insulator conduction band of a thin film metal-insulator-metal (MIM) heterojunction

An “atomic” model of an insulating barrier between two free-electron model metals is used to investigate resonant tunneling across the insulator in the presence of a medium to large, externally applied electric field (bias). The exact numerically calculated tunneling current exhibits a pronounced oscillatory bias dependence superposed on the dominant roughly exponential tunneling characteristic. The interpretation of these results in terms of an internal field emission or Fowler-Nordheim type tunneling subject to “periodic deviations” (or interferences) seems plausible and was suggested by Maserjian. To test this conjecture, a trapezoidal barrier model of our “atomic” model analyzed numerically. As expected, the trapezoidal barrier model could only qualitatively reproduce the oscillatory bias dependence of the barrier transmissivity and of the current. Furthermore this limited agreement depends on allowing the effective mass in the barrier to become a strictly adjustable parameter. This failure of the conventional model of the junction can be interpreted as follows: (i) For moderate external (bias) fields the trapezoidal barrier fails to account for the correct position dependence of the Blochwave vector in the insulator's conduction band, hence the correct interference conditions cannot be reproduced. (ii) For large external fields the band model itself begins to fail. An explanation of oscillatory bias dependence at the tunneling current in terms of splitting of the insulator's conduction band into a set of discrete Stark levels is suggested. It is demonstrated that a fit of the oscillatory tunneling characteristics in the “Fowler-Nordheim regime” is not a reliable technique to determine the effective mass in the thin insulating film of tunneling junctions over the energy interval containing the forbidden gap and the adjoining conduction-band.