Localized scatterers can be expected to give rise to spatial variations in the electric field and in the current distribution. The transport equation allowing for spatial variations is solved by first considering the homogeneous transport equation which omits electric fields. The homogeneous solution gives the purely diffusive motion of current carriers and involves large space charges. The electric field is then found, and approximate space charge neutrality is restored, by adding a particular solution of the transport equation in which the electric field is associated only with space charge but not with a current. The presence of point scatterers leads to a dipole field about each scatterer. The spatial average of a number of these dipole fields is the same as that obtained by the usual approach which does not explicitly consider the spatial variation. Infinite plane obstacles with a reflection coefficient r are also considered. These produce a resistance proportional to r/(1-r).

Spatial variation of currents and fields due to localized scatterers in metallic conduction

Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.

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Scanning tunneling spectroscopy of high-temperature superconductors

Optical rectification and electric-field enhancements in excess of 1,000 are observed when a subnanometre gap between gold electrodes is illuminated with infrared radiation.

Optical rectification and field enhancement in a plasmonic nanogap

Interaction of slow multicharged ions with solid surfaces

A method has been developed for performing fast time-resolved experiments with a scanning tunneling microscope. The method uses the intrinsic nonlinearity in the microscope's current versus voltage characteristics to resolve optically generated transient signals on picosecond time scales. The ability to combine the spatial resolution of tunneling microscopy with the time resolution of ultrafast optics yields a powerful tool for the investigation of dynamic phenomena on the atomic scale.

https://science.sciencemag.org/content/sci/262/5136/1029.full.pdf

Picosecond resolution in scanning tunneling microscopy

Scattering-theoretic approach to elastic one-electron tunneling through localized barriers: Application to scanning tunneling microscopy.

Recent measurements of dc current-voltage characteristics of scanning-tunneling-microscope (STN) junctions have confirmed their expected high rectification property. We propose to exploit this property (i) to study rectification at infrared and optical frequencies and (ii) to arrive at a procedural definition of an electron tunneling time. We envisage focusing a laser beam of linearly polarized light onto a STM junction and detecting the dc bias induced across the junction by the alternating, asymmetrical tunnel current. The asymmetry may be of geometrical, material, and/or thermal origin. The dc rectified bias should vanish at sufficiently high laser frequency for fixed tip-to-surface distance or when withdrawing the tip away from the surface for fixed laser frequency. The average electron tunneling time is then one half of the inverse cutoff frequency observed in such a laser-rectification experiment. Similar experiments have already been conducted successfully with metal-whisker diodes and have resulted in the recent redefinition of the meter. The new opportunity offered by STM is the controllability of the junction and its microscopic size, entailing high responsivity. Additional possible laser-STM studies are proposed.

Proposed use of a scanning-tunneling-microscope tunnel junction for the measurement of a tunneling time.

Energy-loss spectra and electronic-transition probabilities of atomic and molecular ions scattered by surface plasmons are evaluated for experiments in which fast ions are specularly reflected from a metal surface at grazing incidence. The coupling between plasmons and the ion monopole charge governs the scattered-ion loss spectrum. For ion velocities not small compared to plasmon phase velocities and for small grazing angles, large numbers of surface plasmons may be excited, resulting in large average energy losses and broad loss spectra, as observed experimentally. Further, the interaction between the ion multipole moments with the surface plasmons induces transitions between ground and excited states of the ions. For molecular ions this may result in their dissociation when the state produced is unstable. The final electronic states populated by the plasmon scattering mechanism are shown to exhibit a high degree of orientation, as required by the observation of elliptically polarized fluorescence emitted by the scattered beam. As applications, we calculate explicitly the dissociative scattering of H/sup +//sub 2/ and the production of coherently oriented 2P states in He/sup +/ and compare with available experimental results.

A. A. Lucas

Papers

Scattering-theoretic approach to elastic one-electron tunneling through localized barriers: Application to scanning tunneling microscopy.

Proposed use of a scanning-tunneling-microscope tunnel junction for the measurement of a tunneling time.

Self-image excitation mechanism for fast ions scattered by metal surfaces at grazing incidence

Collective Modes of Void-surface Coupled System

New scattering-theoretic approach to elastic one-electron tunneling through spatially localized barriers: Application to scanning tunneling microscopy