Simulation of current in the scanning tunneling microscope

Simulations of infrared and optical rectification by geometrically asymmetric metal-vacuum-metal junctions for applications in energy conversion devices

We use a transfer-matrix methodology to simulate the rectification of infrared and optical radiation by geometrically asymmetric metal-vacuum-metal junctions in which one of the metals is flat while the other is extended by a tip. We determine in particular the power this junction could provide to an external load and the efficiency with which the energy of incident radiations is converted. We consider first situations in which the external radiation is monochromatic, with typical frequencies in the infrared and optical domains. We then consider situations in which the external radiation consists of a full range of frequencies, with amplitudes that are representative of a focused beam of solar radiation. We investigate in particular how the efficiency of the rectification is affected by the aspect ratio of the tip, the work function of the metallic elements and the occurrence of polarization resonances. Our results demonstrate that the rectification of infrared and optical radiation is possible using devices of the type considered in this work. They finally provide a quantitative analysis of the efficiency of this rectification.

Rectification properties of geometrically asymmetric metal-vacuum-metal junctions: a comparison of tungsten and silver tips to determine the effects of polarization resonances

We simulate with a transfer-matrix methodology the rectification properties of geometrically asymmetric metal-vacuum-metal junctions in which one of the metals is flat while the other is extended by a tip. We consider both tungsten and silver as the material for the tip and we study the influence of the dielectric function of these materials on the rectification properties of the junction. We determine in particular the power that these junctions could provide to an external load when subject to a bias whose typical frequency is in the infrared or optical domain. We also study the rectification ratio of these junctions, which characterizes their ability to rectify the external bias by providing currents with a strong dc component. The results show that these quantities exhibit a significant enhancement for frequencies Ω that correspond to a resonant polarization of the tip. With silver and the geometry considered in this paper, this arises for [Formula: see text] values of the order of 3.1 eV in the visible range. Our results hence indicate that the frequency at which the device is the most efficient for the rectification of external signals could be controlled by the geometry or the material used for the tip.

Comparison between experimental and computer simulations of current-voltage (I-V) characteristics of dielectric-coated photon-stimulated field emitters

For the purpose of simulating photon-stimulated field emission by taking account of three-dimensional aspects, a transfer-matrix formulation of electronic scattering was combined with a Floquet expansion of the wave function for taking account of quanta exchanges between the electrons and the external radiation. With specific techniques to preserve numerical stability, this transfer-matrix formalism is well suited to compute the transmission of the field-emitted/photon-stimulated electrons between two electrodes. This theory is applied to the computation of Fowler-Nordheim curves describing the photon-stimulated field emission of a tungsten plane emitter (described by z< or =0), which supports a nanometric protrusion and a dielectric coating. The extraction bias ranges from 12 to 24V, for an inter-electrode distance of 4nm. The electromagnetic radiation has a wavelength of 0.67 microm and a power flux density ranging from 5.96 x 10(10) to 5.96 x 10(12) W/m2. The effects due to the protrusion and the dielectric coating are studied. These theoretical results are compared with the experimental data.

Real-space formulation of the quantum-mechanical electronic scattering under static n-fold axially symmetric electric and magnetic fields in a projection microscope

A numerical technique is developed to compute electronic propagation in three-dimensional potential-energy and vector-potential distributions, as required for the quantum-mechanical modelization of scattering in static electric and magnetic fields in a projection microscope. The technique is implemented in a transfer-matrix and Green's functions general procedure to simulate field-emission and electronic projection microscopy. In particular, simulated observations of a transverse magnetic field confirms the occurrence of diffraction fringes that are oriented in the direction of the field. These diffraction fringes are mainly associated with the detection of a magnetic flux and tend to be more pronounced as this quantity increases. In the conditions of this paper, the smallest magnetic flux that was detected could be associated with a phase shift of around 2pi/8 rad.

Nonsquare transfer-matrix technique applied to the simulation of electronic diffraction by a three-dimensional circular aperture

The transfer-matrix methodology is frequently used to deal with elastic scattering problems that require a solution of the Schrodinger or homogeneous Maxwell equations in the continuous part of their spectra. Until now, this technique was limited to representations associated with square transfer matrices. This paper extends the transfer-matrix methodology to enable consideration of general representations associated with nonsquare matrices. The theory is illustrated by the diffraction of a field-emitted electronic beam by a three-dimensional circular aperture. The application focuses on the dependence of the long-range angular spread on the aperture radius, by highlighting the effects of the field-emission tip shape and dimensions.