In the spectroscopy of atoms and molecules, an asymmetric Fano resonance arises whenever a bound state associated with one electronic configuration is coupled to the ionization continuum of a different configuration. A strikingly similar resonance appears for electronic transport in conductors with more than one subband, independent of the specific details of the system under study. We develop a two-subband approximation which describes the Fano resonances for conduction through an electron waveguide containing donor impurities, for Γ-X-Γ intervalley tunneling in a GaAsAlxGa1-xAsGaAs heterojunction, and for an electron waveguide coupled to a resonant cavity. Interference between the direct and intersubband transmission channels gives rise to the asymmetric Fano resonance. © 1993 The American Physical Society

We developed a formalism within the linear-response theory to investigate the transport through a quantum point contact between two electron-gas reservoirs. It is valid for two-terminal conductance through a constriction of a two-dimensional (2D) or 3D potential and has a wide range of applicability covering ballistic as well as tunneling regimes. We studied the quantization of conductance and examined several effects influencing the quantum transmission. Among these effects we found that the simple phase relation results in resonance structures superimposed on the plateaus between two steps of quantized conductance. These resonances are destroyed by the smooth entrance, finite temperature and bias, and variation of the potential. The simulation of adiabatic transmission in constrictions having smoothly varying widths resulted in the conductance with sharp quantum steps without the resonance structure. The quality of quantization is strongly affected by the length of constriction, Fermi-level smearing, the obstacle at the entrance, impurity scattering, nonuniformities of geometry and potential, and in particular by the variation of the longitudinal potential resulting in a sharp saddle-point structure. The quasibound states may occur in a local widening of the width or in a locally lowered potential. These states give rise to a sudden increase of the transmission prior to the opening of a new conduction channel. We present an extensive analysis of this phenomenon and show that it is due to resonant tunneling through these bound states. Owing to enhanced backscattering, the bound states of an attractive impurity in a constriction can yield dips in the conductance at the threshold of channels. In addition to quantized ballistic transport, we extended our method to treat the transport mechanism in scanning tunneling microscopy and in field emission of collimated electrons from an atomic-size source. The issues of current interest in these fields that we treated are (i) the transition from the tunneling to the ballistic regime and the interpretation of conductance oscillations, and (ii) the anomalous corrugation of Hat metal surfaces. Our results reveal crucial features of the lateral confinement of the currenttransporting states in the constriction of potential between the tip and sample. The effective barriers created from this confinement effect dominate the transmission at small tip-sample distances and inAuence the apparent barrier height.

We investigate the interaction energy, the short-range force components, and the electron potential between two Al(001) slabs, which mimic a blunt tip close to an atomically corrugated sample in scanning force microscopy. The adhesive energy and perpendicular force calculated using the self-consistent-field pseudopotential method in the local-density approximation are site dependent, but can be accurately represented by a universal function in terms of scaled variables in the attractive range. The lateral force which determines friction variations on an atomic scale is not simply proportional to the perpendicular force and is typically one order of magnitude smaller. At larger separations the effect of the total long-range Van der Waals force and of its gradient are estimated to be small for a sharp conical support tip, but quite appreciable for a rounded support tip with a radius as small as 200. By calculating the interaction energy of an Al atom between two slabs, we also study the possibility of single-atom transfer between tip and sample, and show that the double well in the interaction energy collapses into a single minimum at a slab separation larger than two bulk interlayer spacings. The atom is preferentially located on the side of the deeper minimum, but can hop between the two wells at finite temperatures. Moreover, the position of the deeper minimum relative to the electrodes can vary as the tip is scanned against the sample. Finally we explore possible relations between the short-range perpendicular force and the tunneling conductance through the potential barrier between two semi-infinite jellium slabs as a function of their separation. © 1992 The American Physical Society

The effects of the geometry and temperature on the quantum conductance for one-dimensional (1D) ballistic transport through a constriction in a 2D electron gas are investigated by use of a refined formalism. As the length of the constriction increases, weak oscillations around the classical conductance evolve into a steplike structure and the resonances on the plateaus become pronounced. Quantization at integer multiples of 2e2/h occurs only for uniform constriction of finite length. At finite temperature of 0.6 K significantly long uniform constriction is necessary to observe plateaus devoid of resonance structure. © 1989 The American Physical Society