Ballistic Transmission Research Articles

Nonadiabatic transport through quantum dots.

Nonadiabatic transport through a constriction in a quasi-one-dimensional quantum wire or through a quantum dot connected to quantum-wire leads occurs when different lateral modes are coupled by scattering off changes in the geometry of the conducting channel. We present calculations of coherent resonant tunneling through quantum dots that are connected nonadiabatically to the quantum-wire leads either by abrupt junctions or by smoothly tapered junctions. We determine how the nonadiabatic transport is influenced by the type of transport through the dot by studying transmission through dots that are connected to the leads across potential barriers with different heights (ballistic transmission for no barriers, resonant tunneling for large barriers). Nonadiabatic effects are enhanced when the transport through the dot occurs by resonant tunneling rather than ballistic transport. Resonant transport through dots with smoothly tapered connections can be nonadiabatic even when ballistic transport through constrictions with the same connections is adiabatic. These results justify the recent suggestion that the mode mixing that occurs during resonant tunneling through a vertical quantum dot is strong enough to open the additional transport channels needed to understand the fine structure observed in the resonant tunneling.

Elastic scattering in ballistic-electron-emission microscopy studies of the epitaxial NiSi2/Si(111) interface.

We have fabricated epitaxial ${\mathrm{NiSi}}_{2}$ films on Si(111) under ultrahigh-vacuum conditions and conducted in situ ballistic-electron-emission-microscopy (BEEM) measurements on this coherent interface system. We demonstrate the sensitivity of BEEM to mechanisms which broaden the angular distribution of the injected electrons for transport through a Si(111) interface. Spatially localized increases in ballistic transmission rates are observed, attributable to the elastic scattering of the injected electrons by structural defects present on the surface or buried in the silicide film.

Ballistic electron emission microscopy studies of the NiSi2/Si(111) interface

We have performed ballistic electron emission microscopy measurements on the NiSi2/Si(111) system under UHV conditions. Schottky barrier heights have been measured for types A and B interfaces and are found to be in agreement with other techniques. Through correlations between structural defects in the silicide and local increases in collector currents, we find that elastic scattering in the film can increase ballistic electron transmission rates across the interface.

Ballistic Electron Transmission in GaAs–AlGaAs Crossed Wire Junctions

Four-terminal resistances in narrow GaAs–AlGaAs crosses are investigated. The anomalies in the bend and Hall resistances give us information about the transmission of a ballistic electron in the cross. The transmission probabilities turning to the right and left side probes are kept comparable over a wide range of magnetic field, although the transmission probability for the motion straight on the cross is suppressed once the field is applied.

Ballistic magnetoresistance and the Hall effect in a restricted geometry

The authors have measured the magnetoresistance and Hall resistance of an open-cross structure formed in a two-dimensional electron gas by electrostatic depletion. They observe effects similar to those seen by other authors in more closed geometries. They interpret results using the classical model of Beenakker and van Houten (1989). By measuring the ballistic transmission coefficient directly they are able to provide quantitative confirmation of the model.

Ballistic transmission in perpendicular quantum point contacts.

We have measured the series resistance of a pair of quantum point contacts (QPC's) oriented with their axes mutually perpendicular. In zero magnetic field the series resistance of the QPC's is the classical Ohmic sum of the two individual resistances. The degree of ballistic transmission increases sharply as the magnetic field is increased and is also weakly dependent on the width of the QPC. This behavior is interpreted in terms of a simple model involving the classical ballistic trajectory of the electrons.

Nonlocal quantum transport in narrow multibranched electron wave guide of GaAs-AlGaAs

Quantum transport of electrons through a narrow multibranched electron wave guide is investigated. The electron wave guides are fabricated from high and low mobility GaAsAlGaAs wafers. The background average resistance revealed quite different behavior in both samples. It is found in a high mobility (ballistic) sample that the resistance is negative and with applying magnetic field it approaches to zero at B∼0.2T, where the diameter of the cyclotron orbit is comparable to the channel width W∼0.3 μm. In a low mobility (diffusive) sample, the average resistance is positive and non-zero in the entire field range. The positive resistance of the diffusive sample is comparable to a starting film resistivity, while the negative resistance originates from a bend of the current carrying channel and is attributed to the ballistic electron transmission.

Ballistic electron transmission through interfaces.

We report on a new method for calculating ballistic electron transmission through epitaxial interfaces and its application to a silicon twist boundary. This method is based on constructing the electronic states of the infinite system from the generalized Bloch states for each layer in the structure; these states are matched together at the layer boundaries to construct the composite wave function. We find that the conduction-band electrons and the degenerate light and heavy holes within thermal energies of the band gap are strongly scattered by the twist boundary, but that the split-off holes in the same energy range are not. These results, which are unexpected from naive effective-mass considerations, can be simply understood in terms of flux patterns of the band-extrema wave functions on the interface-matching plane.

Angular dependence of phonon transmission through a Fibonacci superlattice

Phonon imaging is employed to examine the propagation of acoustic phonons through a Fibonacci superlattice. Ballistic transmission of phonons with \ensuremath{\nu}>850 GHz through 750 superlattice interfaces is detected. In addition, sharp variations in the phonon intensity with propagation angle are observed. These measurements are consistent with Monte Carlo simulations presented in this paper. Distinct stop bands are expected theoretically, and the angular dependence of these structures is remarkably similar to those predicted for a periodic superlattice.

Ballistic Propagation of Near-Gap Phonons in Bulk Superconducting Tin

Phonon transport in $\ensuremath{\beta}$-Sn crystals was studied by heat-pulse techniques using superconducting tunnel-junction detectors. The temperature-dependent attenuation by the quasiparticles was measured at fixed frequency (\ensuremath{\simeq} 160 GHz), while, at lower temperature (0.6 K), the ballistic transmission was monitored up to the onset of pair-breaking frequencies. A new method of determining the energy gap follows, which is shown to apply to nonequilibrium states of the bulk.