Kondo effect in a triangular triple quantum dots ring with three terminals

For a triangular triple quantum dots (TTQDs) ring with three terminals, when lowering one of the dot-lead coupling to realize the left-right (L-R) reflection symmetry coupling, the internal C-upsilon of the TTQDs is well preserved in the absence of many-body effect for the symmetric distribution of the dot-lead coupling on the molecular orbits. In the presence of Kondo effect, the decrement of one of the dot-lead couplings suppresses the inter-dot hopping. This happens especially for the coupled quantum dot (QD), which decouples with the other two ones gradually to form a localized state near the Fermi level As a result, the internal dynamic symmetry of the TTQDs ring is reduced to L-R reflection symmetry, and simultaneously, the linear conductance is lifted for the new forming molecular orbit near the Fermi level

Rashba electron's ballistic transport in two-dimensional quantum waveguide

The ballistic transport of Rashba electrons in a straight structure in two-dimensional electron gas is studied. It is found that there is no mixing between the wave functions of spin up and spin down states, and the transfer matrix is independent for the spin in every interface. The influence of the structure and Rashba coefficient on the electron transport is investigated. Our results indicate that the transmission probabilities are independent of the sign and magnitude of the Rashba coefficient and it depends on the shape of the structure, especially the stub width. The antiresonance is found, where the quasiconfined state is formed in the center part of the structure.

THEORETICAL INVESTIGATION OF THE DYNAMIC PROCESS OF THE ILLUMINATION OF GAAS

The dynamic process of light illumination of GaAs is studied numerically in this paper to understand the photoquenching characteristics of the material. This peculiar behavior of GaAs is usally ascribed to the existence of EL2 states and their photodriven metastable states. To understand the conductivity quenching, we have introduced nonlinear terms describing the recombination of the nonequilibrium free electrons and holes into the calculation. Though some photoquenching such as photocapacitance, infrared absorption, and electron-paramagnetic-resonance quenching can be explained qualitatively by only considering the internal transfer between the EL2 state and its metastability, it is essential to take the recombination into consideration for a clear understanding of the photoquenching process. The numerical results and approximate analytical approach are presented in this paper for the first time to our knowledge. The calculation gives quite a reasonable explanation for n-type semiconducting GaAs to have infrared absorption quenching while lacking photoconductance quenching. Also, the calculation results have allowed us to interpret the enhanced photoconductance phenomenon following the conductance quenching in typical semi-insulating GaAs and have shown the expected thermal recovery temperature of about 120 K. The numerical results are in agreement with the reported experiments and have diminished some ambiguities in previous works.

ELECTRIC-FIELD-INDUCED EXCITON-LINEWIDTH BROADENING IN SHORT-PERIOD GAAS/GAXAL1-XAS SUPERLATTICES

We have investigated the Wannier-Stark effect in GaAs/GaAl1-xAs superlattices under electric fields by photocurrent spectroscopy measurements in the range of temperatures 10-300 K. The linewidth of the Oh Stark-ladder exciton was found to increase significantly along with an increase in peak intensity when the electric field increases. We present a mechanism based on an enhanced interface roughness scattering of electronic states due to Wannier-Stark localization in order to explain this increased broadening with electric field. This electric-field-related scattering mechanism will weaken the negative differential conductance effects in superlattices predicted by Esaki and Tsu.

A NOVEL ODOR SENSOR COATED WITH A LIPID-MEMBRANE

In this paper, an interdigital electrode lipid film odour sensor (ILOS) is designed, fabricated and tested. It is made from a microfabricated chemiresistor coated with a synthetic multibilayer film. Nine odorants in gas phase at room temperature have been detected using the odour sensor. For most of the odorants, the relation between the response of the ILOS and odorant concentration obeys Stevens' power law, and there is a good correlation between the minimum odorant concentrations that give rise to a change of the sensor's conductance and human olfactory thresholds.

QUANTUM WAVE-GUIDE THEORY FOR MESOSCOPIC STRUCTURES

A one-dimensional quantum waveguide theory for mesoscopic structures is proposed, and the boundary conditions of the wave functions at an intersection are given. The Aharonov-Bohm effect is quantitatively discussed with use of this theory, and the reflection, transmission amplitudes, etc., are given as functions of the magnetic flux, the arm lengths, and the wave vector. It is found that the oscillating current consists of a significant component of the second harmonic. This theory is also applied to investigate quantum-interference devices. The results on the Aharonov-Bohm effect and the quantum-interference devices are found to be in agreement with previous theoretical results.

Electronic states and magnetotransport in quantum waveguides with nonuniform magnetic fields

The electronic states and magnetotransport properties of quantum waveguides (QW's) in the presence of nonuniform magnetic fields perpendicular to the QW plane are investigated theoretically. It is found that the magnetoconductance of those structures as a function of Fermi energy exhibits stepwise variation or square-wave-like oscillations, depending on the specific distributions (both in magnitude and direction) of nonuniform magnetic fields in QW's. We have investigated the dual magnetic strip structures and three magnetic strip structures. The character of the magnetotransport is closely related to the effective magnetic potential and the energy-dispersion spectrum of electron in the structures. It is found that dispersion relations seem to be combined by different sets of dispersion curves that belong to different individual magnetic subwaveguides. The magnetic effective potential leads to the coupling of states and the substantial distortion of the original dispersion curves at the interfaces in which the abrupt change of magnetic fields appears. Magnetic scattering states are created. Only in some three magnetic strip structures, these scattering states produce the dispersion relations with oscillation structures superimposed on the bulk Landau levels. It is the oscillatory behavior in dispersions that leads to the occurrence of square-wave-like modulations in conductance.

The scattering matrix method for quantum waveguides

A scattering matrix method for investigating the electron transport in quantum waveguides is presented. By dividing the structure into a number of transverse slices, the global scattering matrix is obtained by the composition of the individual scattering matrices associated with each interface. Complicated geometries and inhomogeneous external potentials are included in the formulation. It is shown that the proposed scattering matrix method possesses many advantages over the traditional mode-matching and transfer matrix methods, especially in treating the electron wave propagation in complicated geometries. Justification for the method is provided by the unitarity of the calculated scattering matrix, and the consistency of the results with those obtained by the recursive Green's function method.

Quantum coherent networks: A theoretical study

Electron transport in quantum coherent networks (interacting quantum waveguide arrays) is investigated theoretically with use of the scattering-matrix method. The scattering matrix for the basic unit of networks, the cross junction with Square or rounded corners, is derived using the mode-matching technique, The overall scattering matrix for the network is obtained by the composition of the scattering matrices associated with each unit of the network, For a uniform network, the transmission spectra are calculated in the single-mode regime and an found notably dependent on the junction geometry. Small reflection for the input terminal and uniform output for some output ports are obtained, which means that the quantum coherent network can be used as a distributing net for the electron waves. Cross junctions with rounded corners of large radii are found to play a negative role in the device application of quantum coherent networks. (C) 1997 American Institute of Physics.

Suppression of ballistic electron transmission through a semiconductor II-structure by an external transverse electric field

We have conducted numerical studies of ballistic electron transport in a semiconductor II-structure when an external transverse electric field is applied. The device conductance as a function of electron energy and the strength of the transverse electric field is calculated on the basis of tight-binding Green's function formalism. The calculations show that a relatively weak electric field can induce very large decrease in the electron transmission across the structure. When the transverse electric field is sufficiently strong, electrons can hardly be transported through the device. Thus the performance of the device can be greatly improved for it is much easier to control electron transport through the device with an external transverse electric field.

Numerical simulation of quantum directional couplers

A numerical analysis of a quantum directional coupler based on Pi-shaped electron waveguides is presented with use of the scattering-matrix method. After the optimization of the device parameters, uniform output for the two output ports and high directivity are obtained within a wide range of the electron momenta. The electron transfer in the device is found more efficient than that in the previously proposed structures. The study of the shape-dependence of transmission for the device shows that the device structure with smooth boundaries exhibits a much better performance.

Influence of the lateral confining potential on the ballistic electron transport in a quantum wire

A theoretical investigation of ballistic electron transport in a quantum wire with soft wall confinement is presented. A general method of the electron transmission calculation is proposed for structures with complicated geometries. The effects of the lateral guiding potential on ballistic transport are investigated using three soft wall confinement models and the results are compared with those obtained from the hard wall confinement approximation. It is shown that the calculated transmission coefficients are notably dependent on the lateral confining potential especially when the incident electron energy is larger than the energy of the second transverse mode. It is found that the transmission profile obtained from soft wall confinement models exhibits simpler resonance structures than that obtained from the hard wall confinement approximation. Our results suggest that only in the single-channel regime the hard wall confinement approximation can give reasonable results.

A transition layer model and its application to resonant tunneling in heterostructures

A transition layer model is proposed and used to calculate resonant tunneling in a double-barrier quantum well system. Compared with the ideal step of the potential at the interface, the studied system has transition layers that are composed by many thin rectangular barriers with a random height. It is found that these transition layers can improve the peak-to-valley ratio of the tunneling current and change the negative differential conductance.

Temperature dependence of the low-field mobility of a very narrow miniband in the degenerate case

Perpendicular transport in a specially designed, doped and weakly coupled GaAs/AlAs superlattice is investigated. A linear current-voltage at a bias within +/-10 mV and a negative differential velocity effect at a bias of about +/-40 mV are observed at low temperatures. The miniband conductance near zero electric field is studied as a function of temperature. It is found that the measured conductance increases slightly as the temperature increases to about 30 K, decreases as the temperature rises from 30 K to 70 K, and then increases strongly above 70 K, indicating the existence of a mobility gap.