Edge Of Wire Research Articles

LOCAL ELECTRON DENSITY PROFILE NEAR THE SAMPLE BOUNDARY INVESTIGATED BY MAGNETO-CAPACITANCE IN QUANTUM HALL REGIME

The spatial profiles of the electron density near the edge of narrow wires made from GaAs / AlGaAs heterostructure have been investigated using magneto-capacitance measurements in quantum Hall conditions. With decreasing the wire width below one micron, the profile of the electron density becomes steep and the electron density decreases due to the confinement potential at the boundaries of the wire. The extent of the depletion region and the local electron density near the wire edge is strongly influenced by the wire width, especially in sub-micron wires. The evaluation techniques of the local electron density profile near the edge are discussed in detail.

Role of wire imperfections in micromagnetic traps for atoms

We present a quantitative study of roughness in the magnitude of the magnetic fieldproduced by a current carrying microwire, i.e. in the trapping potential for paramagnetic atoms.We show that this potential roughness arises from deviations in the wire current flow due to geometric fluctuations of the edges of the wire : a measurement of the potentialusing cold trapped atoms agrees with the potential computed from the measurement of the wire edge roughness by a scanning electronmicroscope.

Effect of wire width on strain distribution and bandgap in laterally aligned InGaAs/GaAs quantum wire nanostructures

The effect of InGaAs quantum wire width in the GaAs matrix on the strain distribution was studied using the finite element method and the resultant strain-modified bandgap was analysed in light of the ‘model solid’ theory of Van de Walle. The absolute magnitude of the strain in the bulk of the wire was attenuated significantly at the wire end along the width direction. Based on wire centres, the strain-modified direct bandgap increased with the decrease in wire width. The experimentally observed blueshift in photoluminescence spectra reported elsewhere was much higher than the predicted value based on the change in strain-modified bandgap especially at small-width wires. Other sources, such as the association of strain-relieved wire edge to the active region in the small-width wires, were interpreted to contribute significantly to the experimental blueshift.

Charge injection in molecular wires grafted on metallic surfaces: electron-lattice interaction

We study the injection of single charge carrier in one-dimensional molecular wires in the presence of coherent electron-lattice coupling. We show that charge injection at the molecule/metal interface induces lattice distortions in the molecule itself reflecting the formation of polaron-like defect. The behaviour of the charge density of the injected carrier and the mean position of the carrier depend strongly on the value of the injection energy relative to the band edges of the molecular wires.

Features of optical absorption and photoluminescence of porous silicon

Features of optical absorption and emission in porous silicon are discussed. The allowance for the finite-ness of the potential well depth when calculating the shift of the absorption edge of a quantum wire, the probability of a transition between states in the conduction and the valence bands, as well as the radiation lifetime are shown to introduce significant amendments into the dependences of these parameters on the size of a quantum wire in porous silicon.

Spatially resolved dynamics of localized spin-wave modes in ferromagnetic wires.

We have observed localized spin-wave modes in individual thin-film ferromagnetic wires using time-resolved Kerr microscopy as a micron-scale spectroscopic probe. The localization is due to the internal field profile present when an external field is applied in the plane of the film and perpendicular to the long axis of the wire. Spatially resolved spectra demonstrate the existence of distinct modes at the edges of a rectangular wire. Spectral images clearly show the crossover of the two edge modes into a single mode in low applied fields, in agreement with the results of micromagnetic simulations.

Theory of Exciton Formation and Relaxation in Quantum Wires

The relaxation dynamics of electron-hole pairs generated by a short laser pulse near the band edge of a GaAs quantum wire is analyzed by using a microscopic density matrix approach. We account for Coulomb renormalizations of various scattering mechanisms that are of importance near the band gap. The exciton formation dynamics, photoluminescence signals, and the build-up of center-of-mass kinetic energies depend crucially on the excitation energy. Exciton formation and dissociation are shown to change characteristically with temperature.

Spin-dependent resonances in the conduction edge of quantum wires

The conductance through a quantum wire of cylindrical cross section and a weak bulge is solved exactly for two electrons within the Landauer-Buettiker formalism. We show that this 'open' quantum dot exhibits spin-dependent Coulomb blockade resonances resulting in two anomalous structure on the rising edge to the first conductance plateau, one near 0.25(2e^2/h), related to a singlet resonance, and one near 0.7(2e^2/h), related to a triplet resonance. These resonances are generic and robust, occurring for other types of quantum wire and surviving to temperatures of a few degrees.

Raman investigation of CdxZn1−xSe/ZnSe quantum wires: Strain relaxation and excitation profile

CdZnSe/ZnSe quantum wires with two different Cd contents and different wire sizes were investigated by resonance Raman spectroscopy. Depending on the Cd content, the longitudinal optical (LO) phonon signal of the wires shows an asymmetry or a clear separation into two peaks. This separation is caused by an inhomogeneous strain parallel to the layer plane due to the strain relaxation at the wire edges. Thus, the observed signals can be attributed to LO phonons excited in the center and in the edge of the structures. Furthermore, high outgoing resonances for the LO overtones (2LO and 3LO) can be observed. For two wire sizes [(a): length L=100 μm, width W=18 nm; (b): L=49 nm, W=27 nm] excitation profiles of the LO fundamental and of the first overtone of Cd0.2Zn0.8Se quantum wires have been recorded. The excitation profiles clearly show structures attributed to the incoming and outgoing resonances. The intensity ratio 2LO/1LO, which is also a good measure for the electron–phonon coupling in a system, was determined for various wire sizes. The results point to a decrease of the electron–phonon coupling for decreasing wire length (transition from wire to dot).

Length dependence of the longitudinal optical phonon properties in CdZnSe/ZnSe quantum wires

This paper presents the results of Raman spectroscopic investigations on etched CdxZn1−xSe/ZnSe quantum wires with different wire lengths and wire widths. The recorded spectra show intensive signals in the region of the longitudinal optical phonons (LO) of the ZnSe barrier and the CdZnSe wire as well as several overtones. A strong outgoing resonance can be observed when the overtone position matches the luminescence peak position. The asymmetric LO signal can be separated into three distinct peaks: two higher wavenumber peaks, which are observed especially for the long and thick wires for short excitation wavelengths, are attributed to the LO phonon modes of the ZnSe barrier material and the strained CdZnSe wire; a shoulder at the low wavenumber region, which appears in the spectra of the short and thin wires, is attributed to an LO phonon excited in the strain relaxed wire edges.

ニッケル結晶における転位発生の局所不安定クライテリオン

In order to clarify the shear instability condition of local field, which induces a dislocation nucleation, 10 times of molecular dynamics simulations in a tension of nickel nano-wire (cross section: 30a×30a) were carried out. The results obtained are summarized as follows. (1) The partial dislocation nucleated from the edge of wire at ezz=0.093-0.096 (2) Dislocation nucleation site was different in each simulation due to the statistical fluctuation. (3) The strain concentration was observed in a 1/4 sphere of 3.0nm radius from the nucleation site before the dislocation appeared. (4) Detail observation revealed that the dislocation nucleated when the strain distribution in the region exceeded a criterion (critical curve). (5) The local strain concentration disappeared by the fluctuation when the strain distribution was lower than the critical curve. (6) The strain distribution on the critical curve brought about the dislocation nucleation when the rate of number of unstable lattices was higher than 0.1s-1.

Resonance Raman spectroscopy on strain relaxed CdZnSe/ZnSe quantum wires

Cd0.35Zn0.65Se/ZnSe and Cd0.2Zn0.8Se/ZnSe quantum wells grown by molecular beam epitaxy were patterned by electron beam lithography and wet chemical etching to give quantum wires of widths down to the sub-100 nm range. The splitting between the ZnSe LO phonon of the barrier layers and the ZnSe-like LO phonon of the quantum well layer was investigated by applying different excitation wavelengths. For two wire fields with different wire sizes excitation profiles were recorded, which show features belonging to the incoming and outgoing resonance of the LO phonon fundamental vibration and its first overtone. As a result, the LO phonon wavenumber of the wire shows a small shift on changing the excitation wavelength, which is due to changes in the relative intensities of the contributions of the wire center and the wire edges. The wavenumber positions of these two peak contributions are independent of the different resonance conditions at the edge and in the middle of the wire. Copyright © 2000 John Wiley & Sons, Ltd.

On the variability of cross-sectional dimensions and torsional properties of rectangular nickel-titanium arch wires

Twenty-five rectangular superelastic or conventional work-hardened nickel-titanium alloy wires, commonly used in the 0.018-inch edgewise technique, supplied by seven different manufacturers, along with one braided nickel-titanium and two beta-titanium wires, were studied with respect to wire dimensions, edge bevel, and mechanical properties in longitudinal torsion at 37° C. The wires were twisted 25° and studied in deactivation, simulating application of torque to an individual tooth. Standard Siamese brackets. with stated slot heights of 0.018 inches and measured slot heights of 0.0187 inches, were used. Most wires were within ±0.0005 inches of the stated dimensions, but had more edge bevel than previously reported for stainless steel and chrome-cobalt alloy wires. Variations in wire dimensions and edge bevel led to variable torsional (third-order) clearance. The torsional stiffness varied among manufacturers within the various wire sizes, this being the result of differences in actual cross-sectional geometry and material properties. None of the tested wires exhibited superelastic properties under the current conditions, and only one wire had a superelastic tendency.(Am J Orthod Dentofacial Orthop 1998;113:57)

On bracket slot height: A methodologic study

Effective bracket slot height is estimated by using a formula that describes the relationship between bracket slot height, wire dimensions, wire edge bevel, and torsional play (third-order clearance). With a torque measuring instrument, the torsional play was estimated for 10 different brackets (0.018-inch stated slot) of the same manufacturer and type. One arch wire with known dimensions and edge bevel was used for all the measurements. With known torsional play, wire dimensions and edge bevel, the bracket slot height could be calculated. This was performed five times for each bracket and the method error for estimation of torsional play for a single measurement was 0.04 degrees, corresponding to 0.36 mm in slot height. The brackets tested had a mean slot height of 0.476 +/- 0.003 mm, with a range of 0.470 to 0.481 mm (0.0187 +/- 0.0001 inches, range 0.0185 to 0.0189). The variation in bracket slot height was much greater than the method error. The method used to measure bracket slot height seems to have a high degree of accuracy and is easier to implement than conventional methods.

In Situ Monitoring of the Electrochemical Absorption of Deuterium into Palladium by X-Ray Diffraction using Synchrotron-Wiggler Radiation

ABSTRACTWith low energy x-rays, such as those from a Cu x-ray tube, only the outer few microns of a metallic sample can be probed. This low penetrating power prohibits structural studies from being carried out on the interior of an electrode in an electrochemical cell because of absorption by the cell material, electrodes and the electrolyte. The work described in this paper circumvents this problem by utilizing high energy, high brightness x-rays produced on the superconducting wiggler beam line, X-17C, at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The penetrating power of the higher energy x-rays allowed Pd diffraction spectra to be obtained in-situ on a 1 mm diameter Pd wire cathode during electrolysis of heavy water. Moreover, the beam (28 × 28 μm in cross-section) allowed diffraction spectra to be acquired as a function of distance across the sample. Spectra were recorded in 50 μm steps from the edge of the Pd wire to its core. This was done at 2 minute intervals as a function of electrolysis time. The α-β phase transition induced in the Pd while deuterium was electrochemically absorbed was observed by monitoring the Pd-(422) diffraction peaks. Results allowed the diffusion rate and the diffusivity of deuterium atoms in the Pd wire to be determined. Other features of the structural changes associated with the absorption of deuterium into Pd are reported.

Anisotropic domain evolution in epitaxial Fe/GaAs(001) wires

The magnetization reversal in in-plane magnetized epitaxial Fe/GaAs(001) wire elements with dimensions of 15 \ensuremath{\mu}m (width $w)$\ifmmode\times\else\texttimes\fi{}500 \ensuremath{\mu}m (length $l)$\ifmmode\times\else\texttimes\fi{}300 \AA{} (Thickness $t)$ has been studied by scanning Kerr microscopy and Kerr magnetometry. The two-jump switching process is observed which is characteristic for the magnetization reversal in continuous epitaxial Fe(001) films with fourfold in-plane anisotropy. However, in contrast to the continuous film, the domain nucleation and growth processes which mediate the irreversible magnetization jumps at the two critical fields, ${H}_{c1}$ and ${H}_{c2}$ are found to be determined by the orientation of the applied field with respect to the long and the short wire axis. This anisotropy in the domain evolution is a result of the combined effects of local edge dipolar fields, the fourfold magnetocrystalline anisotropy as well as the finite and anisotropic lateral extensions of the wires. Due to the large aspect ratio of $l/w,$ the boundaries of the long and short wire edges restrict the domain expansion differently. Consequently, this ``shape'' anisotropy in the domain evolution contrasts with the conventional shape anisotropy associated with macroscopic (average) demagnetization fields.

Excitation Power Dependent Photoluminescence Behavior in Etched Quantum Wires having Silicon Interlayer-Based Edge Passivation and Its Interpretation

A novel Si interface control layer (ICL)-based passivation technique is applied to the edge passivation of etched In0.53Ga0.47As wires and the effect of the passivation is characterized in terms of the photoluminescence (PL) properties of the wires. The observed PL behavior is found to be strongly excitation power-dependent and inexplicable by the conventionally used PL model assuming a constant surface recombination velocity and an optically “dead layer” having a constant width at the wire edges. A novel one-dimensional computer simulation of wire PL is developed. The result of the simulation indicates that the surface recombination velocity and the surface depletion layer width are excitation power-dependent, providing an explanation for the observed excitation power-dependent PL behavior. The theory and experiments show that the Si ICL edge passivation significantly reduces the surface state density at the edge and improves the PL properties.

Capacitance spectroscopy of compressible and incompressible stripes in a narrow electron channel

We study the capacitance of single electron wires in high quantizing magnetic fields. The capacitance spectra obtained on 300 nm wide electron channels are interpreted with a simple model considering the geometry of the incompressible stripes in the electron channel as function of filling factor. The capacitance spectra directly reflect the potential form of the wire edge. This is experimentally demonstrated by the dependence on the confinement potential as well as by a clear asymmetry of the capacitance minimum at filling factor v=2 that can be well explained with the model. For comparison we present capacitance spectra obtained on very narrow, but otherwise similar quantum wires, in which quantization into one-dimensional subbands dominates the behavior.

Solvable model for an impurity spin coupled to a one-dimensional superconductor.

A solvable model for a magnetic impurity in a one-dimensional superconductor is proposed. The model consists of the impurity spin coupled to the edge of a quantum wire. The electron gas in the wire is dominated by attractive mutual interactions and forms a gapped state with a short-range superconducting order. In a restricted (but nontrivial) region of parameters, the model is solved by means of bosonization methods. The spectrum and thermodynamic functions are calculated. \textcopyright{} 1996 The American Physical Society.

Compressible and incompressible stripes in a narrow electron channel

The capacitance of a single-electron wire at high quantizing magnetic fields is studied. We demonstrate that the capacitance spectroscopy allows us to study the formation of compressible and incompressible stripes directly. The electron wire is field induced in a metal insulator semiconductor heterostructure beneath a center gate. The electrostatic potential defining the wire edge is controlled by a tuning-fork-shaped electrode that encloses the center gate. It is found that the capacitance spectra are very sensitive to the wire edge potential and show a clear asymmetry of the capacitance minimum at filling factor n52. The experimental results are well described by a simple model considering the contribution of compressible and incompressible stripes in the electron channel, to the capacitance signal. The capacitance minima are thus determined in our experiments by the geometry of the compressible and incompressible stripes in the channel and not by the defect-induced density of states. @S0163-1829~96!02143-1#