Narrow Potential Well Research Articles

Large oscillation of electrostatically actuated curved beams

This paper investigates the responses of initially curved micro-beams subjected to an electrostatic excitation. A Euler–Bernoulli beam theory is utilized to derive the governing equation of motion. A reduced-order model was developed by discretizing the equation of motion using straight beam mode shapes as basis functions in a Galerkin expansion. The results show evidence of the superharmonic resonances of order-three and two in addition to primary resonance. The co-existence of multiple stable orbits observed around only one stable equilibrium and under excitation waveforms with RMS voltage less than the snap-back voltage. These branches are a branch of small orbits within a narrow potential well and two branches of medium-sized and large orbits within a wider potential well. The transition between them results in the characteristic of the double peaks appearing in the frequency-response curve. We also report a bubble structure along the medium-sized branch consists of a cascade of period-doubling bifurcations and a cascade of reverse period-doubling bifurcations. A chaotic attractor develops within that structure at larger excitation levels. It demonstrates evidence of chaos with a wide-based spectrum and an elevated noise-floor. Odd-periodic windows appear also within the attractor including period-three (P-3), period-five (P-5) and period-six (P-6) windows. The chaotic attractor terminates in a cascade of reverse period-doubling bifurcations of period-four (P-4) orbits and period-two (P-2) orbits.

Measuring Thermal Rupture Force Distributions from an Ensemble of Trajectories

Rupture, bond breaking, or extraction from a deep and narrow potential well requires considerable force while producing minimal displacement. In thermally fluctuating systems, there is not a single force required to achieve rupture, but a spectrum, as thermal forces can both augment and inhibit the bond breaking. We demonstrate measurement and interpretation of the distribution of rupture forces between pairs of colloidal particles bonded via the van der Waals attraction. The otherwise irreversible bond is broken by pulling the particles apart with optical tweezers. We show that an ensemble of the particle trajectories before, during and after the rupture event may be used to produce a high fidelity description of the distribution of rupture forces. This analysis is equally suitable for describing rupture forces in molecular and biomolecular contexts with a number of measurement techniques.

Growth of nearly one nanometer large silicon particles in silicon carbide and theirquantum-confined photoluminescence features

Silicon particles approaching the size of 1 nm were grown along with the confining SiCfilms by employing a low-temperature chemical vapor deposition procedure. Theresulting amorphous composite structure enables an experimental study of thequantum confinement effect in extremely narrow potential wells, as exemplified hereby photoluminescence measurement. Owing to the enhanced energy fluctuationfor such small particles, strong photoluminescence centered at 450–540 nm, andof comparable profiles, was measured in one single sample with an excitationwavelength selectable within 360–420 nm. Moreover, the typical decay time wasfound to be below 3.0 ns. These properties hold promise for the fabrication ofwide-spectrum photoreceptors, ultraviolet-light detectors, and other optoelectronicdevices.

Field-assisted resonance tunneling through a symmetric double-barrier structure with spin-orbit coupling

We investigate the electron resonance transmission through a symmetric double-barrier structure with the Dresselhaus spin-orbit coupling and an oscillating field applied to the potential-well region. Based on numerical evaluations, it is demonstrated that the multiphoton process results in a two-set multiplet structure of the transmission spectrum. The number of the resonance peaks and the distance between the adjacent peaks can be controlled by adjusting the amplitude and the frequency of the external field, respectively. Moreover, it is shown that a high spin polarization of the photon-mediated transmission probability can be achieved in the case of narrow potential well with a small amplitude of the oscillating field, which may be useful in the tunable spin filters of high efficiency.

The Role of Radial Electric Fields in the Tokamaks TEXTOR-94, CASTOR, and T-10

Radial electric fields (E r) and their role in the establishment of edge transport barriers and improved confinement have been studied in the tokamaks TEXTOR-94 and CASTOR, where E r is externally applied to the plasma in a controlled way using a biased electrode, as well as in the tokamak T-10 where an edge transport barrier (H-mode) is obtained during electron-cyclotron resonance heating (ECRH) of the plasma. The physics of radial currents was studied and the radial conductivity in the edge of TEXTOR-94 (R = 1.75 m, a = 0.46 m) was found to be dominated by recycling (ion-neutral collisions) at the last closed flux surface (LCFS) and by parallel viscosity inside the LCFS. From a performance point of view (edge engineering), such electrode biasing was shown to induce a particle transport barrier, a reduction of particle transport, and a concomitant increase in energy confinement. An H-mode-like behaviour can be induced both with positive and negative electric fields. Positive as well as negative electric fields were shown to strongly affect the exhaust of hydrogen, helium, and impurities, not only in the H-mode-like regime. The impact of sheared radial electric fields on turbulent structures and flows at the plasma edge is investigated on the CASTOR tokamak (R = 0.4 m, a = 0.085 m). A non-intrusive biasing scheme that we call "separatrix biasing" is applied whereby the electrode is located in the scrape-off layer (SOL) with its tip just touching the LCFS. There is evidence of strongly sheared radial electric field and E×B flow, resulting in the formation of a transport barrier at the separatrix. Advanced probe diagnosis of the edge region has shown that the E×B shear rate that arises during separatrix biasing is larger than for standard edge plasma biasing. The plasma flows, especially the poloidal E×B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates (≈ 5×106 s-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements. During ECRH in the tokamak T-10 (R = 1.5 m, a = 0.3 m), a regime of improved confinement is obtained with features resembling those in the H-mode in other tokamaks. Using a heavy ion beam probe, a narrow potential well is observed near the limiter together with the typical features of the L-H transition. The time evolution of the plasma profiles during L-H and H-L transitions is clearly correlated with that of the density profile and the formation of a transport barrier near the limiter. The edge electric field is initially positive after the onset of ECRH. It changes its sign during the L-H transition and grows till a steady condition is reached. Similar to the biasing experiments in TEXTOR-94 and CASTOR, the experimentally observed transport barrier is a barrier for particles.

The effects of thermal annealing on ZnO thin films grown by pulsed laser deposition

ZnO thin films were grown on silicon (100) by pulsed laser deposition. Highly textured crystalline ZnO thin films can be grown at 600 °C. The films were then annealed at 600 °C in oxygen. The effects of annealing on chemical composition of the ZnO films were investigated by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS spectra indicate that water has been adsorbed and then dissociated into H and OH groups. The surface properties of ZnO were studied both by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A narrow potential well has been formed on the surface of the ZnO thin films due to high density of surface states and negatively biasing the ZnO thin films during STS measurement. The discrete energy levels can be measured by STS.

The phase behaviour of short chain molecules: a computer simulation study

Molecular dynamics simulations are reported in which the properties of short semi-flexible chain molecules composed of five hard spheres are studied. The ‘rattling sphere’ model has been used in which spheres are bonded together by means of narrow potential wells and individual spheres are free to rattle between their confining walls. Results for the phase diagrams, translational diffusion, and reorientational motion are presented for three models of differing flexibility. The presence is noted of a metastable smectic phase at high density for the most rigid models, and an absence of other liquid crystalline mesophases.

Molecular dynamics simulation of semi-flexible mesogens

Molecular dynamics simulations are reported for systems of semi-flexible model mesogens composed of seven tangential hard spheres. The simulations employ the ‘rattling spheres’ approach whereby atomic sites are constrained to lie within narrow potential wells, with free-flight molecular dynamics occurring in between elastic collisions of spheres. The phase diagams of four systems with different bonding constraints have been studied. It is found that the addition of a small amount of molecular flexibility to the chains of hard spheres increases the density at which the isotropic to nematic phase transition takes place. Too much flexibility however destroys the nematic phase completely. At high density the most rigid of the systems forms a smectic-A phase and there is strong evidence for ordering within the smectic layers themselves at densities near the fluid-solid transition. There is also evidence that increasing molecular flexibility pushes the formation of the solid phase to higher densities. Calculati...

Computer simulation study of liquid crystal formation in a semi-flexible system of linked hard spheres

Results are reported for a molecular dynamics simulation study of a flexible model mesogen composed of seven tangential spheres. We follow the ‘rattling spheres’ method by which bonded atoms are constrained to lie within narrow potential wells. The dynamics of the system is that of a hard sphere fluid with added constraints. The phase diagram of our model system is calculated as a function of density and shows the presence of three fluid phases. These are assigned to be isotropic, nematic and smectic-A phases. Results are reported for the orientational order, radial distribution functions, structure factors and single-particle structural data within these phases. A small but significant change in shape is measured as the nematic phase is entered from the isotropic liquid. This is interpreted in terms of a quenching of allowed conformations parallel to the director.

Periodic conductance fluctuations in quasi-one-dimensional metal-oxide-semiconductor field-effect transistors with shallow-trench isolations.

Results of conductance measurements in 0.2-\ensuremath{\mu}m-wide and 0.15-\ensuremath{\mu}m-long silicon metal-oxide-semiconductor field-effect transitions with shallow-trench isolations are presented. At T=1.2 K, two kinds of periodic conductance fluctuations are observed; fine structure near threshold and plateaus in the higher conductance regime (\ensuremath{\sim}0.1 mS). The latter are accentuated by applying moderate magnetic fields of 1--2 T, remaining unchanged in their positions. We attribute these features to a quasi-one-dimensional transport mechanism caused by strain-induced narrow potential wells along the trench isolations.

Magnetization reversal in thin magnetic films with perpendicular anisotropy

A one-dimensional theory of magnetization reversal in thin, perpendicularly anisotropic magnetic films is presented. It is postulated that the presence of a defective point creates an infinitely deep, infinitely narrow potential well which inhibits the rotation of local magnetization. The interval D between neighboring defects, the saturation magnetization Ms, the anisotropy constant Ku, and the exchange energy constant A are assumed to be finite and uniform across the film. Starting with an initial state where the film is uniformly magnetized to saturation in the easy direction, we show that a discontinuous change of state occurs when the reverse external field H reaches a critical value Hc. The domains thus nucleated at the critical field expand to cover the entire area of the film as H increases beyond Hc. Using the normalized values of H and D, defined, respectively, as h = H/(2Ku/Ms) and d = D/(4A/Ku)1/2, we show that the critical field hc is a function only of d and that its value decreases significantly as d increases.

Isotope effect on physical adsorption on a non-homogeneous surface

Non-homogeneity of adsorption sites is shown, by a model calculation, to account for a non-monotonic temperature dependence of the isotope separation factor α in adsorption of a monoatomic gas on a solid adsorbent. Such a non-monotonic dependence was experimentally observed by Purer, Kaplan and Smith.The model calculation shows that a non-monotonic temperature dependence of α arises when there are two types of adsorption site, one for which the energy of interaction with the adsorbed atom is characterized by a shallow but narrow potential well and the other for which the well is deeper and wider. It is also shown that wide holes or planar sections in the solid adsorbent and narrow holes correspond, respectively, to these two types of adsorption site which exist when the adsorbent is a porous glass.