Silane pyrolysis

We show that silane pyrolysis is initiated by decomposition on the amorphous silicon surface, with an activation energy E_a of 56 kcal/mole. The observed surface decomposition rate is only weakly dependent on silane pressure. Much faster delayed decomposition rates, approximately independent of surface area and proportional to pressure, are shown to be initiated by surface reactons. A model for surface decomposition is given. Also a model for gas reactions is suggested based on H atom or SiH₃ release by surface decomposition, causing chain reactions that process the gas to higher silanes that decompose rapidly. This model can explain the previous observations that the initial disilane formation rate and the delayed decomposition rate were independent of the surface area to volume ratio A/V, which had misled previous investigators to suggest homogeneous initiation processes.     

Implementing Problem-Based Learning in Science Classrooms

Many new curricular and instructional models must be developed or adapted as the nation moves towards educational reform in science classrooms. This article describes how problem-based learning, an innovative curricular and instructional model developed in medical graduate school programs, has been adapted for use in elementary and high school settings. Included in the integration of problem-based learning and science are components of all problem-based episodes including initiating learning with an ill-structured problem, using the problem to structure the learning agenda, and teacher as metacognitive coach, with important goals of a reformed science curriculum such as learning based on concepts of significance, student-designed experiments, and development of scientific reasoning skills.     

PLE studies of two-dimensional hole systems in the quantum Hall regime

We report a spectroscopic investigation of the densities of both occupied and unoccupied states of a high-quality two-dimensional hole system, in the regime of the quantum Hall effects (QHEs). Photoluminescence and photoluminescence excitation spectroscopies are used to elucidate the complicated valence band structure of the holes, and to establish their optical response to the QHEs.     

Synthesis and characterization of phosphonate‐containing polysiloxanes

Phosphonate‐functionalized polysiloxanes have been prepared with a new siloxane/phosphonate monomer. The reaction of 3‐chloropropylmethyldimethoxysilane with trimethylphosphite or triethylphosphite produces several new monomers containing pendant phosphonate groups. Copolymerization with dimethyldimethoxysilane has produced polymers soluble in most organic solvents. The acid hydrolysis of the phosphoryl esters has produced hydrophilic siloxane polymers containing phosphonic acid groups. The thermal properties of the polymers and several related small molecules have been compared with thermogravimetric analysis. Both the monomers and the resulting polymers have been characterized with ¹H, ¹³C, ³¹P, and ²⁹Si NMR. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 48–59, 2003     

Mn interstitial diffusion in (ga,mn)as

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.     

Fano line shapes reconsidered: symmetric photoionization peaks from pure continuum excitation

In a photoionization spectrum in which there is no excitation of the discrete states, but only the underlying continuum, we have observed resonances which appear as symmetric peaks, not the commonly expected window resonances. Furthermore, since the excitation to the unperturbed continuum vanishes, the cross section expected from Fano's configuration interaction theory is identically zero. This shortcoming is removed by the explicit introduction of the phase shifted continuum, which demonstrates that the shape of a resonance, by itself, provides no information about the relative excitation amplitudes to the discrete state and the continuum.     

Spin reorientation transition in single-domain

We demonstrate that the interplay of in-plane biaxial and uniaxial anisotropy fields in results in a spin reorientation transition and an anisotropic ac susceptibility which is fully consistent with a simple single-domain model. The uniaxial and biaxial anisotropy constants vary, respectively, as the square and fourth power of the spontaneous magnetization across the whole temperature range up to . The weakening of the anisotropy at the transition may be of technological importance for applications involving thermally assisted magnetization switching.     

Large tunneling anisotropic magnetoresistance in (Ga,Mn)As nanoconstrictions

We report a large tunneling anisotropic magnetoresistance (TAMR) in (Ga,Mn)As lateral nanoconstrictions. Unlike previously reported tunneling magnetoresistance effects in nanocontacts, the TAMR does not require noncollinear magnetization on either side of the constriction. The nature of the effect is established by a direct comparison of its phenomenology with that of normal anisotropic magnetoresistance (AMR) measured in the same lateral geometry. The direct link we establish between the TAMR and AMR indicates that TAMR may be observable in other materials showing room temperature AMR and demonstrates that the physics of nanoconstriction magnetoresistive devices can be much richer than previously thought.     

Spontaneous evolution of rydberg atoms into an ultracold plasma

We have observed the spontaneous evolution of a dense sample of Rydberg atoms into an ultracold plasma, in spite of the fact that each of the atoms may initially be bound by up to 100 cm(-1). When the atoms are initially bound by 70 cm(-1), this evolution occurs when most of the atoms are translationally cold, <1 mK, but a small fraction, approximately 1%, is at room temperature. Ionizing collisions between hot and cold Rydberg atoms and blackbody photoionization produce an essentially stationary cloud of cold ions, which traps electrons produced later. The trapped electrons rapidly collisionally ionize the remaining cold Rydberg atoms to form a cold plasma.