Theoretical study of isolated dangling bonds, dangling bond wires and dangling bond clusters on H:Si(100)-(2×\times1) surface

We theoretically study the electronic band structure of isolated unpaired and paired dangling bonds (DB), DB wires and DB clusters on H:Si(100)-(2$\times$1) surface using Extended H\"uckel Theory (EHT) and report their effect on the Si band gap. An isolated unpaired DB introduces a near-midgap state, whereas a paired DB leads to $\pi$ and $\pi^*$ states, similar to those introduced by an unpassivated asymmetric dimer (AD) Si(100)-(2$\times$1) surface. Such induced states have very small dispersion due to their isolation from the other states, which reside in conduction and valence band. On the other hand, the surface state induced due to an unpaired DB wire in the direction along the dimer row (referred to as $[\bar{1}10]$), has large dispersion due to the strong coupling between the adjacent DBs, being 3.84$\AA$ apart. However, in the direction perpendicular to the dimer row (referred to as [110]), due to the reduced coupling between the DBs being 7.68$\AA$ apart, the dispersion in the surface state is similar to that of an isolated unpaired DB. Apart from this, a paired DB wire in $[\bar{1}10]$ direction introduces $\pi$ and $\pi^*$ states similar to those of an AD surface and a paired DB wire in [110] direction exhibits surface states similar to those of an isolated paired DB, as expected. Besides this, we report the electronic structure of different DB clusters, which exhibit states inside the band gap that can be interpreted as superpositions of states due to unpaired and paired DBs.Comment: 7 pages, 10 figure, 1 tabl

Zinc Oxide-on-Silicon Surface Acoustic Wave Devices

A monolithic ZnO-on-silicon surface acoustic wave (SAW) memory correlator has been fabricated which utilizes induced junctions separated by ion implanted regions to store a reference signal. The performance characteristics of this device have been investigated including storage time, dynamic range, and degenerate convolution efficiency. Verification of the existence of charge storage regions is possible prior to completed device fabrication. A theory explaining the charge storage process is developed and applied to the implant-isolated storage correlator. The implant-isolated correlator theory is applied to related structures which employ slightly different storage mechanisms. The ion implanted correlator is used to determine the wave potential associated with a propagating SAW. Characteristics of ZnO-on-Si SAW resonators with sputtered ZnO films limited to the interdigital transducer (IDT) regions are investigated. Upper limits on propagation loss for surface waves on silicon substrates are determined by employing externally coupled limited ZnO SAW resonators. Resonator Q-values are enhanced by restricting the lossy ZnO area and predictions are made as to achievable Q-values for resonators fabricated in the externally coupled configuration. Experimental results for limited ZnO, internally coupled ZnO-on-Si resonators are also given. A complete theory for the mode conversion resonator is presented which predicts the array separation for proper device operation. The theory also gives way to a special condition for spatial ndependence of resonator output with respect to IDT placement. Mode conversion resonators are fabricated which experimentally verify these predictions

Seasonal patterns of fine root production and turnover in a mature rubber tree (Hevea brasiliensis Müll. Arg.) stand- differentiation with soil depth and implications for soil carbon stocks

Fine root dynamics is a main driver of soil carbon stocks, particularly in tropical forests, yet major uncertainties still surround estimates of fine root production and turnover. This lack of knowledge is largely due to the fact that studying root dynamics in situ, particularly deep in the soil, remains highly challenging. We explored the interactions between fine root dynamics, soil depth, and rainfall in mature rubber trees (Hevea brasiliensis Müll. Arg.) exposed to sub-optimal edaphic and climatic conditions. A root observation access well was installed in northern Thailand to monitor root dynamics along a 4.5 m deep soil profile. Image-based measurements of root elongation and lifespan of individual roots were carried out at monthly intervals over 3 years. Soil depth was found to have a significant effect on root turnover. Surprisingly, root turnover increased with soil depth and root half-life was 16, 6–8, and only 4 months at 0.5, 1.0, 2.5, and 3.0 m deep, respectively (with the exception of roots at 4.5 m which had a half-life similar to that found between depths of 1.0 and 2.5 m). Within the first two meters of the soil profile, the highest rates of root emergence occurred about 3 months after the onset of the rainy season, while deeper in the soil, root emergence was not linked to the rainfall pattern. Root emergence was limited during leaf flushing (between March and May), particularly within the first two meters of the profile. Between soil depths of 0.5 and 2.0 m, root mortality appeared independent of variations in root emergence, but below 2.0 m, peaks in root emergence and death were synchronized. Shallow parts of the root system were more responsive to rainfall than their deeper counterparts. Increased root emergence in deep soil toward the onset of the dry season could correspond to a drought acclimation mechanism, with the relative importance of deep water capture increasing once rainfall ceased. The considerable soil depth regularly explored by fine roots, even though significantly less than in surface layers in terms of root length density and biomass, will impact strongly the evaluation of soil carbon stocks

Selective migration of neuralized embryonic stem cells to stem cell factor and media conditioned by glioma cell lines

BACKGROUND: Pluripotent mouse embryonic stem (ES) cells can be induced in vitro to become neural progenitors. Upon transplantation, neural progenitors migrate toward areas of damage and inflammation in the CNS. We tested whether undifferentiated and neuralized mouse ES cells migrate toward media conditioned by glioma cell lines (C6, U87 & N1321) or Stem Cell Factor (SCF). RESULTS: Cell migration assays revealed selective migration by neuralized ES cells to conditioned media as well as to synthetic SCF. Migration of undifferentiated ES cells was extensive, but not significantly different from that of controls (Unconditioned Medium). RT-PCR analysis revealed that all the three tumor cell lines tested synthesized SCF and that both undifferentiated and neuralized ES cells expressed c-kit, the receptor for SCF. CONCLUSION: Our results demonstrate that undifferentiated ES cells are highly mobile and that neural progenitors derived from ES cells are selectively attracted toward factors produced by gliomas. Given that the glioma cell lines synthesize SCF, SCF may be one of several factors that contribute to the selective migration observed

Orientation-dependent perimeter recombination in GaAs diodes

Perimeter recombination currents affect the performance of GaAs-based devices such as solar cells, heterojunction bipolar transistors, and injection lasers. We report that the n SEf 2 perimeter recombination current has a strong orientation dependence. More than a factor of five variation in the surface recombination current at mesa-etched edges has been observed. These results suggest that with proper device design, perimeter recombination currents could be substantially reduced

Effect of Airfoil Parametrization on the Optimization of Counter Rotating Open Rotors

The present study compares two optimizations performed on Counter Rotating Open Rotors (CRORs) running at the same operating condition. The main difference between the two optimizations is the airfoil profile used to construct the blades. The first, uses the NACA 16 family of airfoils, whereas the second one, uses a parametrized airfoil type, CST. Two independent multi-objective optimizations are carried out using approximately the same computational resources. All the design variables except those concerning the airfoil profile, are kept with the same design freedom so that a fair comparison can be made. Both sets of configurations are aerodynamically optimized for maximum thrust coefficient and efficiency at top of climb conditions. The optimization is performed using multi-objective Differential Evolution (DE) coupled with 3D RANS simulations and Radial Basis Function (RBF) meta-modeling

Recombination-current suppression in GaAs p-n junctions grown on AlGaAs buffer layers by molecular-beam epitaxy

n+pp+GaAs and n+pP+ GaAs/GaAs/Al0.3Ga0.7As mesa diodes have been fabricated from films grown by molecular‐beam epitaxy. The diodes made from films employing an AlGaAs buffer layer show marked improvements (a factor of 5 reduction) in recombination current densities. Deep level transient spectroscopy measurements moreover indicate that deep level concentrations are reduced by the AlGaAs buffer

Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism

Aeroengine manufacturers have to demonstrate that critical components such as turbine disks, made of DA Inconel 718, meet the certification requirements in term of fatigue crack growth. In order to be more representative of the in service loading conditions, crack growth under hold time conditions is studied. Modelling crack growth under these conditions is challenging due to the combined effect of fatigue, creep and environment. Under these conditions, established models are often conservative but the degree of conservatism can be reduced by introducing the crack growth threshold in models. Here, the emphasis is laid on the characterization of crack growth rates in the low ?K regime under hold time conditions and in particular, on the involved crack growth mechanism. Crack growth tests were carried out at high temperature (550 °C to 650 °C) under hold time conditions (up to 1200 s) in the low ?K regime using a K-decreasing procedure. Scanning electron microscopy was used to identify the fracture mode involved in the low ?K regime. EBSD analyses and BSE imaging were also carried out along the crack path for a more accurate identification of the fracture mode. A transition from intergranular to transgranular fracture was evidenced in the low ?K regime and slip bands have also been observed at the tip of an arrested crack at low ?K. Transgranular fracture and slip bands are usually observed under pure fatigue loading conditions. At low ?K, hold time cycles are believed to act as equivalent pure fatigue cycles. This change in the crack growth mechanism under hold time conditions at low ?K is discussed regarding results related to intergranular crack tip oxidation and its effect on the crack growth behaviour of Inconel 718 alloy. A concept based on an “effective oxygen partial pressure” at the crack tip is proposed to explain the transition from transgranular to intergranular fracture in the low ?K regime