Alane adsorption and dissociation on the Si(001) surface

We used DFT to study the energetics of the decomposition of alane, AlH3, on the Si(001) surface, as the acceptor complement to PH3. Alane forms a dative bond with the raised atoms of silicon surface dimers, via the Si atom lone pair. We calculated the energies of various structures along the pathway of successive dehydrogenation events following adsorption: AlH2, AlH and Al, finding a gradual, significant decrease in energy. For each stage, we analyse the structure and bonding, and present simulated STM images of the lowest energy structures. Finally, we find that the energy of Al atoms incorporated into the surface, ejecting a Si atom, is comparable to Al adatoms. These findings show that Al incorporation is likely to be as precisely controlled as P incorporation, if slightly less easy to achieve.Comment: Submitted to J. Phys.: Condens. Matte

Design techniques for revealing adolescent memory processes related to information seeking: A preliminary study

This study investigates the effectiveness of design techniques as a means for uncovering metamemory, an attribute of metacognition, and its role in information seeking. A focus group with four adolescents aged 13 and 14 used design techniques such as brainstorming and sketching, metaphorical design and fictional inquiry, to help express their thinking about their own memory processes during the information search process. Results showed that metaphorical design and fictional inquiry are both effective tools for revealing conceptual thinking about metamemory and information seeking. Coupling these techniques with brainstorming and sketching helped the teens to visualize and communicate their ideas. Results from this study will contribute to knowledge about adolescent thinking, metamemory, and information seeking behavior, broaden the range of methodological approaches used in the study of information seeking behavior, and will provide cognitive models for the design of information systems and tools that scaffold metacognition. © 2012 ACM

Feelbook: A social media app for teens designed to foster positive online behavior and prevent cyberbullying

This project presents a prototype for a stand-alone social media application designed for teenage users in order to prevent and mitigate mean and cruel online behavior. The purpose of the app is to create a nurturing environment where teenagers use a variety of features designed to help raise self-awareness of their own online behavior, seek support when needed, and learn to control and, when possible, correct aggressive behavior. The prototype is framed by four design principles: design for reflection, design for empathy, design for empowerment, and design for the whole. We conclude by outlining the next steps in our project to develop an application that helps to improve the online experiences of young people. This work has implications for the CHI community because it applies software solutions to tackle a critical social problem that can affect the health and well being of young people

The environment and host haloes of the brightest z~6 Lyman-break galaxies

By studying the large-scale structure of the bright high-redshift Lyman-break galaxy (LBG) population it is possible to gain an insight into the role of environment in galaxy formation physics in the early Universe. We measure the clustering of a sample of bright (-22.7<M_UV<-21.125) LBGs at z~6 and use a halo occupation distribution (HOD) model to measure their typical halo masses. We find that the clustering amplitude and corresponding HOD fits suggests that these sources are highly biased (b~8) objects in the densest regions of the high-redshift Universe. Coupled with the observed rapid evolution of the number density of these objects, our results suggest that the shape of high luminosity end of the luminosity function is related to feedback processes or dust obscuration in the early Universe - as opposed to a scenario where these sources are predominantly rare instances of the much more numerous M_UV ~ -19 population of galaxies caught in a particularly vigorous period of star formation. There is a slight tension between the number densities and clustering measurements, which we interpret this as a signal that a refinement of the model halo bias relation at high redshifts or the incorporation of quasi-linear effects may be needed for future attempts at modelling the clustering and number counts. Finally, the difference in number density between the fields (UltraVISTA has a surface density ~1.8 times greater than UDS) is shown to be consistent with the cosmic variance implied by the clustering measurements.Comment: 19 pages, 8 figures, accepted MNRAS 23rd March 201

Constraints on the Spin Evolution of Young Planetary-Mass Companions

Surveys of young star-forming regions have discovered a growing population of planetary-mass (<13 M_Jup) companions around young stars. There is an ongoing debate as to whether these companions formed like planets (that is, from the circumstellar disk), or if they represent the low-mass tail of the star formation process. In this study we utilize high-resolution spectroscopy to measure rotation rates of three young (2-300 Myr) planetary-mass companions and combine these measurements with published rotation rates for two additional companions to provide a look at the spin distribution of these objects. We compare this distribution to complementary rotation rate measurements for six brown dwarfs with masses <20 M_Jup, and show that these distributions are indistinguishable. This suggests that either that these two populations formed via the same mechanism, or that processes regulating rotation rates are independent of formation mechanism. We find that rotation rates for both populations are well below their break-up velocities and do not evolve significantly during the first few hundred million years after the end of accretion. This suggests that rotation rates are set during late stages of accretion, possibly by interactions with a circumplanetary disk. This result has important implications for our understanding of the processes regulating the angular momentum evolution of young planetary-mass objects, and of the physics of gas accretion and disk coupling in the planetary-mass regime.Comment: 31 pages, 10 figures, published in Nature Astronomy, DOI:10.1038/s41550-017-0325-

Accelerating Atomic Orbital-based Electronic Structure Calculation via Pole Expansion and Selected Inversion

We describe how to apply the recently developed pole expansion and selected inversion (PEXSI) technique to Kohn-Sham density function theory (DFT) electronic structure calculations that are based on atomic orbital discretization. We give analytic expressions for evaluating the charge density, the total energy, the Helmholtz free energy and the atomic forces (including both the Hellman-Feynman force and the Pulay force) without using the eigenvalues and eigenvectors of the Kohn-Sham Hamiltonian. We also show how to update the chemical potential without using Kohn-Sham eigenvalues. The advantage of using PEXSI is that it has a much lower computational complexity than that associated with the matrix diagonalization procedure. We demonstrate the performance gain by comparing the timing of PEXSI with that of diagonalization on insulating and metallic nanotubes. For these quasi-1D systems, the complexity of PEXSI is linear with respect to the number of atoms. This linear scaling can be observed in our computational experiments when the number of atoms in a nanotube is larger than a few hundreds. Both the wall clock time and the memory requirement of PEXSI is modest. This makes it even possible to perform Kohn-Sham DFT calculations for 10,000-atom nanotubes with a sequential implementation of the selected inversion algorithm. We also perform an accurate geometry optimization calculation on a truncated (8,0) boron-nitride nanotube system containing 1024 atoms. Numerical results indicate that the use of PEXSI does not lead to loss of accuracy required in a practical DFT calculation

Long-range electron transfer in structurally engineered pentaammineruthenium (histidine-62) cytochrome c

In many biological processes, long-range electron transfer (ET) plays a key role. When the three-dimensional structures of proteins are accurately known, use of modified proteins and protein-protein complexes provides an experimental approach to study ET rates between two metal centers. For Ru(His)- modified proteins, the introduction of histidine residues at any desired surface location by site-directed mutagenesis opens the way for systematic investigations of ET pathways

Interdigital Capacitance Local Non-Destructive Examination of Nuclear Power Plant Cable for Aging Management Programs

This Pacific Northwest National Laboratory milestone report describes progress to date on the investigation of non-destructive test methods focusing on local cable insulation and jacket testing using an interdigital capacitance (IDC) approach. Earlier studies have assessed a number of non-destructive examination (NDE) methods for bulk, distributed, and local cable tests. A typical test strategy is to perform bulk assessments of the cable response using dielectric spectroscopy, Tan , or partial discharge followed by distributed tests like time domain reflectometry or frequency domain reflectometry to identify the most likely defect location followed by a local test that can include visual inspection, indenter modulus tests, or Fourier Transform Infrared Spectroscopy (FTIR) or Near Infrared Spectroscopy FTIR (FTNIR). If a cable is covered with an overlaying jacket, the jacket’s condition is likely to be more severely degraded than the underlying insulation. None of the above local test approaches can be used to evaluate insulation beneath a cable jacket. Since the jacket’s function is neither structural nor electrical, a degraded jacket may not have any significance regarding the cable’s performance or suitability for service. IDC measurements offer a promising alternative or complement to these local test approaches including the possibility to test insulation beneath an overlaying jacket