Convergent N2-scaling iterative method of photoelectron diffraction and low-energy electron diffraction for ordered or disordered systems

We present results of a convergent iterative method of photoelectron diffraction and low-energy electron diffraction. The computation time of this method scales as N2, where N is the dimension of the propagator matrix, rather than N3 as in conventional Gaussian substitutional methods. We show that the Rehr-Albers separable-representation cluster approach or slab-type nonseparable methods can all be cast in this iterative form. The convergence of this method is demonstrated for different materials. With the substantial savings in computational time and no loss in numerical accuracy, this method will be very useful in future applications of multiple-scattering theory, particularly for systems either involving very large unit cells (200-700 atoms) or where no long-range order is present. ©1999 The American Physical Society.published_or_final_versio

Surface Patterson function by inversion of low-energy electron diffraction I-V spectra at multiple incident angles

An artifacts-free Patterson function was obtained from the sum of transforms of low energy electron diffraction intensity-energy spectra at multiple incident angle and momenta transfers. Normal incidence measured spectra and calculated spectra at three angles of incidence were used to demonstrate this phenomena. The Patterson function was found to be highly accurate in determining the pairwise atomic distances in the horizontal direction.published_or_final_versio

Adsorption and desorption kinetics of gallium atoms on 6H-SiC(0001) surfaces

Gallium (Ga) surface adsorption and desorption kinetics on 6H-SiC(0001) are investigated using reflection high-energy electron diffraction. It is found that for Ga adsorption, a wetting layer bonds strongly to the SiC(0001) surface. Additional Ga atoms form droplets on top of the wetting layer. The Ga droplets behave like a metallic liquid. The activation energies for desorption are determined to be 3.5 eV for Ga in the wetting layer and 2.5 eV for Ga in the droplets. It is further found that the desorption of Ga atoms from the wetting layer follows a zero-order kinetics, i.e., the desorption rate is independent of the number of adsorbed atoms. ©2000 The American Physical Society.published_or_final_versio

Community Assessment on Colorectal Cancer Screening Knowledge, Attitudes, Behavior, and Health Literacy in American Samoa: Methods and Results

urpose/Background: “Weaving” Indigenous and Western knowledges are understood to advance health equity for the Indigenous community whose own knowledge and values are often subsumed. Partnerships that support Indigenous leadership and bring Indigenous knowledge to greater parity with Western knowledge are imperative. Indigenous people living in Small Island Developing States like the U.S. territories experience health disparities. The WHO has declared non- communicable diseases as a global health epidemic, including cancer disparities. In American Samoa (AS) less than 7% of age-eligible adults participated in colorectal cancer (CRC) screening, while the Healthy People 2020 target is 70% screened. CRC may be prevented through screening and early detection. The relationship between CRC screening with knowledge, attitudes, beliefs (KAB), and health literacy has never been locally assessed in AS. AS based community researchers partnered with regional academic researchers in local research training, instrument development, data collection, and data analysis. Community relevant approaches guided assessment of CRC screening rates, CRC screening KAB, and health literacy. Objective: This abstract describes the community engaged approaches and results through the National Institute of Minority Health Disparities funded INdigenous Samoan Partnership to Initiate Research Excellence (INSPIRE), introduced at the 2015 CTR-IN annual meeting. Materials & Methods: INSPIRE principal Investigators are based in AS, operated through the American Samoa Community Coalition, a community-based organization, partnering with academic co-investigators in Hawaii and California. To assess CRC KAB and health literacy, two validated assessment instruments were adapted: The KAB survey, from self-developed and population-based instruments, and Short Test for Functional Health Literacy in Adults (S-TOFHLA). Community partners translated then back translated both surveys with three focus groups. Twenty cognitive interviews were conducted to test translation cogency. Seven AS INSPIRE research trainees participating in year-long workshops were trained on Respondent Driven Sampling (RDS). Initial participants (seeds) were recruited by social characteristics. Upon survey completion participants received coupons to each recruit three eligible participants. 713 community respondents participated in three months. An INSPIRE researcher (epidemiologist) analyzed data, with technical assistance from an academic biostatistician in Hawaii when needed fostering research capacity training. Bivariate and multivariate statistics analyzed CRC knowledge, attitudes, health literacy as correlates and predictors of self-reported screening. Results: About 65 percent of respondents finished high school; 68.9% had no insurance. Only 2 respondents completed CRC screening, but 63.3% and 82.3% of English vs Samoan survey respondents respectively would get colonoscopy. Among the 11 knowledge questions, 5 items were “don’t know” while 2 items were incorrect responses on English and Samoan surveys respectively. Almost twice as many Samoan compared to English survey respondents (81% vs. 44%) scored “inadequate” in the S-TOFHLA. Correlates and predictors of CRC screening behavior and differences in results between the respondents who took English vs Samoan surveys will be reported. Discussion/Conclusion: Almost all participants have not received CRC screening, lower than other reported AS nationwide figures. This first ever study, led by and for American Samoans obtained robust results on knowledge, attitudes, and health literacy of CRC screening to allow continued research and interventions to increase very low CRC screening rates

Role of scattering-factor anisotropy in electron, positron, and photon holography

We have studied the angular anisotropy in the scattering factor of electrons, positrons, and photons in solids. We show that as a function of angle, the maximum number of dips in the scattering factor's magnitude and jumps of near π in its phase are related to the angular momenta of the bound and resonance states of the potential. The effect of the scattering factor's anisotropy on low-energy electron and positron holographic wave-front reconstruction is discussed. Applying the variable-axis small-cone method, a good-quality reconstructed image is only possible within angular regions where the scattering factor is near isotropic. Thus the usable window for low-energy electron wave-front reconstruction is element dependent; the window size decreases as the atomic number increases. Positrons, on the other hand, are like photons and are not bound by the potential. For positrons or photons, there is no elemental dependence of the usable window and the entire backscattering regime is suitable for holographic reconstruction. We have established two rules that predict the maximum number of magnitude dips and phase jumps in the scattering factor for any element.published_or_final_versio

Methods in angle-resolved photoelectron diffraction: Slab method versus separable propagator cluster approach

We have compared multiple-scattering results of angle-resolved photoelectron diffraction spectra between the exact slab method and the separable propagator perturbation cluster method. In the slab method, the source wave and multiple scattering within strongly scattering layers are expanded in spherical waves while the scattering among different layers is expressed in plane waves. The transformation between spherical waves and plane waves is done exactly. The plane waves are then matched across the solid-vacuum interface to a single outgoing plane wave in the detector's direction. The slab is infinitely extended parallel to the surface. Normal to the surface, enough layers are included to ensure convergence of the calculated intensity. The separable propagator perturbation approach uses two approximations; (i) A separable representation of the Green's-function propagator and (ii) a perturbation expansion of multiple-scattering terms. The cluster size is finite, typically containing 50 atoms or less. Results of this study show that using a cluster of 148 atoms, the largest cluster used to date, the cluster size is still too small for the cluster results on Ni(001) to converge with those of the slab method. Ideas to improve the perturbation expansion cluster method are discussed.published_or_final_versio

A smoothing SQP method for nonlinear programs with stability constraints arising from power systems

This paper investigates a new class of optimization problems arising from power systems, known as nonlinear programs with stability constraints (NPSC), which is an extension of ordinary nonlinear programs. Since the stability constraint is described generally by eigenvalues or norm of Jacobian matrices of systems, this results in the semismooth property of NPSC problems. The optimal conditions of both NPSC and its smoothing problem are studied. A smoothing SQP algorithm is proposed for solving such optimization problem. The global convergence of algorithm is established. A numerical example from optimal power flow (OPF) is done. The computational results show efficiency of the new model and algorithm. © The Author(s) 2010.published_or_final_versionSpringer Open Choice, 21 Feb 201

Initial stage of GaN growth and its implication to defect formation in films

In situ scanning tunneling microscopy (STM) observations of initial growth processes of GaN by molecularbeam epitaxy reveal important differences between growth on vicinal versus flat SiC(0001) substrates. Based on stop-growth STM studies, we explain why there are orders of magnitude reductions in the density of threading screw dislocations in the vicinal films. It is shown that on vicinal surfaces, three-dimensional (3D) islands develop into a characteristic shape. The islands coalesce much sooner than on flat surfaces. Consequently, fewer defects are created at their boundaries.published_or_final_versio

A study of Inx Ga1-x N growth by reflection high-energy electron diffraction

Epitaxial growth of Inx Ga1-x N alloys on GaN (0001) by plasma-assisted molecular-beam epitaxy is investigated using the in situ reflection high-energy electron-diffraction (RHEED) technique. Based on RHEED pattern changes over time, the transition of growth mode from two-dimensional (2D) nucleation to three-dimensional islanding is studied for various indium compositions. RHEED specular-beam intensity oscillations are recorded during the 2D wetting-layer growth, and the dependences of the oscillation period/frequency on the substrate temperature and source flux are established. By measuring the spacing between diffraction spots in RHEED, we also estimated indium composition, x, in alloys grown under different flux combinations. Incorporation coefficients of both gallium and indium are derived. Possible surface segregation of indium atoms is finally examined. © 2005 American Institute of Physics.published_or_final_versio

Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire

Stress and its effects on optical properties of GaN epilayers grown in Si(111), 6H-SiC(0001), and c-plane sapphire were investigated. Large tensile stress was present in GaN epilayers grown on Si and 6H-SiC, and a small compressive stress appeared in the film grown on sapphire. The results showed that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films.published_or_final_versio