Magnetic field dependence of the atomic collapse state in graphene

Quantum electrodynamics predicts that heavy atoms ($Z > Z_c \approx 170$) will undergo the process of atomic collapse where electrons sink into the positron continuum and a new family of so-called collapsing states emerges. The relativistic electrons in graphene exhibit the same physics but at a much lower critical charge ($Z_c \approx 1$) which has made it possible to confirm this phenomenon experimentally. However, there exist conflicting predictions on the effect of a magnetic field on atomic collapse. These theoretical predictions are based on the continuum Dirac-Weyl equation, which does not have an exact analytical solution for the interplay of a supercritical Coulomb potential and the magnetic field. Approximative solutions have been proposed, but because the two effects compete on similar energy scales, the theoretical treatment varies depending on the regime which is being considered. These limitations are overcome here by starting from a tight-binding approach and computing exact numerical results. By avoiding special limit cases, we found a smooth evolution between the different regimes. We predict that the atomic collapse effect persists even after the magnetic field is activated and that the critical charge remains unchanged. We show that the atomic collapse regime is characterized: 1) by a series of Landau level anticrossings and 2) by the absence of $\sqrt{B}$ scaling of the Landau levels with regard to magnetic field strength

Factors influencing development of management strategies for the Abou Ali River in Lebanon. I: Spatial variation and land use

Surface water bodies are progressively subject to increasing stress as a result of environmentally degrading processes primarily related to anthropogenic activities. This study assesses and examines the impact of land use and anthropogenic activities on the spatial variation in water quality of the Abou Ali River in North Lebanon. It is the first detailed study of its kind in Lebanon and adds to the existing Knowledge by shedding light on a relatively small Mediterranean river in a developing country where there is a paucity of such studies. The assessment was conducted at the end of the dry season in 2002 and 2003 and the end of the wet season in 2003 and 2004. The study has demonstrated the importance of anthropogenic influences on the water quality of the Abou Ali River Basin, as concentrations of most contaminants were higher at locations with greatest human activity. The most adversely affected area was the section of the river that flows through an entirely urbanized and highly populated region, the Tripoli conurbation. Upstream rural sites were enriched by contaminants primarily from non-point sources such as agricultural runoff and poultry litter whereas contaminant concentrations at the urban sites were enriched by a combination of sewage discharge and flow of contaminants from upstream. If the Abou Ali River is to be utilized as a managed water resource and its water quality sustained, point source discharges will require treatment and land use management must be planned to minimize the impact of diffuse source pollution on the river. A high priority should be given to the implementation and enforcement of the precautionary and polluter pays principles. Moreover, an effective legal, economic and institutional framework is required to encourage investment in waste reduction and control and to introduce environmentally sound practices

Hybrid and modular multilevel converter designs for isolated HVDC–DC converters

Efficient medium and high-voltage dc-dc conversion is critical for future dc grids. This paper proposes a hybrid multilevel dc-ac converter structure that is used as the kernel of dc-dc conversion systems. Operation of the proposed dc-ac converter is suited to trapezoidal ac-voltage waveforms. Quantitative and qualitative analyses show that said trapezoidal operation reduces converter footprint, active and passive components' size, and on-state losses relative to conventional modular multilevel converters. The proposed converter is scalable to high voltages with controllable ac-voltage slope; implying tolerable dv/dt stresses on the converter transformer. Structural variations of the proposed converter with enhanced modularity and improved efficiency will be presented and discussed with regards to application in front-to-front isolated dc-dc conversion stages, and in light of said trapezoidal operation. Numerical results provide deeper insight of the presented converter designs with emphasis on system design aspects. Results obtained from a proof-of-concept 1-kW experimental test rig confirm the validity of simulation results, theoretical analyses, and simplified design equations presented in this paper. - 2013 IEEE.Scopu

An improved analytic solution for analysis of particle trajectories in fibrous, two‐dimensional filters

The Kuwabara solution for creeping fluid flow through periodic arrangement of cylinders is widely used in analytic and numerical studies of fibrous filters. Numerical solutions have shown that the Kuwabara solution has systematic errors, and when used for the particle trajectories in filters it results in some error in the predicted filter efficiency. The numerical solutions, although accurate, preclude further analytic treatments, and are not as compact and convenient to use as the Kuwabara solution. By reexamining the outer boundary conditions of the Kuwabara solution, a correction term to the Kuwabara solution has been derived to obtain an extended solution that is more accurate and improves prediction of the filter efficiency. By comparison with the numerical solutions, it is shown that the Kuwabara solution is the high porosity asymptote, and that the extended solution has an improved porosity dependence. A rectification is explained that can make particle collection less efficient for periodic, in‐line arrangements of fibers with particle diffusion or body force. This rectification also results in the alignment of particles with inertia (i.e., high Stokes number particles).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70636/2/PHFLE6-6-2-507-1.pd

Land Use Impact on the Spatial and Seasonal Variation of the Contaminant Loads to Abou Ali River and Its Coastal Zone in North Lebanon

Rosana G. Moreira, Editor-in-Chief; Texas A&M UniversityThis is a paper from International Commission of Agricultural Engineering (CIGR, Commission Internationale du Genie Rural) E-Journal Volume 6 (2004): M. Massoud, M. El-Fadel, M. Scrimshaw, and J. Lester. Land Use Impact on the Spatial and Seasonal Variation of the Contaminant Loads to Abou Ali River and Its Coastal Zone in North Lebanon. (May 2004)

Organizational learning, authentic leadership and individual-level resistance to change: a study of Egyptian academics

Purpose – This paper focuses on academics in three private foreign universities located in Cairo (Egypt) to explore the effect of organizational learning on individual-level resistance to change with and without the mediation of authentic leadership.Design/ methodology/ approach – A total of 960 academics were contacted and all of them received a set of questionnaires. After four follow ups, a total of 576 responses were collected with a response rate of 60.00 percent. The author used the chi-square test to determine the association between organizational learning and authentic leadership. Multiple regressions were employed to show how much variation in individual-level resistance to change can be explained by organizational learning and authentic leadership.Findings – The findings highlight a statistical association between organizational learning and authentic leadership. Moreover, another statistical association is explored between authentic leadership and individual-level resistance to change. Furthermore, the statistical analysis proved that having an authentic leadership in the workplace fosters the effect of organizational learning in alleviating individual’s resistance to change.Research limitations/ implications – Data was collected only from academics and did not include rectors and/or heads of academic departments, the matter that may lead to an inflation of statistical relationships. Future research could use a double source method. Moreover, focusing only on private foreign universities working in Egypt diminishes the author’s potential for generalizing his results.Practical implications – The author recommends establishing a unit for knowledge management inside every university. The function of this unit includes but is not limited to examining prospective socio-political, cultural and economic changes/challenges in the surrounding environment and preparing the possible scenarios for dealing with them. This in turn should comprise involvement and learning opportunities for academics work in these universities. The suggested units should also organize monthly meetings between academics and representatives from different Egyptian sectors such as NGOs personnel, CEOs of private and public companies, environmentalists and politicians to address what change those actors seek universities to undertake to guide academics to fulfil their expectations.Originality/ value – This paper contributes by filling a gap in HR management and organization literature in the higher education sector, in which empirical studies on the relationship between organizational learning, authentic leadership and resistance to change have been limited until now