Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies

We have investigated compositional changes on GaNsurfaces under Ar-ion bombardment using synchrotron-based high-resolution x-rayphotoemission (PES) and near-edge x-ray absorption fine structure(NEXAFS)spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemissionspectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-rayabsorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaNsurfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.P.N.K.D. is grateful for the financial support of the Australian Research Council

Neutrino Emission From Direct Urca Processes in Pion Condensed Quark Matter

We study neutrino emission from direct Urca processes in pion condensed quark matter. In compact stars with high baryon density, the emission is dominated by the gapless modes of the pion condensation which leads to an enhanced emissivity. While for massless quarks the enhancement is not remarkable, the emissivity is significantly larger and the cooling of the condensed matter is considerably faster than that in normal quark matter when the mass difference between $u$- and $d$-quarks is sizable.Comment: 12 pages,6 figures, published versio

Effect of Muons on the Phase Transition in Magnetised Proto-Neutron Star Matter

We study the effect of inclusion of muons and the muon neutrinos on the phase transition from nuclear to quark matter in a magnetised proto-neutron star and compare our results with those obtained by us without the muons. We find that the inclusion of muons changes slightly the nuclear density at which transition occurs.However the dependence of this transition density on various chemical potentials, temperature and the magnetic field remains quantitatively the same.Comment: LaTex2e file with four postscript figure

Dynamic crossover scaling in polymer solutions

The crossover region in the phase diagram of polymer solutions, in the regime above the overlap concentration, is explored by Brownian Dynamics simulations, to map out the universal crossover scaling functions for the gyration radius and the single-chain diffusion constant. Scaling considerations, our simulation results, and recently reported data on the polymer contribution to the viscosity obtained from rheological measurements on DNA systems, support the assumption that there are simple relations between these functions, such that they can be inferred from one another.Comment: 4 pages, 6 figures, 1 Table. Revised version to appear in Physical Review Letters. Includes supplemental material

Fabrication of micromirrors with pyramidal shape using anisotropic etching of silicon

Gold micro-mirrors have been formed in silicon in an inverted pyramidal shape. The pyramidal structures are created in the (100) surface of a silicon wafer by anisotropic etching in potassium hydroxide. High quality micro-mirrors are then formed by sputtering gold onto the smooth silicon (111) faces of the pyramids. These mirrors show great promise as high quality optical devices suitable for integration into MOEMS systems

Infectious diseases at the paediatric isolation units of Clairwood and King Edward VIII Hospitals, Durban

Objective. Information on diseases of public health importance is scanty or  unavailable in South Africa as a result of a weak health surveillance system. Large institutional databases of common diseases can, therefore, provide useful ancillary information for planning and policy, despite unavoidable selection bias. We conducted a 12-year retrospective review (1985 - 1996) of all children admitted to the only isolation facility for the Durban metropolitan region. ·Our aim was to document changes in admissions and mortality for common childhood infectious diseases and to detect any impact of the HIV epidemic on these diseases.Results. During these years 19 037 children were admitted and annual admissions decreased by 79%. Measles accounted for the majority of admissions (58%), followed by varicella at 23%. No cases of poliomyelitis, diphtheria or cholera have been seen since 1990. Typhoid fever, mumps, tetanus and pertussis have  decreased, but remain at low endemic levels. Between 1994 and 1996, 1% of measles and 15.3% of varicella cases have been associated with illV-l infection; this has resulted in 56% of measles deaths and 75% of varicella deaths occurring in HIV co-infected children. Overall, 60% of deaths during the past 3 years have been in illV co-infected children. HIV testing based solely on clinical suspicion was performed in 11% and 29% of measles and varicella cases, respectively. Average all-disease mortality was 5.3%, a decrease of 87% over the study period, with measles accounting for most deaths (86%).Conclusions. The changing profile of childhood infectious diseases described at the paediatric isolation units is consistent with available national data. Probable reasons for these changes are the shift in emphasis to primary health care issues, and a gradual improvement in socio-economic conditions of the poor

Kaon Zero-Point Fluctuations in Neutron Star Matter

We investigate the contribution of zero-point motion, arising from fluctuations in kaon modes, to the ground state properties of neutron star matter containing a Bose condensate of kaons. The zero-point energy is derived via the thermodynamic partition function, by integrating out fluctuations for an arbitrary value of the condensate field. It is shown that the vacuum counterterms of the chiral Lagrangian ensure the cancellation of divergences dependent on $\mu$, the charge chemical potential, which may be regarded as an external vector potential. The total grand potential, consisting of the tree-level potential, the zero-point contribution, and the counterterm potential, is extremized to yield a locally charge neutral, beta-equilibrated and minimum energy ground state. In some regions of parameter space we encounter the well-known problem of a complex effective potential. Where the potential is real and solutions can be obtained, the contributions from fluctuations are found to be small in comparison with tree-level contributions.Comment: 40 pages RevTeX, 3 epsf figure