A Photonic Topological Mode Bound to a Vortex

Topological photonics sheds light on some of the surprising phenomena seen in condensed matter physics that arise with the appearance of topological invariants. Optical waveguides provide a well-controlled platform to investigate effects that relate to different topological phases of matter, providing insight into phenomena such as topological insulators and superconductors by direct simulation of the states that are protected by the topology of the system. Here, we observe a mode associated with a topological defect in the bulk of a 2D photonic material by introducing a vortex distortion to an hexagonal lattice and analogous to graphene. These observations are made possible by advances in our experimental methods. We were able to manufacture uniform large two-dimensional photonic crystal structures, containing thousands of identical waveguides arranged in two dimensions, and we developed a new method to excite multiples of these waveguides with a well-defined light field. This allows us to probe the detailed spatial features of topological defect modes for the first time. The observed modes lie mid-gap at zero energy and are closely related to Majorana bound states in superconducting vortices. This is the first experimental demonstration of a mode that is a solution to the Dirac equation in the presence of a vortex, as proposed by Jackiw and Rossi

Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

Stress corrosion cracking (SCC) in Cu-free Al-Zn-Mg (7xxx) aluminium alloys limits its use in many applications. In this work, we study in detail the microstructure of a peak and slightly overaged condition in an AA7003 alloy using transmission- and scanning electron microscopy in order to provide a comprehensive understanding of the microstructural features related to SCC. The SCC properties have been assessed using the double cantilever beam method and slow strain rate tensile tests. Grain boundary particles, precipitate free zones, and matrix precipitates have been studied. A difference in the SCC properties is established between the two ageing conditions. The dominating difference is the size and orientation of the hardening phases. Possible explanations correlating the microstructure and SCC properties are discussed.</jats:p

Tetra­kis(4-cyano­pyridine)­palladium(II) bis­(trifluoro­methane­sulfonate)

The title salt, [Pd(C6H4N2)4](CF3SO3)2, comprises Pd(4-cyano­pyridine)4 dications balanced by two trifluoro­methane­sulfonate anions. The PdII atom lies in a square-planar geometry defined by four N atoms which form equivalent Pd—N inter­actions. The 4-cyano­pyridine ligands are twisted out of the N4 plane, forming dihedral angles ranging from 66.5 (2) to 89.9 (2)°. In the crystal packing, columns of edge-to-edge dications define channels in which reside the anions. A range of C—H⋯N and C—H⋯O hydrogen-bonding interactions stabilizes the crystal packing

Photographic investigation of reflected shock phenomena from decigram explosive charges

An investigation of the primary shock front and Mach Y-stem systems was conducted utilizing decigram explosive charges. Distance and arrival time data of the primary shock front was correlated with that of high yield explosions. A good correlation would indicate the feasibility of conducting laboratory scale tests to obtain information on high yield explosions without the expenditure of time and money involved in large scale field tests. The shock front system was also investigated at the time of first formation of the Mach Y-stem. The critical angle of incidence of the primary shock front for the formation of the Y-stem was compared to the theoretical value. Theoretical calculations of the yield of the explosion was also compared to the actual yield. Basic data were obtained from the explosions by photographing the shadow of the shock front system utilizing a Polaroid camera, a microflash unit and a time delay generator. Correlation of the overpressure was excellent for small to moderate distances from the point of explosion, as was the yield comparison. The critical angle of incidence comparison indicated a marked difference between experimental end theoretical values.http://www.archive.org/details/photographicinve00luskLieutenant, United States NavyLieutenant, United States NavyApproved for public release; distribution is unlimited

The Vibrio cholerae var regulon encodes a metallo-β-lactamase and an antibiotic efflux pump, which are regulated by VarR, a LysR-type transcription factor

The genome sequence of V. cholerae O1 Biovar Eltor strain N16961 has revealed a putative antibiotic resistance (var) regulon that is predicted to encode a transcriptional activator (VarR), which is divergently transcribed relative to the putative resistance genes for both a metallo-β-lactamase (VarG) and an antibiotic efflux-pump (VarABCDEF). We sought to test whether these genes could confer antibiotic resistance and are organised as a regulon under the control of VarR. VarG was overexpressed and purified and shown to have β-lactamase activity against penicillins, cephalosporins and carbapenems, having the highest activity against meropenem. The expression of VarABCDEF in the Escherichia coli (ΔacrAB) strain KAM3 conferred resistance to a range of drugs, but most significant resistance was to the macrolide spiramycin. A gel-shift analysis was used to determine if VarR bound to the promoter regions of the resistance genes. Consistent with the regulation of these resistance genes, VarR binds to three distinct intergenic regions, varRG, varGA and varBC located upstream and adjacent to varG, varA and varC, respectively. VarR can act as a repressor at the varRG promoter region; whilst this repression was relieved upon addition of β-lactams, these did not dissociate the VarR/varRG-DNA complex, indicating that the de-repression of varR by β-lactams is indirect. Considering that the genomic arrangement of VarR-VarG is strikingly similar to that of AmpR-AmpC system, it is possible that V. cholerae has evolved a system for resistance to the newer β-lactams that would prove more beneficial to the bacterium in light of current selective pressures

Prevalence Rates of Mental Disorders in Chilean Prisons

PMCID: PMC371883