Characterization of multiaxial warp knit composites

The objectives were to characterize the mechanical behavior and damage tolerance of two multiaxial warp knit fabrics to determine the acceptability of these fabrics for high performance composite applications. The tests performed included compression, tension, open hole compression, compression after impact and compression-compression fatigue. Tests were performed on as-fabricated fabrics and on multi-layer fabrics that were stitched together with either carbon or Kevlar stitching yarn. Results of processing studies for vacuum impregnation with Hercules 3501-6 epoxy resin and pressure impregnation with Dow Tactix 138/H41 epoxy resin and British Petroleum BP E905L epoxy resin are presented

The distribution of stars around the Milky Way's central black hole II: Diffuse light from sub-giants and dwarfs

This is the second of three papers that search for the predicted stellar cusp around the Milky Way's central black hole, Sagittarius A*, with new data and methods. We aim to infer the distribution of the faintest stellar population currently accessible through observations around Sagittarius A*. We use adaptive optics assisted high angular resolution images obtained with the NACO instrument at the ESO VLT. Through optimised PSF fitting we remove the light from all detected stars above a given magnitude limit. Subsequently we analyse the remaining, diffuse light density. The analysed diffuse light arises from sub-giant and main-sequence stars with KS ~ 19 - 20 with masses of 1 - 2 Msol . These stars can be old enough to be dynamically relaxed. The observed power-law profile and its slope are consistent with the existence of a relaxed stellar cusp around the Milky Way's central black hole. We find that a Nuker law provides an adequate description of the nuclear cluster's intrinsic shape (assuming spherical symmetry). The 3D power-law slope near Sgr A* is \gamma = 1.23 +- 0.05. At a distance of 0.01 pc from the black hole, we estimate a stellar mass density of 2.3 +- 0.3 x 10^7 Msol pc^-3 and a total enclosed stellar mass of 180 +- 20 Msol. These estimates assume a constant mass-to-light ratio and do not take stellar remnants into account. The fact that no cusp is observed for bright (Ks 16) giant stars at projected distances of roughly 0.1-0.3 pc implies that some mechanism has altered their appearance or distribution.Comment: Accepted for publication A&

The distribution of old stars around the Milky Way's central black hole I: Star counts

(abridged) In this paper we revisit the problem of inferring the innermost structure of the Milky Way's nuclear star cluster via star counts, to clarify whether it displays a core or a cusp around the central black hole. Through image stacking and improved PSF fitting we push the completeness limit about one magnitude deeper than in previous, comparable work. Contrary to previous work, we analyse the stellar density in well-defined magnitude ranges in order to be able to constrain stellar masses and ages. The RC and brighter giant stars display a core-like surface density profile within a projected radius R<0.3 pc of the central black hole, in agreement with previous studies, but show a cusp-like surface density distribution at larger R. The surface density of the fainter stars can be described well by a single power-law at R<2 pc. The cusp-like profile of the faint stars persists even if we take into account the possible contamination of stars in this brightness range by young pre-main sequence stars. The data are inconsistent with a core-profile for the faint stars.Finally, we show that a 3D Nuker law provides a very good description of the cluster structure. We conclude that the observed stellar density at the Galactic Centre, as it can be inferred with current instruments, is consistent with the existence of a stellar cusp around the Milky Way's central black hole, Sgr A*. This cusp is well developed inside the influence radius of about 3 pc of Sgr A* and can be described by a single three-dimensional power-law with an exponent gamma=1.23+-0.05. The apparent lack of RC stars and brighter giants at projected distances of R < 0.3 pc (R<8") of the massive black hole may indicate that some mechanism has altered their distribution or intrinsic luminosity.Comment: Accepted for publication A&

Detrimental adsorbate fields in experiments with cold Rydberg gases near surfaces

We observe the shift of Rydberg levels of rubidium close to a copper surface when atomic clouds are repeatedly deposited on it. We measure transition frequencies of rubidium to S and D Rydberg states with principal quantum numbers n between 31 and 48 using the technique of electromagnetically induced transparency. The spectroscopic measurement shows a strong increase of electric fields towards the surface that evolves with the deposition of atoms. Starting with a clean surface, we measure the evolution of electrostatic fields in the range between 30 and 300 \mum from the surface. We find that after the deposition of a few hundred atomic clouds, each containing ~10^6 atoms, the field of adsorbates reaches 1 V/cm for a distance of 30 \mum from the surface. This evolution of the electrostatic field sets serious limitations on cavity QED experiments proposed for Rydberg atoms on atom chips.Comment: 4 pages, 3 figures Submitted to Phys. Rev.

Compact Radio Sources within 30" of Sgr A*: Proper Motions, Stellar Winds and the Accretion Rate onto Sgr A*

Recent broad-band 34 and 44 GHz radio continuum observations of the Galactic center have revealed 41 massive stars identified with near-IR counterparts, as well as 44 proplyd candidates within 30" of Sgr A*. Radio observations obtained in 2011 and 2014 have been used to derive proper motions of eight young stars near Sgr A*. The accuracy of proper motion estimates based on near-IR observations by Lu et al. and Paumard et al. have been investigated by using their proper motions to predict the 2014 epoch positions of near-IR stars and comparing the predicted positions with those of radio counterparts in the 2014 radio observations. Predicted positions from Lu et al. show an rms scatter of 6 mas relative to the radio positions, while those from Paumard et al. show rms residuals of 20 mas, which is mainly due to uncertainties in the IR-based proper motions. Under the assumption of homogeneous ionized winds, we also determine the mass-loss rates of 11 radio stars, finding rates that are on average $\sim$2 times smaller than those determined from model atmosphere calculations and near-IR data. Clumpiness of ionized winds would reduce the mass loss rate of WR and O stars by additional factors of 3 and 10, respectively. One important implication of this is a reduction in the expected mass accretion rate onto Sgr A* from stellar winds by nearly an order of magnitude to a value of few$\times10^{-7}$ \msol\ yr$^{-1}$. Finally, we present the positions of 318 compact 34.5 GHz radio sources within 30\arcs\ of Sgr A*. At least 45 of these have stellar counterparts in the near-IR $K_s$ (2.18 $\mu$m) and $L'$ (3.8$\mu$m) bands.Comment: 30 pages, 4 figures, ApJ (in press

Magnetotransport in the Kondo model with ferromagnetic exchange interaction

We consider the transport properties in an applied magnetic field of the spin S=1/2 Kondo model with ferromagnetic exchange coupling to electronic reservoirs, a description relevant for the strong coupling limit of underscreened spin S=1 Kondo impurities. Because the ferromagnetic Kondo interaction is marginally irrelevant, perturbative methods should prove accurate down to low energies. For the purpose of this study, we use a combination of Majorana diagrammatic theory with Density Matrix Numerical Renormalization Group simulations. In the standard case of antiferromagnetic Kondo exchange, we first show that our technique recovers previously obtained results for the T-matrix and spin relaxation at weak coupling (above the Kondo temperature). Considering then the ferromagnetic case, we demonstrate how the low-energy Kondo anomaly splits for arbitrary small values of the Zeeman energy, in contrast to fully screened Kondo impurities near the strong coupling Fermi liquid fixed point, and in agreement with recent experimental findings for spin S=1 molecular quantum dots.Comment: 14 pages, 13 figures, minor changes in V

Cold atoms near superconductors: Atomic spin coherence beyond the Johnson noise limit

We report on the measurement of atomic spin coherence near the surface of a superconducting niobium wire. As compared to normal conducting metal surfaces, the atomic spin coherence is maintained for time periods beyond the Johnson noise limit. The result provides experimental evidence that magnetic near field noise near the superconductor is strongly suppressed. Such long atomic spin coherence times near superconductors open the way towards the development of coherently coupled cold atom / solid state hybrid quantum systems with potential applications in quantum information processing and precision force sensing.Comment: Major revisions of the text for submission to New Journal of Physics 8 pages, 4 figure