Growth and characterisation of MnSb thin films and interfaces

The deposition of Mn on to reconstructed InSb and GaAs surfaces has been studied by re ection high-energy electron diffraction (RHEED), atomic force microscopy and scanning tunnelling microscopy. On both Ga- and As-terminated GaAs(001), a Mn-induced (2x2) reconstruction is observed. In contrast, there are no well defined Mn-induced surface reconstructions on InSb. Islands are observed to form on all of the surfaces studied, with islands on the Group III-rich surfaces composed of elemental Mn and of an alloy on the Group V-rich surfaces. The conversion from Group III(V)-rich to Group V(III)-rich surfaces are discussed in terms of basic thermodynamic quantities and a number of models for surface atom substitution are proposed as pathways for MnAs and MnSb island formation. A high resolution X-ray diffraction study (HRXRD) has been performed on niccolite, cubic and wurtzite crystallites present within MnSb thin films grown on GaAs(111) substrates. It is observed that the lattice parameters of the polymorphs do not depend on the film thickness or the time-corrected beam ux ratio, J. The niccolite phase is found to relax rapidly (within 3 nm) and the average c lattice parameter of these films is 5.791(1) A. Variations in the c lattice parameter indicate that the average stoichiometry of the films varies on a per sample basis and this may act to promote the formation of polymorphs. Cubic MnSb crystallites exhibit a large strain dispersion of approximately 1 % and a rhombohedral or trigonal distortion is believed to be the origin. Quantitative analysis of asymmetric reciprocal space maps reveals that films grown using the optimised conditions have the highest concentration of the cubic polymorph, with lower values of J in the optimised range promoting this polymorph. The growth of MnSb on Ge(001) and Ge(111) substrates has been investigated. On Ge(001) growth proceeds through the formation of three dimensional islands and no dependence on the growth conditions is observed. Evidence for (1102) and (1120) crystallites is seen in XRD and RHEED, respectively. The interface between the MnSb islands and the Ge(001) substrate is sharp with no evidence for interfacial reactivity. The epitaxial growth of MnSb on Ge(111) is reported for the first time. The growth orientation is confirmed to be (0001) by X-ray diffraction while the layers are found to be ferromagnetic with a Curie temperature in excess of 300 K

Epitaxial growth and surface reconstruction of CrSb(0001)

Smooth CrSb(0001) films have been grown by molecular beam epitaxy on MnSb(0001) – GaAs(111) substrates. CrSb(0001) shows (2 × 2), triple domain (1 × 4) and (√3×√3)R30° reconstructed surfaces as well as a (1 × 1) phase. The dependence of reconstruction on substrate temperature and incident fluxes is very similar to MnSb(0001)

Bulk crystal growth and surface preparation of NiSb, MnSb, and NiMnSb

Bulk single crystal and polycrystalline samples of NiSb, MnSb, and NiMnSb have been grown and characterized. The lattice parameter of NiMnSb was found to be 5.945 ± 0.001 Å, around 0.25% larger than previous reports. The surface preparation of these materials was investigated using x-ray photoelectron spectroscopy. Wet etching with HCl and argon ion sputtering were used in tandem with vacuum annealing. For both binary materials, a clean and stoichiometric surface could be regained by HCl etching and annealing alone. However, clean and stoichiometric ternary NiMnSb was not successfully prepared by these methods. The transition metal 2p and 3p levels are analyzed for all three materials

Epitaxial growth of cubic MnSb on GaAs AND InGaAs(111)

The cubic polymorph of the binary transition metal pnictide (TMP) MnSb, c-MnSb, has been predicted to be a robust half-metallic ferromagnetic (HMF) material with minority spin gap ≳1 eV. Here, MnSb epilayers are grown by molecular beam epitaxy (MBE) on GaAs and In0.5Ga0.5As(111) substrates and analyzed using synchrotron radiation X-ray diffraction. We find polymorphic growth of MnSb on both substrates, where c-MnSb co-exists with the ordinary niccolite n-MnSb polymorph. The grain size of the c-MnSb is of the order of tens of nanometer on both substrates and its appearance during MBE growth is independent of the very different epitaxial strain from the GaAs (3.1%) and In0.5Ga0.5As (0.31%) substrates

WFPC2 Observations of the Cooling Flow Elliptical in Abell 1795

We present WFPC2 images of the core of the cooling flow cD galaxy in Abell 1795. An irregular, asymmetric dust lane extends 7 \h75 kpc in projection to the north-northwest. The dust shares the morphology observed in the H$\alpha$ and excess UV emission. We see both diffuse and knotty blue emission around the dust lane, especially at the ends. The dust and emission features lie on the edge of the radio lobes, suggesting star formation induced by the radio source or the deflection of the radio jets off of pre-existing dust and gas. We measure an apparent R$_V$ significantly less than 3.1, implying that the extinction law is not Galactic in the dust lane, or the presence of line emission which is proportional to the extinction. The dust mass is at least 2$\times10^{5} h_{75}^{-2}$ M\solar\ and is more likely to be 6.5$\times10^{5} h_{75}^{-2}$ M\solar.Comment: 14 pages, LaTeX, Figure 4 included, Postscript Figs. 1-3 available at ftp://astro.nmsu.edu/pub/JASON/A1795/, accepted for publication in ApJ Letter

Hubble Space Telescope Imaging of the Circumstellar Nebulosity of T Tauri

Short-exposure Planetary Camera images of T Tauri have been obtained using broadband filters spanning the wavelength range 0.55-0.80 μm. The optically visible star lies very close to an arc of reflection nebulosity. The arc's northern arm extends approximately 5" from the star, while its southwestern arm appears brighter and extends only 2". The arc shows an approximate symmetry along an axis toward the west-northwest, the direction of Hind's Nebula and the blueshifted molecular outflow. The morphology of the reflected light is similar to models of scattered light within an illuminated, axisymmetric outflow cavity in a circumbinary envelope, viewed ≈ 45° from the outflow axis. However, our model images do not successfully account for the amount of limb brightening that is seen. No optical counterpart to the infrared companion is seen to a limiting magnitude of V = 19.6, which suggests A_V > 7 mag toward this source. There is no evidence for an optical tertiary, to a limiting ΔV = 5.1 mag fainter than the primary, at the position where such an object has been previously reported

Hubble Space Telescope Observations of the Draco Dwarf Spheroidal

We present an F606W-F814W color-magnitude diagram for the Draco dwarf spheroidal galaxy based on Hubble Space Telescope WFPC2 images. The luminosity function is well-sampled to 3 magnitudes below the turn-off. We see no evidence for multiple turnoffs and conclude that, at least over the field of the view of the WFPC2, star formation was primarily single-epoch. If the observed number of blue stragglers is due to extended star formation, then roughly 6% (upper limit) of the stars could be half as old as the bulk of the galaxy. The color difference between the red giant branch and the turnoff is consistent with an old population and is very similar to that observed in the old, metal-poor Galactic globular clusters M68 and M92. Despite its red horizontal branch, Draco appears to be older than M68 and M92 by 1.6 +/- 2.5 Gyrs, lending support to the argument that the ``second parameter'' which governs horizontal branch morphology must be something other than age. Draco's observed luminosity function is very similar to that of M68, and the derived initial mass function is consistent with that of the solar neighborhood.Comment: 16 pages, AASTeX, 9 postscript figures, figures 1 and 2 available at ftp://bb3.jpl.nasa.gov/pub/draco/. Accepted for publication in the Astronomical Journa

Observations and Implications of the Star Formation History of the LMC

We present derivations of star formation histories based on color-magnitude diagrams of three fields in the LMC from HST/WFPC2 observations. A significant component of stars older than 4 Gyr is required to match the observed color-magnitude diagrams. Models with a dispersion-free age-metallicity relation are unable to reproduce the width of the observed main sequence; models with a range of metallicity at a given age provide a much better fit. Such models allow us to construct complete ``population boxes'' for the LMC based entirely on color-magnitude diagrams; remarkably, these qualitatively reproduce the age-metallicity relation observed in LMC clusters. We discuss some of the uncertainties in deriving star formation histories. We find, independently of the models, that the LMC bar field has a larger relative component of older stars than the outer fields. The main implications suggested by this study are: 1) the star formation history of field stars appears to differ from the age distribution of clusters, 2) there is no obvious evidence for bursty star formation, but our ability to measure bursts shorter in duration than $\sim$ 25% of any given age is limited by the statistics of the observed number of stars, 3) there may be some correlation of the star formation rate with the last close passage of the LMC/SMC/Milky Way, but there is no dramatic effect, and 4) the derived star formation history is probably consistent with observed abundances, based on recent chemical evolution models.Comment: Accepted by AJ, 36 pages including 12 figure

Inclusion and release of ant alarm pheromones from metal–organic frameworks

Zinc(II) and zirconium(IV) metal–organic frameworks show uptake and slow release of the ant alarm pheromones 3-octanone and 4-methyl-3-heptanone. Inclusion of N-propyl groups on the MOFs allows for enhanced uptake and release over several months. In preliminary field trials, leaf cutting ants show normal behavioural responses to the released pheromones