Quantitative in situ nanomechanical characterization by combining scanning probe microscope and scanning electron microscope

The great effort is being placed in the design of new characterization techniques and the construction of new instruments capable of characterizing the mechanical response of nanoscale materials. However, property measurements of nanoscale object, such as nanowires and nanothin film, are extremely challenging because of their miniscale size. The main challenges in the experimental study of nanoobject include: (1) real observation of the nanoscale object during mechanical testing; (2) manipulation and positioning of specimens with nanometer scale accuracy; and (3) measurement of force and deformation with nano-Newton and nanometer level resolution. Hence, it is necessary to develop a new experimental testing setup to perform fast and accurate mechanical characterization of nanoobject. The SEM is powerful tool to perform online observation of nanoscale object with large chamber which provides the opportunity to integrate physical or mechanical property measuring system for in situ experimentation. On the other hand, scanning probe microscopy (SPM) or atomic force microscopy has been employed extensively as a high resolution force and displacement measurement tool to characterize the mechanical property of nanoscale object. In this study, we introduce a quantitative nanomechanical in situ test platform by implementing a home-made SPM head in SEM chamber. The SPM/SEM hybrid system has been capable of a wide range of quantitative nanomechanical characterized functions, including three-point bending, uniaxial tension, compression, and nanoindentation. The methodlogies to obtain force versus displacement measurements to extract quantitative material properties are also presented in detail. We show the efficacy of this integrated system by the in situ three-point bending and nanoindentation experiments for individual nanostructure. KEY WORDS scanning probe microscopy, nanomechanical, individual nanostructure, scanning electron microscop

LAMOST 1: A Disrupted Satellite in the Constellation Draco

Using LAMOST spectroscopic data, we find a strong signal of a comoving group of stars in the constellation of Draco. The group, observed near the apocenter of its orbit, is 2.6 kpc from the Sun with a metallicity of -0.64 dex. The system is observed as a streaming population of unknown provenance with mass of about 2.1E4 solar masses and an absolute V band magnitude of about -3.6. Its high metallicity, diffuse physical structure, and eccentric orbit may indicate that the progenitor satellite was a globular cluster rather than a dwarf galaxy or an open cluster.Comment: 6 pages, 4 Figures, 1 Table, Accepted to ApJ

Determining the local dark matter density with LAMOST data

Measurement of the local dark matter density plays an important role in both Galactic dynamics and dark matter direct detection experiments. However, the estimated values from previous works are far from agreeing with each other. In this work, we provide a well-defined observed sample with 1427 G \& K type main-sequence stars from the LAMOST spectroscopic survey, taking into account selection effects, volume completeness, and the stellar populations. We apply a vertical Jeans equation method containing a single exponential stellar disk, a razor thin gas disk, and a constant dark matter density distribution to the sample, and obtain a total surface mass density of $\rm {78.7 ^{+3.9}_{-4.7}\ M_{\odot}\ pc^{-2}}$up to 1 kpc and a local dark matter density of$0.0159^{+0.0047}_{-0.0057}\,\rm M_{\odot}\,\rm pc^{-3}$. We find that the sampling density (i.e. number of stars per unit volume) of the spectroscopic data contributes to about two-thirds of the uncertainty in the estimated values. We discuss the effect of the tilt term in the Jeans equation and find it has little impact on our measurement. Other issues, such as a non-equilibrium component due to perturbations and contamination by the thick disk population, are also discussed.Comment: 11 pages, 10 figure

The Milky Way's rotation curve out to 100 kpc and its constraint on the Galactic mass distribution

The rotation curve (RC) of the Milky Way out to $\sim$ 100 kpc has been constructed using $\sim$ 16,000 primary red clump giants (PRCGs) in the outer disk selected from the LSS-GAC and the SDSS-III/APOGEE survey, combined with $\sim$ 5700 halo K giants (HKGs) selected from the SDSS/SEGUE survey. To derive the RC, the PRCG sample of the warm disc population and the HKG sample of halo stellar population are respectively analyzed using a kinematical model allowing for the asymmetric drift corrections and re-analyzed using the spherical Jeans equation along with measurements of the anisotropic parameter $\beta$ currently available. The typical uncertainties of RC derived from the PRCG and HKG samples are respectively 5-7 km/s and several tens km/s. We determine a circular velocity at the solar position, $V_c (R_0)$ = 240 $\pm$ 6 km/s and an azimuthal peculiar speed of the Sun, $V_{\odot}$ = 12.1 $\pm$ 7.6 km/s, both in good agreement with the previous determinations. The newly constructed RC has a generally flat value of 240 km/s within a Galactocentric distance $r$ of 25 kpc and then decreases steadily to 150 km/s at $r$ $\sim$ 100 kpc. On top of this overall trend, the RC exhibits two prominent localized dips, one at $r$ $\sim$ 11 kpc and another at $r$ $\sim$ 19 kpc. From the newly constructed RC, combined with other constraints, we have built a parametrized mass model for the Galaxy, yielding a virial mass of the Milky Way's dark matter halo of $0.90^{+0.07}_{-0.08} \times 10^{12}$ ${\rm M}_{\odot}$ and a local dark matter density, $\rho_{\rm \odot, dm} = 0.32^{+0.02}_{-0.02}$ GeV cm$^{-3}$.Comment: MNRAS accepted, 18 pages, 15 figures, 4 table