250 research outputs found
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 $\rm {78.7 ^{+3.9}_{-4.7}\
M_{\odot}\ pc^{-2}}0.0159^{+0.0047}_{-0.0057}\,\rm M_{\odot}\,\rm pc^{-3}$. 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 100 kpc has been
constructed using 16,000 primary red clump giants (PRCGs) in the outer
disk selected from the LSS-GAC and the SDSS-III/APOGEE survey, combined with
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 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, = 240 6 km/s and an
azimuthal peculiar speed of the Sun, = 12.1 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 of 25 kpc
and then decreases steadily to 150 km/s at 100 kpc. On top of this
overall trend, the RC exhibits two prominent localized dips, one at
11 kpc and another at 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
and a local dark matter
density, GeV cm.Comment: MNRAS accepted, 18 pages, 15 figures, 4 table
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