24,621 research outputs found
Understanding habitat selection of wild yak Bos mutus on the Tibetan Plateau
We tested a series of hypotheses on drivers of habitat selection by the Vulnerable wild yak Bos mutus, combining distribution-wide sighting data with species distribution modelling approaches. The results indicate that climatic conditions are of paramount importance in shaping the wild yak's distribution on the Tibetan Plateau. Habitat selection patterns were seasonal, with yaks appearing to select areas closer to villages during the vegetation-growing season. Unexpectedly, our index of forage quantity had a limited effect in determining the distribution of the species. Overall, our results suggest that expected changes in climate for this region could have a significant impact on habitat availability for wild yaks, and we call for more attention to be focused on the unique wildlife in this ecosystem
Random Access Analysis for Massive IoT Networks Under a New Spatio-Temporal Model: A Stochastic Geometry Approach
Massive Internet of Things (mIoT) has provided an auspicious opportunity to
build powerful and ubiquitous connections that faces a plethora of new
challenges, where cellular networks are potential solutions due to their high
scalability, reliability, and efficiency. The Random Access CHannel (RACH)
procedure is the first step of connection establishment between IoT devices and
Base Stations (BSs) in the cellular-based mIoT network, where modelling the
interactions between static properties of physical layer network and dynamic
properties of queue evolving in each IoT device are challenging. To tackle
this, we provide a novel traffic-aware spatio-temporal model to analyze RACH in
cellular-based mIoT networks, where the physical layer network is modelled and
analyzed based on stochastic geometry in the spatial domain, and the queue
evolution is analyzed based on probability theory in the time domain. For
performance evaluation, we derive the exact expressions for the preamble
transmission success probabilities of a randomly chosen IoT device with
different RACH schemes in each time slot, which offer insights into
effectiveness of each RACH scheme. Our derived analytical results are verified
by the realistic simulations capturing the evolution of packets in each IoT
device. This mathematical model and analytical framework can be applied to
evaluate the performance of other types of RACH schemes in the cellular-based
networks by simply integrating its preamble transmission principle
Effect of selective post-aging treatment on subsurface damage of quasicrystal reinforced Al composite manufactured by selective laser melting
In this work, Al-Fe-Cr quasicrystal reinforced Al matrix composite was in-situ prepared by using selective laser melting from powder mixture of Al-Cu-Fe-Cr quasicrystal and pure Al. The effect of selective post-aging treatment on microstructure and mechanical properties were determined with focus on the metastable phases. The microstructural analysis, which was determined by X-ray diffraction and scanning electron microscopy, indicates that the Al-based intermetallic is precipitated from supersaturated α-Al after the aging process. Moreover, the compression tests were performed on the samples in form of dense and lattice structures (50% porosity). The elastic modules of dense and lattice structural samples reduce from 21.3 GPa and 4.4–14.6 GPa and 3.6 GPa by using a low cooling-rated aging process. After aging process, the compressive deformation behavior of dense part changes from elastic-plastic-fracture mode to elastic-plastic-densification mode. On the other hand, the failure mechanism of lattice structural sample changes from rapid-single-stage to slow-double-stage with an improvement of the strain at failure
Penetration Depth and Anisotropy in MgB2
The penetration depth lambda of MgB2 was deduced from both the ac
susceptibility chi and the magnetization M(H) of sorted powders. The good
agreement between the two sets of data without geometric correction for the
grain orientation suggests that MgB2 is an isotropic superconductor.Comment: 9 pages, 5 figures; submitted to Physical Review B (February 28,
2001; revised June 28, 2001); reference list update
Separation of spin and charge in paired spin-singlet quantum Hall states
We propose a series of paired spin-singlet quantum Hall states, which exhibit
a separation of spin and charge degrees of freedom. The fundamental excitations
over these states, which have filling fraction \nu=2/(2m+1) with m an odd
integer, are spinons (spin-1/2 and charge zero) or fractional holons (charge
+/- 1/(2m+1) and spin zero). The braid statistics of these excitations are
non-abelian. The mechanism for the separation of spin and charge in these
states is topological: spin and charge excitations are liberated by binding to
a vortex in a p-wave pairing condensate. We briefly discuss related, abelian
spin-singlet states and possible transitions.Comment: 4 pages, uses revtex
Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB samples
The real and imaginary parts of the surface impedance
in polycrystalline MgB samples of different density
with the critical temperature K are measured at the frequency
of 9.4 GHz and in the temperature range K. The normal skin-effect
condition at holds only for the samples of the
highest density with roughness sizes not more than 0.1 m. For such samples
extrapolation of the linear at temperature dependences
and results in values of the London
penetration depth \AA and residual surface resistance
m. In the entire temperature range the dependences
and are well described by the modified two-fluid model.Comment: 7 pages, 3 figures. Europhysics Letters, accepted for publicatio
Superconducting properties and c-axis superstructure of Mg1-xAlxB2
The superconducting and structural properties of a series of Mg1-xAlxB2
samples have been investigated. X-ray diffraction results confirmed the
existence of a structural transition associated with the significant change in
inter-boron layer distance as reported previously by Slusky et al.
Moreover,transmission-electron-microscopy observations revealed the existence
of a superstructure with doubled lattice constant along the c-axis direction.
We propose that this superstructure is essentially related to the structural
transition. The modifications of superconducting transition temperature Tc, the
normal state resistivity, and the upper critical field Bc2 by Al doping are
discussed in terms of Al-substitution induced changes in the electronic
structure at the Fermi energy.Comment: 15 pages, 7 figure
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