89,695 research outputs found
Heavy-tailed statistics in short-message communication
Short-message (SM) is one of the most frequently used communication channels
in the modern society. In this Brief Report, based on the SM communication
records provided by some volunteers, we investigate the statistics of SM
communication pattern, including the interevent time distributions between two
consecutive short messages and two conversations, and the distribution of
message number contained by a complete conversation. In the individual level,
the current empirical data raises a strong evidence that the human activity
pattern, exhibiting a heavy-tailed interevent time distribution, is driven by a
non-Poisson nature.Comment: 4 pages, 4 figures and 1 tabl
Effect and Compensation of Timing Jitter in Through-Wall Human Indication via Impulse Through-Wall Radar
Impulse through-wall radar (TWR) is considered as one of preferred choices for through-wall human indication due to its good penetration and high range resolution. Large bandwidth available for impulse TWR results in high range resolution, but also brings an atypical adversity issue not substantial in narrowband radars — high timing jitter effect, caused by the non-ideal sampling clock at the receiver. The fact that impulse TWR employs very narrow pulses makes little jitter inaccuracy large enough to destroy the signal correlation property and then degrade clutter suppression performance. In this paper, we focus on the timing jitter impact on clutter suppression in through-wall human indication via impulse TWR. We setup a simple timing jitter model and propose a criterion namely average range profile (ARP) contrast is to evaluate the jitter level. To combat timing jitter, we also develop an effective compensation method based on local ARP contrast maximization. The proposed method can be implemented pulse by pulse followed by exponential average background subtraction algorithm to mitigate clutters. Through-wall experiments demonstrate that the proposed method can dramatically improve through-wall human indication performance
Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study
Rhenium diboride is a recently recognized ultra-incompressible superhard
material. Here we report the electronic (e), phonon (p), e-p coupling and
thermal properties of ReB from first-principles density-functional theory
(DFT) calculations and neutron scattering measurements. Our calculated elastic
constants ( = 641 GPa, = 159 GPa, = 128 GPa,
= 1037 GPa, and = 271 GPa), bulk modulus ( 350 GPa) and
hardness ( 46 GPa) are in good agreement with the reported
experimental data. The calculated phonon density of states (DOS) agrees very
well with our neutron vibrational spectroscopy result. Electronic and phonon
analysis indicates that the strong covalent B-B and Re-B bonding is the main
reason for the super incompressibility and hardness of ReB. The thermal
expansion coefficients, calculated within the quasi-harmonic approximation and
measured by neutron powder diffraction, are found to be nearly isotropic in
and directions and only slightly larger than that of diamond in terms of
magnitude. The excellent agreement found between calculations and experimental
measurements indicate that first-principles calculations capture the main
interactions in this class of superhard materials, and thus can be used to
search, predict, and design new materials with desired properties.Comment: submitted to pr
Spin-Hall and Anisotropic Magnetoresistance in Ferrimagnetic Co-Gd / Pt layers
We present the Co-Gd composition dependence of the spin-Hall
magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) for
ferrimagnetic Co100-xGdx / Pt bilayers. With Gd concentration x, its magnetic
moment increasingly competes with the Co moment in the net magnetization. We
find a nearly compensated ferrimagnetic state at x = 24. The AMR changes sign
from positive to negative with increasing x, vanishing near the magnetization
compensation. On the other hand, the SMR does not vary significantly even where
the AMR vanishes. These experimental results indicate that very different
scattering mechanisms are responsible for AMR and SMR. We discuss a possible
origin for the alloy composition dependence.Comment: 31 Pages, 9 figure
Transition Metal-Ethylene Complexes as High-Capacity Hydrogen Storage Media
From first-principles calculations, we predict that a single ethylene
molecule can form a stable complex with two transition metals (TM) such as Ti.
The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules,
reaching to gravimetric storage capacity of 14 wt%. Dimerization,
polymerizations and incorporation of the TM-ethylene complexes in nanoporous
carbon materials have been also discussed. Our results are quite remarkable and
open a new approach to high-capacity hydrogen storage materials discovery.Comment: 5 pages, 4 figures, additional content, Phys. Rev. Lett. in pres
Ground-state configuration space heterogeneity of random finite-connectivity spin glasses and random constraint satisfaction problems
We demonstrate through two case studies, one on the p-spin interaction model
and the other on the random K-satisfiability problem, that a heterogeneity
transition occurs to the ground-state configuration space of a random
finite-connectivity spin glass system at certain critical value of the
constraint density. At the transition point, exponentially many configuration
communities emerge from the ground-state configuration space, making the
entropy density s(q) of configuration-pairs a non-concave function of
configuration-pair overlap q. Each configuration community is a collection of
relatively similar configurations and it forms a stable thermodynamic phase in
the presence of a suitable external field. We calculate s(q) by the
replica-symmetric and the first-step replica-symmetry-broken cavity methods,
and show by simulations that the configuration space heterogeneity leads to
dynamical heterogeneity of particle diffusion processes because of the entropic
trapping effect of configuration communities. This work clarifies the fine
structure of the ground-state configuration space of random spin glass models,
it also sheds light on the glassy behavior of hard-sphere colloidal systems at
relatively high particle volume fraction.Comment: 26 pages, 9 figures, submitted to Journal of Statistical Mechanic
Detection of weak-order phase transitions in ferromagnets by ac resistometry
It is shown that ac resistometry can serve as an effective tool for the
detection of phase transitions, such as spin reorientation or premartensitic
phase transitions, which generally are not disclosed by dc resistivity
measurement. Measurement of temperature dependence of impedance, , allows
one to unmask the anomaly, corresponding to a weak-order phase transition. The
appearance of such an anomaly is accounted for by a change in the effective
permeability of a sample upon the phase transition. Moreover, frequency
dependence of makes it possible to use the frequency of the applied ac
current as an adjusting parameter in order to make this anomaly more
pronounced. The applicability of this method is tested for the rare earth Gd
and Heusler alloy NiMnGa.Comment: 4 pages, 2 figures, to be published in J. Appl. Phys., v.94(5
The electronic structures and magnetic properties of perovskite ruthenates from constrained orbital hybridization calculations
We introduce a method to analyze the effect of hybridization by shifting
corresponding atomic levels using external potentials. Based on this approach,
we study perovskite ruthenates,\ and unambiguously identify that the covalency
between the \textit{A}-site cation and O ion will modify the Ru-O hybridization
and change the density of state at Fermi level, consequently affect the
magnetic properties significantly. We also study the effect of pressure and
reveal that hydrostatic pressure has a small effect on the Ru-O-Ru bond angle
of SrRuO, while it will decrease the Ru-O length and increase the band
width significantly. Therefore, the magnetic ordering temperature will decrease
monotonically with pressure
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