26,122 research outputs found
Defending against Sybil Devices in Crowdsourced Mapping Services
Real-time crowdsourced maps such as Waze provide timely updates on traffic,
congestion, accidents and points of interest. In this paper, we demonstrate how
lack of strong location authentication allows creation of software-based {\em
Sybil devices} that expose crowdsourced map systems to a variety of security
and privacy attacks. Our experiments show that a single Sybil device with
limited resources can cause havoc on Waze, reporting false congestion and
accidents and automatically rerouting user traffic. More importantly, we
describe techniques to generate Sybil devices at scale, creating armies of
virtual vehicles capable of remotely tracking precise movements for large user
populations while avoiding detection. We propose a new approach to defend
against Sybil devices based on {\em co-location edges}, authenticated records
that attest to the one-time physical co-location of a pair of devices. Over
time, co-location edges combine to form large {\em proximity graphs} that
attest to physical interactions between devices, allowing scalable detection of
virtual vehicles. We demonstrate the efficacy of this approach using
large-scale simulations, and discuss how they can be used to dramatically
reduce the impact of attacks against crowdsourced mapping services.Comment: Measure and integratio
Identifying Urban Functional Areas and Their Dynamic Changes in Beijing: Using Multiyear Transit Smart Card Data
A growing number of megacities have been experiencing changes to their landscape due to rapid urbanisation trajectories and travel behaviour dynamics. Therefore, it is of great significance to investigate the distribution and evolution of a city’s urban functional areas over different periods of time. Although the smart card automated fare collection system (SCAFC) is already widely used, few studies have used smart card data to infer information about changes in urban functional areas, particularly in developing countries. Thus, this research aims to delineate the dynamic changes that have occurred in urban functional areas based on passengers’ travel patterns, using Beijing as a case study. We established a Bayesian framework and applied a Gaussian mixture model (GMM) derived from transit smart card data in order to gain insight into passengers’ travel patterns at station level and then identify the dynamic changes in their corresponding urban functional areas. Our results show that Beijing can be clustered into five different functional areas based on the analysis of corresponding transit station functions, namely: multimodal interchange hub and leisure area; residential area; employment area; mixed but mainly residential area; and a mixed residential and employment area. In addition, we found that urban functional areas have experienced slight changes between 2014 and 2017. The findings can be used to inform urban planning strategies designed to tackle urban spatial structure issues, as well as guiding future policy evaluation of urban landscape pattern use
Superconductivity induced by Ni doping in BaFeAs
A series of 122 phase BaFeNiAs ( = 0, 0.055, 0.096, 0.18,
0.23) single crystals were grown by self flux method and a dome-like Ni doping
dependence of superconducting transition temperature is discovered. The
transition temperature reaches a maximum of 20.5 K at = 0.096,
and it drops to below 4 K as 0.23. The negative thermopower in the
normal state indicates that electron-like charge carrier indeed dominates in
this system. This Ni-doped system provides another example of superconductivity
induced by electron doping in the 122 phase.Comment: 7 pages, 5 figures, revised version, added EDX result, accepted for
special issue of NJ
Quasiparticle Scattering Interference in (K,Tl)FexSe2 Superconductors
We model the quasiparticle interference (QPI) pattern in the recently
discovered (K,Tl)Fe_xSe2 superconductors. We show in the superconducting state
that, due to the absence of hole pockets at the Brillouin zone center, the
quasiparticle scattering occurs around the momentum transfer q=(0,0) and (\pm
\pi, \pm \pi) between electron pockets located at the zone boundary. More
importantly, although both d_{x^2-y^2}-wave and s-wave pairing symmetry lead to
nodeless quasiparticle excitations, distinct QPI features are predicted between
both types of pairing symmetry. In the presence of a nonmagnetic impurity
scattering, the QPI exhibits strongest scattering with q=(\pm \pi, \pm \pi) for
the d_{x^2-y^2}-wave pairing symmetry; while the strongest scattering exhibits
a ring-like structure centered around both q=(0,0) and (\pm \pi, \pm \pi) for
the isotropic s-wave pairing symmetry. A unique QPI pattern has also been
predicted due to a local pair-potential-type impurity scattering. The
significant contrast in the QPI pattern between the d_{x^2-y^2}-wave and the
isotropic s-wave pairing symmetry can be used to probe the pairing symmetry
within the Fourier-transform STM technique.Comment: 4+ pages, 3 embedded eps figure
Multi-wavelength emissions from the millisecond pulsar binary PSR J1023+0038 during an accretion active state
Recent observations strongly suggest that the millisecond pulsar binary PSR
J1023+0038 has developed an accretion disk since 2013 June. We present a
multi-wavelength analysis of PSR J1023+0038, which reveals that 1) its
gamma-rays suddenly brightened within a few days in June/July 2013 and has
remained at a high gamma-ray state for several months; 2) both UV and X-ray
fluxes have increased by roughly an order of magnitude, and 3) the spectral
energy distribution has changed significantly after the gamma-ray sudden flux
change. Time variabilities associated with UV and X-rays are on the order of
100-500 seconds and 50-100 seconds, respectively. Our model suggests that a
newly formed accretion disk due to the sudden increase of the stellar wind
could explain the changes of all these observed features. The increase of UV is
emitted from the disk, and a new component in gamma-rays is produced by inverse
Compton scattering between the new UV component and pulsar wind. The increase
of X-rays results from the enhancement of injection pulsar wind energy into the
intra-binary shock due to the increase of the stellar wind. We also predict
that the radio pulses may be blocked by the evaporated winds from the disk and
the pulsar is still powered by rotation.Comment: 8 pages, 3 figures; accepted for publication in Ap
Superconductivity at the Border of Electron Localization and Itinerancy
The superconducting state of iron pnictides and chalcogenides exists at the
border of antiferromagnetic order. Consequently, these materials could provide
clues about the relationship between magnetism and unconventional
superconductivity. One explanation, motivated by the so-called bad-metal
behaviour of these materials, proposes that magnetism and superconductivity
develop out of quasi-localized magnetic moments which are generated by strong
electron-electron correlations. Another suggests that these phenomena are the
result of weakly interacting electron states that lie on nested Fermi surfaces.
Here we address the issue by comparing the newly discovered alkaline iron
selenide superconductors, which exhibit no Fermi-surface nesting, to their iron
pnictide counterparts. We show that the strong-coupling approach leads to
similar pairing amplitudes in these materials, despite their different Fermi
surfaces. We also find that the pairing amplitudes are largest at the boundary
between electronic localization and itinerancy, suggesting that new
superconductors might be found in materials with similar characteristics.Comment: Version of the published manuscript prior to final journal-editting.
Main text (23 pages, 4 figures) + Supplementary Information (14 pages, 7
figures, 3 tables). Calculation on the single-layer FeSe is added.
Enhancement of the pairing amplitude in the vicinity of the Mott transition
is highlighted. Published version is at
http://www.nature.com/ncomms/2013/131115/ncomms3783/full/ncomms3783.htm
LHC diphoton Higgs signal and top quark forward-backward asymmetry in quasi-inert Higgs doublet model
In the quasi-inert Higgs doublet model, we study the LHC diphoton rate for a
standard model-like Higgs boson and the top quark forward-backward asymmetry at
Tevatron. Taking into account the constraints from the vacuum stability,
unitarity, electroweak precision tests, flavor physics and the related
experimental data of top quark, we find that compared with the standard model
prediction, the diphoton rate of Higgs boson at LHC can be enhanced due to the
light charged Higgs contributions, while the measurement of the top quark
forward-backward asymmetry at Tevatron can be explained to within due
to the non-standard model neutral Higgs bosons contributions. Finally, the
correlations between the two observables are discussed.Comment: 14 pages, 5 figues. Version to appear in JHEP, some references adde
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