125 research outputs found
Joint Optimization for Pedestrian, Information and Energy Flows in Emergency Response Systems With Energy Harvesting and Energy Sharing
The rapid progress in informatisation and electrification in transportation has gradually transferred public transport junctions such as metro stations into the nexus of pedestrian flows, information flows, computation flows and energy flows. These smart environments that are efficient in handling large volume passenger flows in routine circumstances can become even more vulnerable during emergency situations and amplify the losses in lives and property owing to power outage triggered service degradation and destructive crowed behaviours. On the bright side, the increasingly abundant resources contained in smart environments have enlarged the optimisation space of an evacuation process, yet little research has concentrated on the joint optimal resource allocation between transportation infrastructures and pedestrians. Hence, in the paper, we propose a queueing network based resource allocation model to comprehensively optimise various types of resources during emergency evacuations. Experiments are conducted in a simulated metro station environment with realistic settings. The simulation results show that the proposed model can considerably improve the evacuation efficiency as well as the robustness of the emergency response system during emergency situations
Strong Interplay between Stripe Spin Fluctuations, Nematicity and Superconductivity in FeSe
Elucidating the microscopic origin of nematic order in iron-based
superconducting materials is important because the interactions that drive
nematic order may also mediate the Cooper pairing. Nematic order breaks
fourfold rotational symmetry in the iron plane, which is believed to be driven
by either orbital or spin degrees of freedom. However, as the nematic phase
often develops at a temperature just above or coincides with a stripe magnetic
phase transition, experimentally determining the dominant driving force of
nematic order is difficult. Here, we use neutron scattering to study
structurally the simplest iron-based superconductor FeSe, which displays a
nematic (orthorhombic) phase transition at K, but does not order
antiferromagnetically. Our data reveal substantial stripe spin fluctuations,
which are coupled with orthorhombicity and are enhanced abruptly on cooling to
below . Moreover, a sharp spin resonance develops in the superconducting
state, whose energy (~4 meV) is consistent with an electron boson coupling mode
revealed by scanning tunneling spectroscopy, thereby suggesting a spin
fluctuation-mediated sign-changing pairing symmetry. By normalizing the dynamic
susceptibility into absolute units, we show that the magnetic spectral weight
in FeSe is comparable to that of the iron arsenides. Our findings support
recent theoretical proposals that both nematicity and superconductivity are
driven by spin fluctuations.Comment: 19 pages, 8 figure
A Mott insulator continuously connected to iron pnictide superconductors
Iron-based superconductivity develops near an antiferromagnetic order and out
of a bad metal normal state, which has been interpreted as originating from a
proximate Mott transition. Whether an actual Mott insulator can be realized in
the phase diagram of the iron pnictides remains an open question. Here we use
transport, transmission electron microscopy, X-ray absorption spectroscopy, and
neutron scattering to demonstrate that NaFeCuAs near
exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators
with the insulating behavior persisting above the N\'eel temperature,
indicative of a Mott insulator. Upon decreasing from , the
antiferromagnetic ordered moment continuously decreases, yielding to
superconductivity around . Our discovery of a Mott insulating state in
NaFeCuAs thus makes it the only known Fe-based material in which
superconductivity can be smoothly connected to the Mott insulating state,
highlighting the important role of electron correlations in the high- superconductivity.Comment: in press, Nat. Commun., 4 figures, supplementary information
available upon reques
Spin excitations in metallic kagome lattice FeSn and CoSn
In two-dimensional (2D) metallic kagome lattice materials, destructive
interference of electronic hopping pathways around the kagome bracket can
produce nearly localized electrons, and thus electronic bands that are flat in
momentum space. When ferromagnetic order breaks the degeneracy of the
electronic bands and splits them into the spin-up majority and spin-down
minority electronic bands, quasiparticle excitations between the spin-up and
spin-down flat bands should form a narrow localized spin-excitation Stoner
continuum coexisting with well-defined spin waves in the long wavelengths. Here
we report inelastic neutron scattering studies of spin excitations in 2D
metallic Kagome lattice antiferromagnetic FeSn and paramagnetic CoSn, where
angle resolved photoemission spectroscopy experiments found spin-polarized and
nonpolarized flat bands, respectively, below the Fermi level. Although our
initial measurements on FeSn indeed reveal well-defined spin waves extending
well above 140 meV coexisting with a flat excitation at 170 meV, subsequent
experiments on CoSn indicate that the flat mode actually arises mostly from
hydrocarbon scattering of the CYTOP-M commonly used to glue the samples to
aluminum holder. Therefore, our results established the evolution of spin
excitations in FeSn and CoSn, and identified an anomalous flat mode that has
been overlooked by the neutron scattering community for the past 20 years
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