178 research outputs found
Online Station Assignment for Electric Vehicle Battery Swapping
This paper investigates the online station assignment for (commercial) electric vehicles (EVs) that request battery swapping from a central operator, i.e., in the absence of future information a battery swapping service station has to be assigned instantly to each EV upon its request. Based on EVs' locations, the availability of fully-charged batteries at service stations in the system, as well as traffic conditions, the assignment aims to minimize cost to EVs and congestion at service stations. Inspired by a polynomial-time offline solution via a bipartite matching approach, we develop an efficient and implementable online station assignment algorithm that provably achieves the tight (optimal) competitive ratio under mild conditions. Monte Carlo experiments on a real transportation network by Baidu Maps show that our algorithm performs reasonably well on realistic inputs, even with a certain amount of estimation error in parameters
Online Station Assignment for Electric Vehicle Battery Swapping
This paper investigates the online station assignment for (commercial) electric vehicles (EVs) that request battery swapping from a central operator, i.e., in the absence of future information a battery swapping service station has to be assigned instantly to each EV upon its request. Based on EVs' locations, the availability of fully-charged batteries at service stations in the system, as well as traffic conditions, the assignment aims to minimize cost to EVs and congestion at service stations. Inspired by a polynomial-time offline solution via a bipartite matching approach, we develop an efficient and implementable online station assignment algorithm that provably achieves the tight (optimal) competitive ratio under mild conditions. Monte Carlo experiments on a real transportation network by Baidu Maps show that our algorithm performs reasonably well on realistic inputs, even with a certain amount of estimation error in parameters
Universal quasiparticle decoherence in hole- and electron-doped high-Tc cuprates
We use angle-resolved photoemission to unravel the quasiparticle decoherence
process in the high- cuprates. The coherent band is highly renormalized,
and the incoherent part manifests itself as a nearly vertical ``dive'' in the
- intensity plot that approaches the bare band bottom. We find that the
coherence-incoherence crossover energies in the hole- and electron-doped
cuprates are quite different, but scale to their corresponding bare bandwidth.
This rules out antiferromagnetic fluctuations as the main source for
decoherence. We also observe the coherent band bottom at the zone center, whose
intensity is strongly suppressed by the decoherence process. Consequently, the
coherent band dispersion for both hole- and electron-doped cuprates is
obtained, and is qualitatively consistent with the framework of Gutzwiller
projection.Comment: 4 pages, 4 figure
Synchrotron x-ray-diffraction study of the structure and growth of Xe films adsorbed on the Ag(111) surface
URL:http://link.aps.org/doi/10.1103/PhysRevB.59.15464
DOI:10.1103/PhysRevB.59.15464Synchrotron x-ray scattering has been used to investigate the structure and growth of perhaps the simplest of all films: xenon physisorbed on the Ag(111) surface. High-resolution x-ray scans of the in-plane structure and lower-resolution scans (specular and nonspecular) of the out-of-plane order were performed. The Xe films were prepared under both quasiequilibrium and kinetic growth conditions, and have fewer structural defects than those investigated previously by others on graphite substrates. Under quasiequilibrium conditions, the bulk Xe-Xe spacing is reached at monolayer completion, and the monolayer and bilayer lattice constants at coexistence are inferred equal to within 0.005 Å, consistent with theoretical calculations. The Xe/vacuum interface profile for a complete monolayer and bilayer grown at quasiequilibrium is found to be sharper than for kinetically grown films. At coverages above two layers, diffraction scans along the Xe(01l) rod for quasiequilibrated films are consistent with the presence of two domains having predominantly an ABC stacking sequence and rotated 60° with respect to each other about the surface normal. Annealing of these films alters neither the population of the two domains nor the fraction of ABA stacking faults. The thickest film grown under quasiequilibrium conditions exceeds 220 Å (resolution limited). Under kinetic growth conditions, x-ray intensity oscillations at the Xe anti-Bragg position of the specular rod are observed as a function of time, indicating nearly layer-by-layer growth. Up to four complete oscillations corresponding to a film of eight layers have been observed before the intensity is damped out; the number of oscillations is found to depend on the substrate temperature, the growth rate, and the quality of the Ag(111) substrate. The specular reflectivity from kinetically grown films at nominal coverages of three and four layers has been analyzed using a Gaussian model which gives a film thickness standard deviation of 0.5 and 1.0 layers, respectively. Diffraction scans along the Xe(01l) rod of these films indicate a larger fraction of ABA stacking faults than found for thicker films. These results demonstrate the difficulty of kinetically growing Xe films thicker than two layers which have an ideal slab geometry.This work was partially supported by U.S. National Science Foundation Grant Nos. DMR-8704938, DMR-9011069, and DMR-9314235 and the U.S. Department of Energy Grant
No. DE-FG02-85ER45183 of the MATRIX Participating Research Team
Anisotropic but nodeless superconducting gap in the presence of spin density wave in iron-pnictide superconductor NaFe1-xCoxAs
The coexisting regime of spin density wave (SDW) and superconductivity in the
iron pnictides represents a novel ground state. We have performed high
resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x =
0.0175) in this regime and revealed its distinctive electronic structure, which
provides some microscopic understandings of its behavior. The SDW signature and
the superconducting gap are observed on the same bands, illustrating the
intrinsic nature of the coexistence. However, because the SDW and
superconductivity are manifested in different parts of the band structure,
their competition is non-exclusive. Particularly, we found that the gap
distribution is anisotropic and nodeless, in contrast to the isotropic
superconducting gap observed in an SDW-free NaFe1-xCoxAs (x=0.045), which puts
strong constraints on theory.Comment: 5 pages, 4 figures + supplementary informatio
Signature of Magnetic Phase Separation in the Ground State of Pr1-xCaxMnO3
Neutron scattering has been used to investigate the evolution of the long-
and short-range charge-ordered (CO), ferromagnetic (FM), and antiferromagnetic
(AF) correlations in single crystals of Pr1-xCaxMnO3. The existence and
population of spin clusters as refected by short-range correlations are found
to drastically depend on the doping (x) and temperature (T). Concentrated spin
clusters coexist with long-range canted AF order in a wide temperature range in
x = 0.3 while clusters do not appear in x = 0.4 crystal. In contrast, both CO
and AF order parameters in the x = 0.35 crystal show a precipitous decrease
below ~ 35 K where spin clusters form. These results provide direct evidence of
magnetic phase separation and indicate that there is a critical doping x_c
(close to x = 0.35) that divides the phase-separated site-centered from the
homogeneous bond-centered or charge-disproportionated CO ground state.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Letter
Observation of momentum-confined in-gap impurity state in BaKFeAs: evidence for anti-phase pairing
We report the observation by angle-resolved photoemission spectroscopy of an
impurity state located inside the superconducting gap of
BaKFeAs and vanishing above the superconducting
critical temperature, for which the spectral weight is confined in momentum
space near the Fermi wave vector positions. We demonstrate, supported by
theoretical simulations, that this in-gap state originates from weak
non-magnetic scattering between bands with opposite sign of the superconducting
gap phase. This weak scattering, likely due to off-plane Ba/K disorders, occurs
mostly among neighboring Fermi surfaces, suggesting that the superconducting
gap phase changes sign within holelike (and electronlike) bands. Our results
impose severe restrictions on the models promoted to explain high-temperature
superconductivity in these materials.Comment: 8 pages, 5 figures. Accepted for publication in Physical Review
Inelastic neutron scattering studies of Crystal Field Levels in PrOsAs
We use neutron scattering to study the Pr crystalline electric field
(CEF) excitations in the filled skutterudite PrOsAs. By comparing
the observed levels and their strengths under neutron excitation with the
theoretical spectrum and neutron excitation intensities, we identify the
Pr CEF levels, and show that the ground state is a magnetic
triplet, and the excited states ,
and are at 0.4, 13 and 23 meV, respectively. A comparison of the
observed CEF levels in PrOsAs with the heavy fermion superconductor
PrOsSb reveals the microscopic origin of the differences in the
ground states of these two filled skutterudites.Comment: 7 pages, 7 figure
Critical Quadrupole Fluctuations and Collective Modes in Iron Pnictide Superconductors
The multiband nature of iron pnictides gives rise to a rich
temperature-doping phase diagram of competing orders and a plethora of
collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural
transition is closely followed by a spin density wave transition both being in
close proximity to the superconducting phase. A key question is the nature of
high- superconductivity and its relation to orbital ordering and
magnetism. Here we study the NaFeCoAs superconductor using
polarization-resolved Raman spectroscopy. The Raman susceptibility displays
critical enhancement of non-symmetric charge fluctuations across the entire
phase diagram which are precursors to a -wave Pomeranchuk instability at
temperature \theta(\mbox{x}). The charge fluctuations are interpreted in
terms of quadrupole inter-orbital excitations in which the electron and hole
Fermi surfaces breathe in-phase. Below , the critical fluctuations acquire
coherence and undergo a metamorphosis into a coherent ingap mode of
extraordinary strength.Comment: 21 pages, 15 figures. To appear in Phys. Rev.
Three dimensionality of band structure and a large residual quasiparticle population in BaKFeAs as revealed by the c-axis polarized optical measurement
We report on a c-axis polarized optical measurement on a
BaKFeAs single crystal. We find that the c-axis
optical response is significantly different from that of high-T cuprates.
The experiments reveal an anisotropic three-dimensional optical response with
the absence of the Josephson plasma edge in R() in the superconducting
state. Furthermore, different from the ab-plane optical response, a large
residual quasiparticle population down to was observed in
the c-axis polarized reflectance measurement. We elaborate that there exist
nodes for the superconducting gap in regions of the 3D Fermi surface that
contribute dominantly to the c-axis optical conductivity.Comment: 4 figure
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