6,784 research outputs found
Direct equivalence between quantum phase transition phenomena in radiation-matter and magnetic systems: scaling of entanglement
We show that the quantum phase transition arising in a standard
radiation-matter model (Dicke model) belongs to the same universality class as
the infinitely-coordinated, transverse field XY model. The effective
qubit-qubit exchange interaction is shown to be proportional to the square of
the qubit-radiation coupling. A universal finite-size scaling is derived for
the corresponding two-qubit entanglement (concurrence) and a size-consistent
effective Hamiltonian is proposed for the qubit subsystem.Comment: 4 pages, 3 figures. Minor changes. Published versio
Soil water balance models for determining crop water and irrigation requirements and irrigation scheduling focusing on the FAO56 method and the dual Kc approach
This study reviews soil water balance (SWB) model approaches to determine crop irrigation requirements and
scheduling irrigation adopting the FAO56 method. The Kc-ETo approach is discussed with consideration of
baseline concepts namely standard vs. actual Kc concepts, as well as single and dual Kc approaches.
Requirements for accurate SWB and appropriate parameterization and calibration are introduced. The one-step
vs. the two-step computational approaches is discussed before the review of the FAO56 method to compute and
partition crop evapotranspiration and related soil water balance. A brief review on transient state models is also
included. Baseline information is concluded with a discussion on yields prediction and performance indicators
related to water productivity. The study is continued with an overview on models development and use after
publication of FAO24, essentially single Kc models, followed by a review on models following FAO56, particularly
adopting the dual Kc approach. Features of dual Kc modeling approaches are analyzed through a few
applications of the SWB model SIMDualKc, mainly for derivation of basal and single Kc, extending the basal Kc
approach to relay intercrop cultivation, assessing alternative planting dates, determining beneficial and nonbeneficial
uses of water by an irrigated crop, and assessing the groundwater contribution to crop ET in the
presence of a shallow water table. The review finally discusses the challenges placed to SWB modeling for real
time irrigation scheduling, particularly the new modeling approaches for large scale multi-users application, use
of cloud computing and adopting the internet of things (IoT), as well as an improved wireless association of
modeling with soil and plant sensors. Further challenges refer to the use of remote sensing energy balance and
vegetation indices to map Kc, ET and crop water and irrigation requirements. Trends are expected to change
research issues relative to SWB modeling, with traditional models mainly used for research while new, fastresponding
and multi-users models based on cloud and IoT technologies will develop into applications to the
farm practice. Likely, the Kc-ETo will continue to be used, with ETo from gridded networks, re-analysis and other
sources, and Kc data available in real time from large databases and remote sensinginfo:eu-repo/semantics/publishedVersio
Swift/XRT monitoring of five orbital cycles of LSI +61 303
LSI +61 303 is one of the most interesting high-mass X-ray binaries owing to
its spatially resolved radio emission and its TeV emission, generally
attributed to non-thermal particles in an accretion-powered relativistic jet or
in the termination shock of the relativistic wind of a young pulsar. Also, the
nature of the compact object is still debated. Only LS 5039 and PSR B1259-63
(which hosts a non-accreting millisecond pulsar) have similar characteristics.
We study the X-ray emission from LSI +61 303 covering both short-term and
orbital variability. We also investigate the source spectral properties in the
soft X-ray (0.3-10 keV) energy range. 25 snapshot observations of LSI +61 303
have been collected in 2006 with the XRT instrument on-board the Swift
satellite over a period of four months, corresponding to about five orbital
cycles. Since individual data sets have too few counts for a meaningful
spectral analysis, we extracted a cumulative spectrum. The count rate folded at
the orbital phase shows a clear modulation pattern at the 26.5 days period and
suggests that the X-ray peak occurs around phase 0.65. Moreover, the X-ray
emission appears to be variable on a timescale of ~1 ks. The cumulative
spectrum is well described by an absorbed power-law model, with hydrogen column
density Nh=(5.7+/-0.3)E+21 cm^-2 and photon index 1.78+/-0.05. No accretion
disk signatures, such as an iron line, are found in the spectrum.Comment: Revised to match the A&A versio
Free expansion of lowest Landau level states of trapped atoms: a wavefunction microscope
We show that for any lowest-Landau-level state of a trapped, rotating,
interacting Bose gas, the particle distribution in coordinate space in a free
expansion (time of flight) experiment is related to that in the trap at the
time it is turned off by a simple rescaling and rotation. When the
lowest-Landau-level approximation is valid, interactions can be neglected
during the expansion, even when they play an essential role in the ground state
when the trap is present. The correlations in the density in a single snapshot
can be used to obtain information about the fluid, such as whether a transition
to a quantum Hall state has occurred.Comment: 5 pages, no figures. v2: discussion of neglect of interactions during
expansion improved, refs adde
Strength–duration relationship for intra- versus 3 extracellular stimulation with microelectrodes
Abstract—Chronaxie, a historically introduced excitability
time parameter for electrical stimulation, has been assumed
to be closely related to the time constant of the cell membrane.
Therefore, it is perplexing that significantly larger
chronaxies have been found for intracellular than for extracellular
stimulation. Using compartmental model analysis,
this controversy is explained on the basis that extracellular
stimulation also generates hyperpolarized regions of the
cell membrane hindering a steady excitation as seen in the
intracellular case. The largest inside/outside chronaxie ratio
for microelectrode stimulation is found in close vicinity of
the cell. In the case of monophasic cathodic stimulation,
the length of the primarily excited zone which is situated
between the hyperpolarized regions increases with
electrode–cell distance. For distant electrodes this results
in an excitation process comparable to the temporal behavior
of intracellular stimulation. Chronaxie also varies along
the neural axis, being small for electrode positions at the
nodes of Ranvier and axon initial segment and larger at
the soma and dendrites. As spike initiation site can change
for short and long pulses, in some cases strength–duration
curves have a bimodal shape, and thus, they deviate from a
classical monotonic curve as described by the formulas of
Lapicque or Weis
Errors and Artefacts in Agent-Based Modelling
The objectives of this paper are to define and classify different types of errors and artefacts that can appear in the process of developing an agent-based model, and to propose activities aimed at avoiding them during the model construction and testing phases. To do this in a structured way, we review the main concepts of the process of developing such a model – establishing a general framework that summarises the process of designing, implementing, and using agent-based models. Within this framework we identify the various stages where different types of errors and artefacts may appear. Finally we propose activities that could be used to detect (and hence eliminate) each type of error or artefact.Verification, Replication, Artefact, Error, Agent-Based Modelling, Modelling Roles
Lattice bosons in quartic confinement
We present a theoretical study of bose condensation of non-interacting bosons
in finite lattices in quartic potentials in one, two, and three dimensions. We
investigate dimensionality effects and quartic potential effects on single
boson density of energy states, condensation temperature, condensate fraction,
and specific heat. The results obtained are compared with corresponding results
for lattice bosons in harmonic traps.Comment: revised version, 11 pages including figures, accepted in EPJ
Alignment Timescale of the Microquasar GRO J1655-40
The microquasar GRO J1655-40 has a black hole with spin angular momentum
apparently misaligned to the orbital plane of its companion star. We
analytically model the system with a steady state disc warped by Lense-Thirring
precession and find the timescale for the alignment of the black hole with the
binary orbit. We make detailed stellar evolution models so as to estimate the
accretion rate and the lifetime of the system in this state. The secondary can
be evolving at the end of the main sequence or across the Hertzsprung gap. The
mass-transfer rate is typically fifty times higher in the latter case but we
find that, in both cases, the lifetime of the mass transfer state is at most a
few times the alignment timescale. The fact that the black hole has not yet
aligned with the orbital plane is therefore consistent with either model. We
conclude that the system may or may not have been counter-aligned after its
supernova kick but that it is most likely to be close to alignment rather than
counteralignment now.Comment: Accepted for publication in MNRA
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