3,329 research outputs found
Guiding of Rydberg atoms in a high-gradient magnetic guide
We study the guiding of Rb 59D Rydberg atoms in a linear,
high-gradient, two-wire magnetic guide. Time delayed microwave ionization and
ion detection are used to probe the Rydberg atom motion. We observe guiding of
Rydberg atoms over a period of 5 ms following excitation. The decay time of the
guided atom signal is about five times that of the initial state. We attribute
the lifetime increase to an initial phase of -changing collisions and
thermally induced Rydberg-Rydberg transitions. Detailed simulations of Rydberg
atom guiding reproduce most experimental observations and offer insight into
the internal-state evolution
Time and energy constraints and the relationships between currencies in foraging theory
Measured foraging strategies often cluster around values that maximize the ratio of energy gained over energy spent while foraging (efficiency), rather than values that would maximize the long-term net rate of energy gain (rate). The reasons for this are not understood. This paper focuses on time and energy constraints while foraging to illustrate the relationship between efficiency and rate-maximizing strategies and develops models that provide a simple framework to analyze foraging strategies in two distinct foraging contexts. We assume that while capturing and ingesting food for their own use (which we term feeding), foragers behave so as to maximize the total net daily energetic gain. When gathering food for others or for storage (which we term provisioning), we assume that foragers behave so as to maximize the total daily delivery, subject to meeting their own energetic requirements. In feeding contexts, the behavior maximizing total net daily gain also maximizes efficiency when daily intake is limited by the assimilation capacity. In contrast, when time available to forage sets the limit to gross intake, the behavior maximizing total net daily gain also maximizes rate. In provisioning contexts, when daily delivery is constrained by the energy needed to power self-feeding, maximizing efficiency ensures the highest total daily delivery. When time needed to recoup energetic expenditure limits total delivery, a low self-feeding rate relative to the rate of energy expenditure favors efficient strategies. However, as the rate of self-feeding increases, foraging behavior deviates from efficiency maximization in the direction predicted by rate maximization. Experimental manipulations of the rate of self-feeding in provisioning contexts could be a powerful tool to explore the relationship between rate and efficiency-maximizing behavio
Quantum simulation of the Klein paradox with trapped ions
We report on quantum simulations of relativistic scattering dynamics using
trapped ions. The simulated state of a scattering particle is encoded in both
the electronic and vibrational state of an ion, representing the discrete and
continuous components of relativistic wave functions. Multiple laser fields and
an auxiliary ion simulate the dynamics generated by the Dirac equation in the
presence of a scattering potential. Measurement and reconstruction of the
particle wave packet enables a frame-by-frame visualization of the scattering
processes. By precisely engineering a range of external potentials we are able
to simulate text book relativistic scattering experiments and study Klein
tunneling in an analogue quantum simulator. We describe extensions to solve
problems that are beyond current classical computing capabilities.Comment: 3 figures, accepted for publication in PR
Intrinsic anomalous Hall effect in nickel: An GGA+U study
The electronic structure and intrinsic anomalous Hall conductivity of nickel
have been calculated based on the generalized gradient approximation (GGA) plus
on-site Coulomb interaction (GGA+U) scheme. It is found that the intrinsic
anomalous Hall conductivity () obtained from the GGA+U
calculations with eV and eV, is in nearly perfect agreement
with that measured recently at low temperatures while, in contrast, the
from the GGA calculations is about 100% larger than the
measured one. This indicates that, as for the other spin-orbit interaction
(SOI)-induced phenomena in 3 itinerant magnets such as the orbital magnetic
magnetization and magnetocrystalline anisotropy, the on-site electron-electron
correlation, though moderate only, should be taken into account properly in
order to get the correct anomalous Hall conductivity. The intrinsic
and the number of valence electrons () have also been
calculated as a function of the Fermi energy (). A sign change is
predicted at eV (), and this explain qualitatively
why the theoretical and experimental values for Fe and Co are
positive. It is also predicted that fcc NiCo(Fe,Cu) alloys with
being small, would also have the negative with the
magnitude being in the range of cm. The most
pronounced effect of including the on-site Coulomb interaction is that all the
-dominant bands are lowered in energy relative to the by about 0.3 eV,
and consequently, the small minority spin X hole pocket disappears. The
presence of the small X hole pocket in the GGA calculations is attributed
to be responsible for the large discrepancy in the between
theory and experiment.Comment: 7 pages, 3 figures; Accepted for publication in Physical Review
The Strange Prospects for Astrophysics
The implications of the formation of strange quark matter in neutron stars
and in core-collapse supernovae is discussed with special emphasis on the
possibility of having a strong first order QCD phase transition at high baryon
densities. If strange quark matter is formed in core-collapse supernovae
shortly after the bounce, it causes the launch of a second outgoing shock which
is energetic enough to lead to a explosion. A signal for the formation of
strange quark matter can be read off from the neutrino spectrum, as a second
peak in antineutrinos is released when the second shock runs over the
neutrinosphere.Comment: 10 pages, 8 figures, invited talk given at the international
conference on strangeness in quark matter (SQM2008), Beijing, October 6-10,
Beijing, China, version to appear in J. Phys.
Strange quark matter in explosive astrophysical systems
Explosive astrophysical systems, such as supernovae or compact star binary
mergers, provide conditions where strange quark matter can appear. The high
degree of isospin asymmetry and temperatures of several MeV in such systems may
cause a transition to the quark phase already around saturation density.
Observable signals from the appearance of quark matter can be predicted and
studied in astrophysical simulations. As input in such simulations, an equation
of state with an integrated quark matter phase transition for a large
temperature, density and proton fraction range is required. Additionally,
restrictions from heavy ion data and pulsar observation must be considered. In
this work we present such an approach. We implement a quark matter phase
transition in a hadronic equation of state widely used for astrophysical
simulations and discuss its compatibility with heavy ion collisions and pulsar
data. Furthermore, we review the recently studied implications of the QCD phase
transition during the early post-bounce evolution of core-collapse supernovae
and introduce the effects from strong interactions to increase the maximum mass
of hybrid stars. In the MIT bag model, together with the strange quark mass and
the bag constant, the strong coupling constant provides a parameter
to set the beginning and extension of the quark phase and with this the mass
and radius of hybrid stars.Comment: 6 pages, 5 figures, talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
2, 2009, to be published in Journal Phys.
Milky Way Demographics with the VVV Survey II. Color Transformations and Near-Infrared Photometry for 136 Million Stars in the Southern Galactic Disk
The new multi-epoch near-infrared VVV survey (VISTA Variables in the Via
Lactea) is sampling 562 sq. deg of the Galactic bulge and adjacent regions of
the disk. Accurate astrometry established for the region surveyed allows the
VVV data to be merged with overlapping surveys (e.g., GLIMPSE, WISE, 2MASS,
etc.), thereby enabling the construction of longer baseline spectral energy
distributions for astronomical targets. However, in order to maximize use of
the VVV data, a set of transformation equations are required to place the VVV
JHKs photometry onto the 2MASS system. The impetus for this work is to develop
those transformations via a comparison of 2MASS targets in 152 VVV fields
sampling the Galactic disk. The transformation coefficients derived exhibit a
reliance on variables such as extinction. The transformed data were
subsequently employed to establish a mean reddening law of
E_{J-H}/E_{H-Ks}=2.13 +/- 0.04, which is the most precise determination to date
and merely emphasizes the pertinence of the VVV data for determining such
important parameters.Comment: 24 pages, 12 figures, published in A&
Strangeness in Astrophysics and Cosmology
Some recent developments concerning the role of strange quark matter for
astrophysical systems and the QCD phase transition in the early universe are
addressed. Causality constraints of the soft nuclear equation of state as
extracted from subthreshold kaon production in heavy-ion collisions are used to
derive an upper mass limit for compact stars. The interplay between the
viscosity of strange quark matter and the gravitational wave emission from
rotation-powered pulsars are outlined. The flux of strange quark matter nuggets
in cosmic rays is put in perspective with a detailed numerical investigation of
the merger of two strange stars. Finally, we discuss a novel scenario for the
QCD phase transition in the early universe, which allows for a small
inflationary period due to a pronounced first order phase transition at large
baryochemical potential.Comment: 8 pages, invited talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
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COSMOGRAIL XVIII: time delays of the quadruply lensed quasar WFI2033-4723
We present new measurements of the time delays of WFI2033-4723. The data sets
used in this work include 14 years of data taken at the 1.2m Leonhard Euler
Swiss telescope, 13 years of data from the SMARTS 1.3m telescope at Las
Campanas Observatory and a single year of high-cadence and high-precision
monitoring at the MPIA 2.2m telescope. The time delays measured from these
different data sets, all taken in the R-band, are in good agreement with each
other and with previous measurements from the literature. Combining all the
time-delay estimates from our data sets results in Dt_AB = 36.2-0.8+0.7 days
(2.1% precision), Dt_AC = -23.3-1.4+1.2 days (5.6%) and Dt_BC = -59.4-1.3+1.3
days (2.2%). In addition, the close image pair A1-A2 of the lensed quasars can
be resolved in the MPIA 2.2m data. We measure a time delay consistent with zero
in this pair of images. We also explore the prior distributions of microlensing
time-delay potentially affecting the cosmological time-delay measurements of
WFI2033-4723. There is however no strong indication in our measurements that
microlensing time delay is neither present nor absent. This work is part of a
H0LiCOW series focusing on measuring the Hubble constant from WFI2033-4723.Comment: Submitted to Astronomy and Astrophysic
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