998 research outputs found
Many-body theory calculations of positron binding to negative ions
A many-body theory approach developed by the authors [Phys. Rev. A 70, 032720
(2004)] is applied to positron bound states and annihilation rates in atomic
systems. Within the formalism, full account of virtual positronium (Ps)
formation is made by summing the electron-positron ladder diagram series, thus
enabling the theory to include all important many-body correlation effects in
the positron problem. Numerical calculations have been performed for positron
bound states with the hydrogen and halogen negative ions, also known as Ps
hydride and Ps halides. The Ps binding energies of 1.118, 2.718, 2.245, 1.873
and 1.393 eV and annihilation rates of 2.544, 2.482, 1.984, 1.913 and 1.809
ns, have been obtained for PsH, PsF, PsCl, PsBr and PsI, respectively.Comment: 19 pages, 13 figures, submitted to International Review of Atomic and
Molecular Physic
Positron scattering and annihilation on noble gas atoms
Positron scattering and annihilation on noble gas atoms below the positronium
formation threshold is studied ab initio using many-body theory methods. The
many-body theory provides a near-complete understanding of the
positron-noble-gas-atom system at these energies and yields accurate numerical
results. It accounts for positron-atom and electron-positron correlations,
e.g., polarization of the atom by the incident positron and the
non-perturbative process of virtual positronium formation. These correlations
have a large effect on the scattering dynamics and result in a strong
enhancement of the annihilation rates compared to the independent-particle
mean-field description. Computed elastic scattering cross sections are found to
be in good agreement with recent experimental results and Kohn variational and
convergent close-coupling calculations. The calculated values of the
annihilation rate parameter (effective number of electrons
participating in annihilation) rise steeply along the sequence of noble gas
atoms due to the increasing strength of the correlation effects, and agree well
with experimental data.Comment: 24 pages, 17 figure
Relativistic coupled-cluster single-double calculations of positron-atom bound states
Relativistic coupled-cluster single-double approximation is used to calculate
positron-atom bound states. The method is tested on closed-shell atoms such as
Be, Mg, Ca, Zn, Cd, and Hg where a number of accurate calculations is
available. It is then used to calculate positron binding energies for a range
of open-shell transition metal atoms from Sc to Cu, from Y to Pd, and from Lu
to Pt. These systems possess Feshbach resonances, which can be used to search
for positron-atom binding experimentally through resonant annihilation or
scattering.Comment: submitted to Phys. Rev.
The Mass Profile and Accretion History of Cold Dark Matter Halos
We use the Millennium Simulation series to study the relation between the
accretion history (MAH) and mass profile of cold dark matter halos. We find
that the mean density within the scale radius, r_{-2} (where the halo density
profile has isothermal slope), is directly proportional to the critical density
of the Universe at the time when the main progenitor's virial mass equals the
mass enclosed within r_{-2}. Scaled to these characteristic values of mass and
density, the mean MAH, expressed in terms of the critical density of the
Universe, M(\rho_{crit}(z)), resembles that of the enclosed density profile,
M(), at z=0. Both follow closely the NFW profile, suggesting that the
similarity of halo mass profiles originates from the mass-independence of halo
MAHs. Support for this interpretation is provided by outlier halos whose
accretion histories deviate from the NFW shape; their mass profiles show
correlated deviations from NFW and are better approximated by Einasto profiles.
Fitting both M() and M(\rho_{crit}) with either NFW or Einasto profiles
yield concentration and shape parameters that are correlated, confirming and
extending earlier work linking the concentration of a halo with its accretion
history. These correlations also confirm that halo structure is insensitive to
initial conditions: only halos whose accretion histories differ greatly from
the NFW shape show noticeable deviations from NFW in their mass profiles. As a
result, the NFW profile provides acceptable fits to hot dark matter halos,
which do not form hierarchically, and for fluctuation power spectra other than
CDM. Our findings, however, predict a subtle but systematic dependence of mass
profile shape on accretion history which, if confirmed, would provide strong
support for the link between accretion history and halo structure we propose
here.Comment: 12 pages, 8 figures, MNRAS 432 1103L (2013
Hyper-Ramsey Spectroscopy of Optical Clock Transitions
We present non-standard optical Ramsey schemes that use pulses individually
tailored in duration, phase, and frequency to cancel spurious frequency shifts
related to the excitation itself. In particular, the field shifts and their
uncertainties of Ramsey fringes can be radically suppressed (by 2-4 orders of
magnitude) in comparison with the usual Ramsey method (using two equal pulses)
as well as with single-pulse Rabi spectroscopy. Atom interferometers and
optical clocks based on two-photon transitions, heavily forbidden transitions,
or magnetically induced spectroscopy could significantly benefit from this
method. In the latter case these frequency shifts can be suppressed
considerably below a fractional level of 10^{-17}. Moreover, our approach opens
the door for the high-precision optical clocks based on direct frequency comb
spectroscopy.Comment: 5 pages, 4 figure
Strangers in the night: Discovery of a dwarf spheroidal galaxy on its first Local Group infall
We present spectroscopic observations of the AndXII dwarf spheroidal galaxy
using DEIMOS/Keck-II, showing it to be moving rapidly through the Local Group
(-556 km/s heliocentric velocity, -281 km/s relative to Andromeda from the MW),
falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. AndXII
therefore represents a dwarf galaxy plausibly falling into the Local Group for
the first time, and never having experienced a dense galactic environment. From
Green Bank Telescope observations, a limit on the H{I} gas mass of <3000 Msun
suggests that AndXII's gas could have been removed prior to experiencing the
tides of the Local Group galaxies. Orbit models suggest the dwarf is close to
the escape velocity of M31 for published mass models. AndXII is our best direct
evidence for the late infall of satellite galaxies, a prediction of
cosmological simulations.Comment: 4 pages 5 figures 1 table, accepted in ApJ, july issu
Hyperpolarizability and operational magic wavelength in an optical lattice clock
Optical clocks benefit from tight atomic confinement enabling extended
interrogation times as well as Doppler- and recoil-free operation. However,
these benefits come at the cost of frequency shifts that, if not properly
controlled, may degrade clock accuracy. Numerous theoretical studies have
predicted optical lattice clock frequency shifts that scale nonlinearly with
trap depth. To experimentally observe and constrain these shifts in an
Yb optical lattice clock, we construct a lattice enhancement cavity
that exaggerates the light shifts. We observe an atomic temperature that is
proportional to the optical trap depth, fundamentally altering the scaling of
trap-induced light shifts and simplifying their parametrization. We identify an
"operational" magic wavelength where frequency shifts are insensitive to
changes in trap depth. These measurements and scaling analysis constitute an
essential systematic characterization for clock operation at the
level and beyond.Comment: 5 + 2 pages, 3 figures, added supplementa
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