16,395 research outputs found
Explicitly correlated plane waves: Accelerating convergence in periodic wavefunction expansions
We present an investigation into the use of an explicitly correlated plane
wave basis for periodic wavefunction expansions at the level of second-order
M{\o}ller-Plesset perturbation theory (MP2). The convergence of the electronic
correlation energy with respect to the one-electron basis set is investigated
and compared to conventional MP2 theory in a finite homogeneous electron gas
model. In addition to the widely used Slater-type geminal correlation factor,
we also derive and investigate a novel correlation factor that we term
Yukawa-Coulomb. The Yukawa-Coulomb correlation factor is motivated by analytic
results for two electrons in a box and allows for a further improved
convergence of the correlation energies with respect to the employed basis set.
We find the combination of the infinitely delocalized plane waves and local
short-ranged geminals provides a complementary, and rapidly convergent basis
for the description of periodic wavefunctions. We hope that this approach will
expand the scope of discrete wavefunction expansions in periodic systems.Comment: 15 pages, 13 figure
X-ray Absorption Fine Structure in Embedded Atoms
Oscillatory structure is found in the atomic background absorption in
x-ray-absorption fine structure (XAFS). This atomic-XAFS or AXAFS arises from
scattering within an embedded atom, and is analogous to the Ramsauer-Townsend
effect. Calculations and measurements confirm the existence of AXAFS and show
that it can dominate contributions such as multi-electron excitations. The
structure is sensitive to chemical effects and thus provides a new probe of
bonding and exchange effects on the scattering potential.Comment: 4 pages plus 2 postscript figures, REVTEX 3.
Observation of blue-shifted ultralong-range Cs Rydberg molecules
We observe ultralong-range blue-shifted Cs molecular states near
Rydberg states in an optical dipole trap, where .
The accidental near degeneracy of and Rydberg states for in
Cs, due to the small fractional quantum defect, leads to non-adiabatic
coupling among these states, producing potential wells above the
thresholds. Two important consequences of admixing high angular momentum states
with states are the formation of large permanent dipole moments, Debye, and accessibility of these states via two-photon association.
The observed states are in excellent agreement with theory. Both projections of
the total angular momentum on the internuclear axis are visible in the
experiment
Is U3Ni3Sn4 best described as near a quantum critical point?
Although most known non-Fermi liquid (NFL) materials are structurally or
chemically disordered, the role of this disorder remains unclear. In
particular, very few systems have been discovered that may be stoichiometric
and well ordered. To test whether U3Ni3Sn4 belongs in this latter class, we
present measurements of the x-ray absorption fine structure (XAFS) of
polycrystalline and single-crystal U3Ni3Sn4 samples that are consistent with no
measurable local structural disorder. We also present temperature-dependent
specific heat data in applied magnetic fields as high as 8 T that show features
that are inconsistent with the antiferromagnetic Griffiths' phase model, but do
support the conclusion that a Fermi liquid/NFL crossover temperature increases
with applied field. These results are inconsistent with theoretical
explanations that require strong disorder effects, but do support the view that
U3Ni3Sn4 is a stoichiometric, ordered material that exhibits NFL behavior, and
is best described as being near an antiferromagnetic quantum critical point.Comment: 9 pages, 8 figures, in press with PR
Spectroscopic accuracy directly from quantum chemistry: application to ground and excited states of beryllium dimer
We combine explicit correlation via the canonical transcorrelation approach
with the density matrix renormalization group and initiator full configuration
interaction quantum Monte Carlo methods to compute a near-exact beryllium dimer
curve, {\it without} the use of composite methods. In particular, our direct
density matrix renormalization group calculations produce a well-depth of
=931.2 cm which agrees very well with recent experimentally derived
estimates =929.7~cm [Science, 324, 1548 (2009)] and
=934.6~cm [Science, 326, 1382 (2009)]], as well the best composite
theoretical estimates, =938~cm [J. Phys. Chem. A, 111,
12822 (2007)] and =935.1~cm [Phys. Chem. Chem. Phys., 13,
20311 (2011)]. Our results suggest possible inaccuracies in the functional form
of the potential used at shorter bond lengths to fit the experimental data
[Science, 324, 1548 (2009)]. With the density matrix renormalization group we
also compute near-exact vertical excitation energies at the equilibrium
geometry. These provide non-trivial benchmarks for quantum chemical methods for
excited states, and illustrate the surprisingly large error that remains for
1 state with approximate multi-reference configuration
interaction and equation-of-motion coupled cluster methods. Overall, we
demonstrate that explicitly correlated density matrix renormalization group and
initiator full configuration interaction quantum Monte Carlo methods allow us
to fully converge to the basis set and correlation limit of the
non-relativistic Schr\"odinger equation in small molecules
Quantifying structural damage from self-irradiation in a plutonium superconductor
The 18.5 K superconductor PuCoGa5 has many unusual properties, including
those due to damage induced by self-irradiation. The superconducting transition
temperature decreases sharply with time, suggesting a radiation-induced Frenkel
defect concentration much larger than predicted by current radiation damage
theories. Extended x-ray absorption fine-structure measurements demonstrate
that while the local crystal structure in fresh material is well ordered, aged
material is disordered much more strongly than expected from simple defects,
consistent with strong disorder throughout the damage cascade region. These
data highlight the potential impact of local lattice distortions relative to
defects on the properties of irradiated materials and underscore the need for
more atomic-resolution structural comparisons between radiation damage
experiments and theory.Comment: 7 pages, 5 figures, to be published in PR
Molecular clouds in the centers of galaxies: Constraints from HCN and CO-13 line emission
We have searched for HCN J=1-0 line emission in the centers of 12 galaxies and have detected it in 10 of them. We have obtained complementary data on J=1-0 and 2-1 transitions of CO-12 and CO-13 in these systems. The ratio of integrated intensities, I(CO 1-0)/I(HCN 1-0) = 25 +/- 11 for this sample. We find that HCN emission of this strength can be produced under conditions of subthermal excitation. In combination with the line ratios in CO and CO-13, HCN puts constraints on the mean conditions of molecular clouds and on the mix of cloud types within the projected beam
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