3,470 research outputs found
Nuclear energy density optimization: Large deformations
A new Skyrme-like energy density suitable for studies of strongly elongated
nuclei has been determined in the framework of the Hartree-Fock-Bogoliubov
theory using the recently developed model-based, derivative-free optimization
algorithm POUNDerS. A sensitivity analysis at the optimal solution has revealed
the importance of states at large deformations in driving the parameterization
of the functional. The good agreement with experimental data on masses and
separation energies, achieved with the previous parameterization UNEDF0, is
largely preserved. In addition, the new energy density UNEDF1 gives a much
improved description of the fission barriers in 240Pu and neighboring nuclei.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.
Helianthus maximiliani and Species Fine-Scale Spatial Pattern Affect Diversity Interactions in Reconstructed Tallgrass Prairies
1. Biodiversity and Ecosystem Function analyses aim to explain how individual speâ cies and their interactions affect ecosystem function. With this study, we asked in what ways do species interact, are these interactions affected by species planting pattern, and are initial (planted) proportions or previous year (realized) proporâ tions a better reference point for characterizing grassland diversity effects?
2. We addressed these questions with experimental communities compiled from a pool of 16 tallgrass prairie species. We planted all species in monocultures and mixtures that varied in their species richness, evenness, and spatial pattern. We recorded speciesâspecific biomass production over three growing seasons and fitâ ted DiversityâInteractions (DI) models to annual plot biomass yields.
3. In the establishment season, all species interacted equally to form the diversity effect. In years 2 and 3, each species contributed a unique additive coefficient to its interaction with every other species to form the diversity effect. These interâ actions were affected by Helianthus maximiliani and the species planting pattern. Models based on species planted proportions betterâfit annual plot yield than models based on species previous contributions to plot biomass.
4. Outcomes suggest that efforts to plant tallgrass prairies to maximize diversity efâ fects should focus on the specific species present and in what arrangement they are planted. Furthermore, for particularly diverse grasslands, the effort of collectâ ing annual species biomass data may not be necessary when quantifying diversity effects with DI models
Search for correlation effects in linear chains of trapped ions
We report a precise search for correlation effects in linear chains of 2 and
3 trapped Ca+ ions. Unexplained correlations in photon emission times within a
linear chain of trapped ions have been reported, which, if genuine, cast doubt
on the potential of an ion trap to realize quantum information processing. We
observe quantum jumps from the metastable 3d 2D_{5/2} level for several hours,
searching for correlations between the decay times of the different ions. We
find no evidence for correlations: the number of quantum jumps with separations
of less than 10 ms is consistent with statistics to within errors of 0.05%; the
lifetime of the metastable level derived from the data is consistent with that
derived from independent single-ion data at the level of the experimental
errors 1%; and no rank correlations between the decay times were found with
sensitivity to rank correlation coefficients at the level of |R| = 0.024.Comment: With changes to introduction. 5 pages, including 4 figures. Submitted
to Europhys. Let
Nuclear energy density optimization: Shell structure
Nuclear density functional theory is the only microscopical theory that can
be applied throughout the entire nuclear landscape. Its key ingredient is the
energy density functional. In this work, we propose a new parameterization
UNEDF2 of the Skyrme energy density functional. The functional optimization is
carried out using the POUNDerS optimization algorithm within the framework of
the Skyrme Hartree-Fock-Bogoliubov theory. Compared to the previous
parameterization UNEDF1, restrictions on the tensor term of the energy density
have been lifted, yielding a very general form of the energy density functional
up to second order in derivatives of the one-body density matrix. In order to
impose constraints on all the parameters of the functional, selected data on
single-particle splittings in spherical doubly-magic nuclei have been included
into the experimental dataset. The agreement with both bulk and spectroscopic
nuclear properties achieved by the resulting UNEDF2 parameterization is
comparable with UNEDF1. While there is a small improvement on single-particle
spectra and binding energies of closed shell nuclei, the reproduction of
fission barriers and fission isomer excitation energies has degraded. As
compared to previous UNEDF parameterizations, the parameter confidence interval
for UNEDF2 is narrower. In particular, our results overlap well with those
obtained in previous systematic studies of the spin-orbit and tensor terms.
UNEDF2 can be viewed as an all-around Skyrme EDF that performs reasonably well
for both global nuclear properties and shell structure. However, after adding
new data aiming to better constrain the nuclear functional, its quality has
improved only marginally. These results suggest that the standard Skyrme energy
density has reached its limits and significant changes to the form of the
functional are needed.Comment: 18 pages, 13 figures, 12 tables; resubmitted for publication to Phys.
Rev. C after second review by refere
Deterministic entanglement and tomography of ion spin qubits
We have implemented a universal quantum logic gate between qubits stored in
the spin state of a pair of trapped calcium 40 ions. An initial product state
was driven to a maximally entangled state deterministically, with 83% fidelity.
We present a general approach to quantum state tomography which achieves good
robustness to experimental noise and drift, and use it to measure the spin
state of the ions. We find the entanglement of formation is 0.54.Comment: 3 figures, 4 pages, footnotes fixe
Long-lived mesoscopic entanglement outside the Lamb-Dicke regime
We create entangled states of the spin and motion of a single Ca
ion in a linear ion trap. The motional part consists of coherent states of
large separation and long coherence time. The states are created by driving the
motion using counterpropagating laser beams. We theoretically study and
experimentally observe the behaviour outside the Lamb-Dicke regime, where the
trajectory in phase space is modified and the coherent states become squeezed.
We directly observe the modification of the return time of the trajectory, and
infer the squeezing. The mesoscopic entanglement is observed up to with coherence time 170 microseconds and mean phonon excitation
\nbar = 16.Comment: 5 pages, 3 figures. Revised version after editor comment
Advancing understanding on industrial relations in multinational companies: key research challenges and the INTREPID contribution
peer-reviewedThis paper has three principal aims. It firstly provides some theoretical background on
the key current research issues and challenges in regard to industrial relations in multinational
companies. It then presents a concise review of scholarship to date on industrial
relations in multinational companies using INTREPID (Investigation of Transnationals Employment Practices: an International Database) data. Finally, the paper identifies some of
the main industrial relations issues that remain to be addressed, in effect charting a form
of research agenda for future work using the INTREPID data, with particular focus on
the potential contribution from âlate joinersâ to the INTREPID project.ACCEPTEDpeer-reviewe
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