Constrained Molecular Dynamics Simulations of Atomic Ground-States

Constrained molecular dynamics(CoMD) model, previously introduced for nuclear dynamics, has been extended to the atomic structure and collision calculations. Quantum effects corresponding to the Pauli and Heisenberg principle are enforced by constraints, in a parameter-free way. Our calculations for small atomic system, H, He, Li, Be, F reproduce the ground-state binding energies within 3%, compared with the results of quantum mechanical Hartree-Fock calculations.Comment: 3 pages, 2 figure

Meteorites on Mars observed with the Mars Exploration Rovers

Reduced weathering rates due to the lack of liquid water and significantly greater typical surface ages should result in a higher density of meteorites on the surface of Mars compared to Earth. Several meteorites were identified among the rocks investigated during Opportunity’s traverse across the sandy Meridiani plains. Heat Shield Rock is a IAB iron meteorite and has been officially recognized as ‘‘Meridiani Planum.’’ Barberton is olivine-rich and contains metallic Fe in the form of kamacite, suggesting a meteoritic origin. It is chemically most consistent with a mesosiderite silicate clast. Santa Catarina is a brecciated rock with a chemical and mineralogical composition similar to Barberton. Barberton, Santa Catarina, and cobbles adjacent to Santa Catarina may be part of a strewn field. Spirit observed two probable iron meteorites from its Winter Haven location in the Columbia Hills in Gusev Crater. Chondrites have not been identified to date, which may be a result of their lower strengths and probability to survive impact at current atmospheric pressures. Impact craters directly associated with Heat Shield Rock, Barberton, or Santa Catarina have not been observed, but such craters could have been erased by eolian-driven erosion.Additional co-authors: DW Ming, RV Morris, PA de Souza Jr, SW Squyres, C Weitz, AS Yen, J Zipfel, T Economo

Atomic response to strong laser pulses

We formulate a semiclassical model for the collective response of an atom to a strong laser pulse and use it to study the behavior of the valence shell of xenon. We find that, in a monochromatic wave, a fraction of the shell ionizes rapidly and the rest is driven coherently. The energy distributions of the emitted electrons show reasonable agreement with experimental distributions that are generated via single-particle ionization, indicating that the spectra are independent of the ionization dynamics. The remaining part of the shell is driven coherently and expands with time, thus exerting minimal effect on the core. We also analyze the effects of pulse shape, pulse duration, and the ponderomotive potential. In particular, we present results for femtosecond pulses

Molecular-dynamics simulations of atomic ionization by strong laser fields

We use molecular-dynamics simulations to study the competition between sequential and collective ionization in low-Z atoms by strong laser fields. The model shows sequential ionization at low fields, changing to collective ionization at large fields. The field strength at which this occurs is lower for higher frequencies. We also study the ionization as a function of pulse length and find that collective ionization is favored for shorter pulses

Theoretical Evaluations of the Fission Cross Section of the 77 eV Isomer of 235-U

We have developed models of the fission barrier (barrier heights and transition state spectra) that reproduce reasonably well the measured fission cross section of $^{235}$U from neutron energy of 1 keV to 2 MeV. From these models we have calculated the fission cross section of the 77 eV isomer of $^{235}$U over the same energy range. We find that the ratio of the isomer cross section to that of the ground state lies between about 0.45 and 0.55 at low neutron energies. The cross sections become approximately equal above 1 MeV. The ratio of the neutron capture cross section to the fission cross section for the isomer is predicted to be about a factor of 3 larger for the isomer than for the ground state of $^{235}$U at keV neutron energies. We have also calculated the cross section for the population of the isomer by inelastic neutron scattering form the $^{235}$U ground state. We find that the isomer is strongly populated, and for $E_n = 1 MeV$ the $(n,n'\gamma)$ cross section leading to the population of the isomer is of the order of 0.5 barn. Thus, neutron reaction network calculations involving the uranium isotopes in a high neutron fluence are likely to be affected by the 77 eV isomer of $^{235}$U. With these same models the fission cross sections of $^{233}$U and $^{237}$U can be reproduced approximately using only minor adjustments to the barrier heights. With the significant lowering of the outer barrier that is expected for the outer barrier the general behavior of the fission cross section of $^{239}$Pu can also be reproduced.Comment: 17 pages including 8 figure

Phi^{zero}-N Bound State

We show that the QCD van der Waals attractive potential is strong enough to bind a $\phi^{0}$ meson onto a nucleon inside a nucleus to form a bound state. The direct experimental signature for such an exotic state is proposed in the case of subthreshold $\phi^{0}$ meson photoproduction from nuclear targets. The production rate is estimated and such an experiment is found to be feasible at the Jefferson Laboratory.Comment: 4 pages, 2 figure

Physical Activity and the Health of Wheelchair Users: A Systematic Review in Multiple Sclerosis, Cerebral Palsy, and Spinal Cord Injury

Objective To understand the benefits and harms of physical activity in people who may require a wheelchair with a focus on people with multiple sclerosis (MS), cerebral palsy (CP), and spinal cord injury (SCI). Data Sources Searches were conducted in MEDLINE, Cumulative Index to Nursing and Allied Health, PsycINFO, Cochrane CENTRAL, and Embase (January 2008 through November 2020). Study Selection Randomized controlled trials, nonrandomized trials, and cohort studies of observed physical activity (at least 10 sessions on 10 days) in participants with MS, CP, and SCI. Data Extraction We conducted dual data abstraction, quality assessment, and strength of evidence. Measures of physical functioning are reported individually where sufficient data exist and grouped as “function” where data are scant. Data Synthesis No studies provided evidence for prevention of cardiovascular conditions, development of diabetes, or obesity. Among 168 included studies, 44% enrolled participants with MS (38% CP, 18% SCI). Studies in MS found walking ability may be improved with treadmill training and multimodal exercises; function may be improved with treadmill, balance exercises, and motion gaming; balance is likely improved with balance exercises and may be improved with aquatic exercises, robot-assisted gait training (RAGT), motion gaming, and multimodal exercises; activities of daily living (ADL), female sexual function, and spasticity may be improved with aquatic therapy; sleep may be improved with aerobic exercises and aerobic fitness with multimodal exercises. In CP, balance may be improved with hippotherapy and motion gaming; function may be improved with cycling, treadmill, and hippotherapy. In SCI, ADL may be improved with RAGT. Conclusions Depending on population and type of exercise, physical activity was associated with improvements in walking, function, balance, depression, sleep, ADL, spasticity, female sexual function, and aerobic capacity. Few harms of physical activity were reported in studies. Future studies are needed to address evidence gaps and to confirm findings

Search for Magnetic Monopoles Trapped in Matter

There have been many searches for magnetic monopoles in flight, but few for monopoles in matter. We have searched for magnetic monopoles in meteorites, schists, ferromanganese nodules, iron ores and other materials. The detector was a superconducting induction coil connected to a SQUID (Superconducting Quantum Interference Device) with a room temperature bore 15 cm in diameter. We tested a total of more than 331 kg of material including 112 kg of meteorites. We found no monopole and conclude the overall monopole/nucleon ratio in the samples is $<1.2 \times 10^{-29}$ with a 90\% confidence level.Comment: 6 pages, rev tex, no figure

A two-dimensional, two-electron model atom in a laser pulse: exact treatment, single active electron-analysis, time-dependent density functional theory, classical calculations, and non-sequential ionization

Owing to its numerical simplicity, a two-dimensional two-electron model atom, with each electron moving in one direction, is an ideal system to study non-perturbatively a fully correlated atom exposed to a laser field. Frequently made assumptions, such as the ``single active electron''- approach and calculational approximations, e.g. time dependent density functional theory or (semi-) classical techniques, can be tested. In this paper we examine the multiphoton short pulse-regime. We observe ``non-sequential'' ionization, i.e.\ double ionization at lower field strengths as expected from a sequential, single active electron-point of view. Since we find non-sequential ionization also in purely classical simulations, we are able to clarify the mechanism behind this effect in terms of single particle trajectories. PACS Number(s): 32.80.RmComment: 10 pages, 16 figures (gzipped postscript), see also http://www.physik.tu-darmstadt.de/tqe

Quantum solitons at strong coupling

We examine the effect of one loop quantum corrections on the formation of nontopological solitons in a strongly coupled scalar-fermionic Yukawa theory. The exact one fermion loop contribution is incorporated by using a nonlocal method to correct the local derivative expansion approximation (DE) of the effective action. As the Yukawa coupling is increased we find that the nonlocal corrections play an increasingly important role. The corrections cause the scalar field to increase in depth while maintaining its size. This increases the energy of the bag configuration, but this is compensated for by more tightly bound fermionic states with lower energy. In contrast to the semi-classical picture without quantum corrections, the binding energy is small, and the total energy scales directly with the Yukawa coupling. This confirms the qualitative behavior found in earlier work using the second order DE, although the quantitative solutions differ.Comment: 17 pages, 4 Postscript figures, REVTe