2,183,778 research outputs found
Doubly magic nuclei from Lattice QCD forces at 469 MeV/c
We perform ab initio self-consistent Green's function calculations of the
closed shell nuclei He, O and Ca, based on
two-nucleon potentials derived from Lattice QCD simulations, in the flavor
SU(3) limit and at the pseudo-scalar meson mass of 469~MeV/c. The
nucleon-nucleon interaction is obtained using the HAL QCD method and its
short-distance repulsion is treated by means of ladder resummations outside the
model space. Our results show that this approach diagonalises ultraviolet
degrees of freedom correctly. Therefore, ground state energies can be obtained
from infrared extrapolations even for the relatively hard potentials of HAL
QCD. Comparing to previous Brueckner Hartree-Fock calculations, the total
binding energies are sensibly improved by the full account of many-body
correlations. The results suggest an interesting possible behaviour in which
nuclei are unbound at very large pion masses and islands of stability appear at
first around the traditional doubly-magic numbers when the pion mass is lowered
toward its physical value. The calculated one-nucleon spectral distributions
are qualitatively close to those of real nuclei even for the pseudo-scalar
meson mass considered here.Comment: 7 pages, 4 figures, RIKEN-QHP-286, RIKEN-iTHEMS-Report-1
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Modelling Emotion Based Reward Valuation with Computational Reinforcement Learning
We show that computational reinforcement learning can model human decision making in the Iowa Gambling Task (IGT). The IGT is a card game, which tests decision making under uncertainty. In our experiments, we found that modulating learning rate decay in Q-learning, enables the approximation of both the behaviour of normal subjects and those who are emotionally impaired by ventromedial prefrontal lesions. Outcomes observed in impaired subjects are modeled by high learning rate decay, while low learning rate decay replicates healthy subjects under otherwise identical conditions. The ventromedial prefrontal cortex has been associated with emotion based reward valuation, and, the value function in reinforcement learning provides an analogous assessment mechanism. Thus reinforcement learning can provide a good model for the role of emotional reward as a modulator of the learning rate
Characterization of Si/Si_(1-y)C_y superlattices grown by surfactant assisted molecular beam epitaxy
Si/Si_(0.97)C_(0.03) superlattices grown on Si(001) substrates by Sb surfactant assisted molecular beam epitaxy are characterized by in situ reflection high energy electron diffraction (RHEED), atomic force microscopy, transmission electron microscopy (TEM), and high resolution xâray diffraction. The RHEED shows that, in the absence of Sb, the growth front roughens during Si_(0.97)C_(0.03) growth and smooths during subsequent Si growth. In contrast, when Sb is present, the growth front remains smooth throughout the growth. This observation is confirmed by crossâsectional TEM, which reveals that for samples grown without the use of Sb, the Si/Si_(0.97)C_(0.03) interfaces (Si_(0.97)C_(0.03) on Si) are much more abrupt than the Si_(0.97)C_(0.03)/Si interfaces. In the case of Sb assisted growth, there is no observable difference in abruptness between the two types of interfaces. Atomic force microscopy micrographs of the Si_(0.97)C_(0.03) surface reveal features that could be the source of the roughness observed by RHEED and TEM
Equations to assess the impact resistance of fiber composites
Numerical analysis of impact resistance of composite materials containing fibers is discussed. Mathematical model of longitudinal impact resistance is presented. Potential impact resistance of various fiber composites as obtained by numerical analysis is presented as plotted curve
Observation of a (2X8) surface reconstruction on Si_(1-x)Ge_x alloys grown on (100) Si by molecular beam epitaxy
We present evidence supporting the formation of a new, (2Ă8) surface reconstruction on Si_(1âx)Ge_x alloys grown on (100) Si substrates by molecularâbeam epitaxy. Surfaces of Si_(1âx)Ge_x alloys were studied using reflection highâenergy electron diffraction (RHEED) and lowâenergy electron diffraction (LEED) techniques. RHEED patterns from samples with Ge concentrations, x, falling within the range 0.10â0.30 and grown at temperatures between 350 and 550â°C, exhibit n/8 fractionalâorder diffraction streaks in addition to the normal (2Ă1) pattern seen on (100) Si. The presence of fractionalâorder diffracted beams is indicative of an eightâfoldâperiodic modulation in electron scattering factor across the alloy surface. LEED patterns from surfaces of samples grown under similar conditions are entirely consistent with these results. In addition, the LEED patterns support the conclusion that the modulation is occurring in the direction of the dimer chains of a (2Ă1) reconstruction. We have examined the thermal stability of the (2Ă8) reconstruction and have found that it reverts to (2Ă1) after annealing to 700â°C and reappears after the sample temperature is allowed to cool below 600â°C. Such behavior suggests that the reconstruction is a stable, ordered phase for which the pairâcorrelation function of surface Ge atoms exhibits an eightfold periodicity in the "1" direction of a Siâlike (2Ă1) reconstruction. We also present a simulation in the kinematic approximation, confirming the validity of our interpretation of these finding
Sb-surfactant-mediated growth of Si/Si1âyCy superlattices by molecular-beam epitaxy
Si/Si0.97C0.03 superlattices were grown on Si(001) substrates by molecular beam epitaxy (MBE) to study the use of Sb as a surfactant during Si1âyCy growth. In situ reflection high energy electron diffraction (RHEED) shows that while carbon easily disrupts the two-dimensional growth of homoepitaxial Si, such disruption is suppressed for layers grown on Sb-terminated Si(001) surfaces. Cross-sectional transmission electron microscopy (TEM) reveals that for samples grown without the use of Sb, the Si/Si0.97C0.03 interfaces (Si0.97C0.03 on Si) were much more abrupt than Si0.97C0.03/Si interfaces. In the case of Sb-mediated growth, differences in abruptness between the two types of interfaces were not readily observable
Low error measurement-free phase gates for qubus computation
We discuss the desired criteria for a two-qubit phase gate and present a
method for realising such a gate for quantum computation that is
measurement-free and low error. The gate is implemented between qubits via an
intermediate bus mode. We take a coherent state as the bus and use cross-Kerr
type interactions between the bus and the qubits. This new method is robust
against parameter variations and is thus low error. It fundamentally improves
on previous methods due its deterministic nature and the lack of approximations
used in the geometry of the phase rotations. This interaction is applicable
both to solid state and photonic qubit systems.Comment: 6 pages, 4 figures. Published versio
Bonding of strain gages to fiber reinforced composite plastic materials
Strain gage is installed during molding of composite and utilizes the adhesive properties of the matrix resin in the composite to bond the strain gage in place. Gages thus embedded provide data at all temperatures that the matrix can withstand
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