6,230 research outputs found
Semiclassical approaches to nuclear dynamics
The extended Gutzwiller trajectory approach is presented for the
semiclassical description of nuclear collective dynamics, in line with the main
topics of the fruitful activity of V.G. Solovjov. Within the Fermi-liquid
droplet model, the leptodermous effective surface approximation was applied to
calculations of energies, sum rules and transition densities for the
neutron-proton asymmetry of the isovector giant-dipole resonance and found to
be in good agreement with the experimental data. By using the Strutinsky shell
correction method, the semiclassical collective transport coefficients such as
nuclear inertia, friction, stiffness, and moments of inertia can be derived
beyond the quantum perturbation approximation of the response function theory
and the cranking model.The averaged particle-number dependence of the low-lying
collective vibrational states are described in good agreement with basic
experimental data, mainly due to an enhancement of the collective inertia as
compared to its irrotational flow value. Shell components of the moment of
inertia are derived in terms of the periodic-orbit free-energy shell
corrections. A good agreement between the semiclassical extended Thomas-Fermi
moments of inertia with shell corrections and the quantum results is obtained
for different nuclear deformations and particle numbers. Shell effects are
shown to be exponentially dampted out with increasing temperature in all the
transport coefficients.Comment: 83 pages, 39 figures, 4 tables, corrected typos and improved Englis
A critical examination of compound stability predictions from machine-learned formation energies
Machine learning has emerged as a novel tool for the efficient prediction of material properties, and claims have been made that machine-learned models for the formation energy of compounds can approach the accuracy of Density Functional Theory (DFT). The models tested in this work include five recently published compositional models, a baseline model using stoichiometry alone, and a structural model. By testing seven machine learning models for formation energy on stability predictions using the Materials Project database of DFT calculations for 85,014 unique chemical compositions, we show that while formation energies can indeed be predicted well, all compositional models perform poorly on predicting the stability of compounds, making them considerably less useful than DFT for the discovery and design of new solids. Most critically, in sparse chemical spaces where few stoichiometries have stable compounds, only the structural model is capable of efficiently detecting which materials are stable. The nonincremental improvement of structural models compared with compositional models is noteworthy and encourages the use of structural models for materials discovery, with the constraint that for any new composition, the ground-state structure is not known a priori. This work demonstrates that accurate predictions of formation energy do not imply accurate predictions of stability, emphasizing the importance of assessing model performance on stability predictions, for which we provide a set of publicly available tests
A Monte Carlo Study of the Dynamical-Flucautation Property of the Hadronic System Inside Jets
A study of the dynamical fluctuation property of jets is carried out using
Monte Carlo method. The results suggest that, unlike the average properties of
the hadronic system inside jets, the anisotropy of dynamical fluctuations in
these systems changes abruptly with the variation of the cut parameter \yct.
A transition point exists, where the dynamical fluctuations in the hadronic
system inside jet behave like those in soft hadronic collisions, i.e. being
circular in the transverse plan with repect to dynamical fluctuations. This
finding obtained from Jetset and Herwig Monte Carlo is encouraged to be checked
by experiments.Comment: 8 pages, 3 figure
Thermodynamically self-consistent non-stochastic micromagnetic model for the ferromagnetic state
In this work, a self-consistent thermodynamic approach to micromagnetism is
presented. The magnetic degrees of freedom are modeled using the
Landau-Lifshitz-Baryakhtar theory, that separates the different contributions
to the magnetic damping, and thereby allows them to be coupled to the electron
and phonon systems in a self-consistent way. We show that this model can
quantitatively reproduce ultrafast magnetization dynamics in Nickel.Comment: 5 pages, 3 figure
Quantum calculations of Coulomb reorientation for sub-barrier fusion
Classical mechanics and Time Dependent Hartree-Fock (TDHF) calculations of
heavy ions collisions are performed to study the rotation of a deformed nucleus
in the Coulomb field of its partner. This reorientation is shown to be
independent on charges and relative energy of the partners. It only depends
upon the deformations and inertias. TDHF calculations predict an increase by
30% of the induced rotation due to quantum effects while the nuclear
contribution seems negligible. This reorientation modifies strongly the fusion
cross-section around the barrier for light deformed nuclei on heavy collision
partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.Comment: accepted for publication in Physical Review Lette
Propfan Test Assessment (PTA): Flight test report
The Propfan Test Assessment (PTA) aircraft was flown to obtain glade stress and noise data for a 2.74m (9 ft.) diameter single rotation propfan. Tests were performed at Mach numbers to 0.85 and altitudes to 12,192m (40,000 ft.). The propfan was well-behaved structurally over the entire flight envelope, demonstrating that the blade design technology was completely adequate. Noise data were characterized by strong signals at blade passage frequency and up to 10 harmonics. Cabin noise was not so high as to preclude attainment of comfortable levels with suitable wall treatment. Community noise was not excessive
- …