65,907 research outputs found
Fast synchrotron X-ray tomographic quantification of dendrite evolution during the solidification of Mg-Sn alloys
The evolution of dendritic microstructures during the solidification of a Mg-15 wt%Sn alloy was investigated in situ via fast synchrotron X-ray microtomography. To enable these in situ observations a novel encapsulation method was developed and integrated into a fast, pink beam, imaging beamline at Diamond Light Source. The dendritic growth was quantified with time using: solid volume fraction, tip velocity, interface specific surface area, and surface curvature. The influence of cooling rate upon these quantities and primary phase nucleation was investigated. The primary dendrites grew with an 18-branch, 6-fold symmetry structure, accompanied by coarsening. The coarsening process was assessed by the specific surface area and was compared with the existing models. These results provide the first quantification of dendritic growth during the solidification of Mg alloys, confirming existing analytic models and providing experimental data to inform and validate more complex numeric models
Microscopic DC-TDHF study of heavy-ion potentials and fusion cross sections
We study heavy-ion fusion reactions at energies near the Coulomb barrier, in
particular with neutron-rich radioactive ion beams. Dynamic microscopic
calculations are carried out on a three-dimensional lattice using the
Density-Constrained Time-Dependent Hartree-Fock (DC-TDHF) method. New results
are presented for the Sn+Ca system which are compared to
Sn+Ca studied earlier. Our theoretical fusion cross-sections
agree surprisingly well with recent data measured at HRIBF. We also study the
near- and sub-barrier fusion of O on C which is important to
determine the composition and heating of the crust of accreting neutron stars.Comment: Talk given by . Volker E. Oberacker at the 11th International
Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA,
May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of
Physics: Conference Series (JPCS
The monoclinic phase in PZT: new light on morphotropic phase boundaries
A summary of the work recently carried out on the morphotropic phase boundary
(MPB) of PZT is presented. By means of x-ray powder diffraction on ceramic
samples of excellent quality, the MPB has been successfully characterized by
changing temperature in a series of closely spaced compositions. As a result,
an unexpected monoclinic phase has been found to exist in between the
well-known tetragonal and rhombohedral PZT phases. A detailed structural
analysis, together with the investigation of the field effect in this region of
compositions, have led to an important advance in understanding the mechanisms
responsible for the physical properties of PZT as well as other piezoelectric
materials with similar morphotropic phase boundaries.Comment: 5 pages REVTeX file, 6 figures embedded. Presented at the Workshop on
"Fundamental Physics of Ferroelectrics" held in Aspen, February 00. To appear
in the proceeding
The Cauchy problem for the 3-D Vlasov-Poisson system with point charges
In this paper we establish global existence and uniqueness of the solution to
the three-dimensional Vlasov-Poisson system in presence of point charges in
case of repulsive interaction. The present analysis extends an analogeous
two-dimensional result by Caprino and Marchioro [On the plasma-charge model, to
appear in Kinetic and Related Models (2010)].Comment: 28 page
Eliashberg theory of excitonic insulating transition in graphene
A sufficiently strong Coulomb interaction may open an excitonic fermion gap
and thus drive a semimetal-insulator transition in graphene. In this paper, we
study the Eliashberg theory of excitonic transition by coupling the fermion gap
equation self-consistently to the equation of vacuum polarization function.
Including the fermion gap into polarization function increases the effective
strength of Coulomb interaction because it reduces the screening effects due to
the collective particle-hole excitations. Although this procedure does not
change the critical point, it leads to a significant enhancement of the
dynamical fermion gap in the excitonic insulating phase. The validity of the
Eliashberg theory is justified by showing that the vertex corrections are
suppressed at large limit.Comment: 8 pages, 6 figure
Analytical infrared intensities for periodic systems with local basis sets
We report a method for the efficient evaluation of analytic infrared (IR)
intensities within generalized Kohn-Sham density functional theory using
Gaussian orbitals and periodic boundary conditions. A discretized form of the
Berry phase is used to evaluate a periodic dipole moment and its derivatives
with respect to in-phase nuclear coordinate displacements. Benchmark
calculations are presented for one-dimensional chains of water molecules and
poly(paraphenylenevinylene).Comment: 16 pages, 3 figures, 4 tables, submitted to Phys. Rev.
Three Kinds of Special Relativity via Inverse Wick Rotation
Since the special relativity can be viewed as the physics in an inverse Wick
rotation of 4-d Euclid space, which is at almost equal footing with the 4-d
Riemann/Lobachevski space, there should be important physics in the inverse
Wick rotation of 4-d Riemann/Lobachevski space. Thus, there are three kinds of
special relativity in de Sitter/Minkowski/anti-de Sitter space at almost equal
footing, respectively. There is an instanton tunnelling scenario in the
Riemann-de Sitter case that may explain why \La be positive and link with the
multiverse.Comment: 3 pages, no figures, to appear in Chin. Phys. Let
Remote polarization entanglement generation by local dephasing and frequency upconversion
We introduce a scheme for remote entanglement generation for the photon
polarization. The technique is based on transferring the initial frequency
correlations to specific polarization-frequency correlations by local dephasing
and their subsequent removal by frequency up-conversion. On fundamental level,
our theoretical results show how to create and transfer entanglement, to
particles which never interact, by means of local operations. This possibility
stems from the multi-path interference and its control in frequency space. For
applications, the developed techniques and results allow for the remote
generation of entanglement with distant parties without Bell state measurements
and opens the perspective to probe frequency-frequency entanglement by
measuring the polarization state of the photons.Comment: 8 page
\Lambda_b \to \Lambda_c P(V) Nonleptonic Weak Decays
The two-body nonleptonic weak decays of \Lambda_b \to \Lambda_c P(V) (P and V
represent pseudoscalar and vector mesons respectively) are analyzed in two
models, one is the Bethe-Salpeter (B-S) model and the other is the hadronic
wave function model. The calculations are carried out in the factorization
approach. The obtained results are compared with other model calculations.Comment: 18 pages, Late
Snyder's Quantized Space-time and De Sitter Special Relativity
There is a one-to-one correspondence between Snyder's model in de Sitter
space of momenta and the \dS-invariant special relativity. This indicates that
physics at the Planck length and the scale should be
dual to each other and there is in-between gravity of local \dS-invariance
characterized by a dimensionless coupling constant .Comment: 8 page
- …