1,823 research outputs found
Bulk Aluminum at High Pressure: A First-Principles Study
The behavior of metals at high pressure is of great importance to the fields
of shock physics, geophysics, astrophysics, and nuclear materials. In order to
further understand the properties of metals at high pressures we studied the
equation of state of aluminum using first-principles techniques up to 2500 GPa,
pressures within reach of the planned L.L.N.L. National Ignition Facility. Our
simulations use density-functional theory and density-functional perturbation
theory in the generalized gradient approximation at 0K. We found core overlaps
to become relevant beyond pressures of 1200 GPa. The equations of state for
three phases (fcc, bcc, and hcp) were calculated predicting the fcc-hcp,
fcc-bcc, and hcp-bcc transitions to occur at 215 GPa, 307 GPa, and 435 GPa
respectively. From the phonon dispersions at increasing pressure, we predict a
softening of the lowest transverse acoustic vibrational mode along the [110]
direction, which corresponds to a Born instability of the fcc phase at 725 GPa.Comment: 4 pages, 5 figures, accepted to Phys. Rev. B as a Brief Report. This
version has update many figures. Moreover we provided updated and more
accurate numbers based on further in-depth analyses of potential
computational error
Angular distributions in decays
The differential decay rates of the processes and
close to the threshold are calculated with
the help of the optical potential. The same calculations are made
for the decays of . We use the potential which has been suggested to
fit the cross sections of scattering together with and
six pion production in annihilation close to the
threshold. The invariant mass spectra is in agreement with the
available experimental data. The anisotropy of the angular distributions, which
appears due to the tensor forces in the interaction, is predicted
close to the threshold. This anisotropy is large enough to be
investigated experimentally. Such measurements would allow one to check the
accuracy of the model of interaction.Comment: 10 pages, 8 figure
Relativistic effects in the processes of heavy quark fragmentation
In the framework based on the quasipotential method and relativistic quark
model a new covariant expression for the heavy quark fragmentation amplitude to
fragment into the pseudoscalar and vector S-wave heavy mesons is obtained. It
contains all possible relativistic corrections including the terms connected
with the transformation law of the bound state wave function to the reference
frame of the moving meson. Relativistic corrections of order {\bf p}^2/m^2 to
the heavy quark fragmentation distributions into (\bar c c), (\bar b c) and
(\bar b b) states are calculated as functions of the longitudinal momentum
fraction z and the transverse momentum p_T relative to the jet axis.Comment: 23 pages, 6 figure
Relativistic Coulomb Green's function in -dimensions
Using the operator method, the Green's functions of the Dirac and
Klein-Gordon equations in the Coulomb potential are derived for
the arbitrary space dimensionality . Nonrelativistic and quasiclassical
asymptotics of these Green's functions are considered in detail.Comment: 9 page
High-energy expansion of Coulomb corrections to the e+e- photoproduction cross section
First correction to the high-energy asymptotics of the total
photoproduction cross section in the electric field of a heavy atom is derived
with the exact account of this field. The consideration is based on the use of
the quasiclassical electron Green function in an external electric field. The
next-to-leading correction to the cross section is discussed. The influence of
screening on the Coulomb corrections is examined in the leading approximation.
It turns out that the high-energy asymptotics of the corresponding correction
is independent of the photon energy. In the region where both produced
particles are relativistic, the corrections to the high-energy asymptotics of
the electron (positron) spectrum are derived. Our results for the total cross
section are in good agreement with experimental data for photon energies down
to a few . In addition, the corrections to the bremsstrahlung spectrum are
obtained from the corresponding results for pair production.Comment: 22 pages, 7 figures, RevTeX.Typos are corrected. The numerical
results, figures and conclusions remain unchanged as they were obtained using
correct formula
Ab initio Investigation of Elasticity and Stability of Metal Aluminum
On the basis of the pseudopotential plane-wave(PP-PW) method in combination
with the local-density-functional theory(LDFT), complete stress-strain curves
for the uniaxial loading and uniaxial deformation along the [001] and [111]
directions, and the biaxial proportional extension along [010] and [001] of
aluminium are obtained. During the uniaxial loading, certain general behaviors
of energy versus stretch and the load versus the stretch are confirmed; in each
acse, there exist three special unstressed structures: f.c.c., b.c.c. and
f.c.t. for [001]; f.c.c., s.c. and b.c.c. for [111]. Using stability criteria,
we find that all of these state are unstable, and always occur together with
shear instability, except the natural f.c.c. structure. A Bain transformation
from the stable f.c.c. structure to the stable b.c.c. configuration cannot be
obtained by uniaxial compression along any equivalent [001] and [111]
direction. The tensile strength are similar for the two directions. For the
higher energy barrier of [111] direction, the compressive strength is greater
than that for the [001] direction. With increase in the ratio of the biaxial
proportional extension, the stress and tensile strength increase; however, the
critical strain does not change significantly. Our results add to the existing
ab initio database for use in fitting and testing interatomic potentials.Comment: 9 Pages in Revtex and 11 Eps figure
Corrections to deuterium hyperfine structure due to deuteron excitations
We consider the corrections to deuterium hyperfine structure originating from
the two-photon exchange between electron and deuteron, with the deuteron
excitations in the intermediate states. In particular, the motion of the two
intermediate nucleons as a whole is taken into account. The problem is solved
in the zero-range approximation. The result is in good agreement with the
experimental value of the deuterium hyperfine splitting.Comment: 7 pages, LaTe
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