149,311 research outputs found
Accelerating Atomic Orbital-based Electronic Structure Calculation via Pole Expansion and Selected Inversion
We describe how to apply the recently developed pole expansion and selected
inversion (PEXSI) technique to Kohn-Sham density function theory (DFT)
electronic structure calculations that are based on atomic orbital
discretization. We give analytic expressions for evaluating the charge density,
the total energy, the Helmholtz free energy and the atomic forces (including
both the Hellman-Feynman force and the Pulay force) without using the
eigenvalues and eigenvectors of the Kohn-Sham Hamiltonian. We also show how to
update the chemical potential without using Kohn-Sham eigenvalues. The
advantage of using PEXSI is that it has a much lower computational complexity
than that associated with the matrix diagonalization procedure. We demonstrate
the performance gain by comparing the timing of PEXSI with that of
diagonalization on insulating and metallic nanotubes. For these quasi-1D
systems, the complexity of PEXSI is linear with respect to the number of atoms.
This linear scaling can be observed in our computational experiments when the
number of atoms in a nanotube is larger than a few hundreds. Both the wall
clock time and the memory requirement of PEXSI is modest. This makes it even
possible to perform Kohn-Sham DFT calculations for 10,000-atom nanotubes with a
sequential implementation of the selected inversion algorithm. We also perform
an accurate geometry optimization calculation on a truncated (8,0)
boron-nitride nanotube system containing 1024 atoms. Numerical results indicate
that the use of PEXSI does not lead to loss of accuracy required in a practical
DFT calculation
Lifshitz Transition in Underdoped Cuprates
Recent studies show that quantum oscillations thought to be associated with a
density wave reconstructed Fermi surface disappear at a critical value of the
doping for YBa2Cu3O6+y, and the cyclotron mass diverges as the critical value
is approached from the high doping side. We argue that the phenomenon is due to
a Lifshitz transition where the pockets giving rise to the quantum oscillations
connect to form an open (quasi-1d) Fermi surface. The estimated critical doping
is close to that found by experiment, and the theory predicts a logarithmic
divergence of the cyclotron mass with a coefficient comparable to that observed
in experiment.Comment: 4 pages, 4 figure
Tunneling and delocalization in hydrogen bonded systems: a study in position and momentum space
Novel experimental and computational studies have uncovered the proton
momentum distribution in hydrogen bonded systems. In this work, we utilize
recently developed open path integral Car-Parrinello molecular dynamics
methodology in order to study the momentum distribution in phases of high
pressure ice. Some of these phases exhibit symmetric hydrogen bonds and quantum
tunneling. We find that the symmetric hydrogen bonded phase possesses a
narrowed momentum distribution as compared with a covalently bonded phase, in
agreement with recent experimental findings. The signatures of tunneling that
we observe are a narrowed distribution in the low-to-intermediate momentum
region, with a tail that extends to match the result of the covalently bonded
state. The transition to tunneling behavior shows similarity to features
observed in recent experiments performed on confined water. We corroborate our
ice simulations with a study of a particle in a model one-dimensional double
well potential that mimics some of the effects observed in bulk simulations.
The temperature dependence of the momentum distribution in the one-dimensional
model allows for the differentiation between ground state and mixed state
tunneling effects.Comment: 14 pages, 13 figure
Polarization and ellipticity of high-order harmonics from aligned molecules generated by linearly polarized intense laser pulses
We present theoretical calculations for polarization and ellipticity of
high-order harmonics from aligned N, CO, and O molecules generated
by linearly polarized lasers. Within the rescattering model, the two
polarization amplitudes of the harmonics are determined by the
photo-recombination amplitudes for photons emitted parallel and perpendicular
to the direction of the {\em same} returning electron wave packet. Our results
show clear species-dependent polarization states, in excellent agreement with
experiments. We further note that the measured polarization ellipse of the
harmonic furnishes the needed parameters for a "complete" experiment in
molecules.Comment: 4 pages, 4 figure
Effect of temperature-dependent shape anisotropy on coercivity with aligned Stoner-Wohlfarth soft ferromagnets
The temperature variation effect of shape anisotropy on the coercivity,
HC(T), for the aligned Stoner-Wohlfarth (SW) soft ferromagnets, such as fcc Ni,
fcc Co and bcc Fe, are investigated within the framework of Neel-Brown (N-B)
analysis. An extended N-B equation is thus proposed,by introducing a single
dimensionless correction function, the reduced magnetization, m(\tao) =
MS(T)/MS(0), in which \tao = T/TC is the reduced temperature, MS(T) is the
saturation magnetization, and TC is the Curie temperature. The factor, m(\tao),
accounts for the temperature-dependent effect of the shape anisotropy. The
constants, H0 and E0, are for the switching field at zero temperature and the
potential barrier at zero field, respectively. According to this newly derived
equation, the blocking temperature above which the properties of
superparamagnetism show up is described by the expression, TB =
E0m^2(\tao)/[kBln(t/t0)], with the extra correction factor m^2(\tao). The
possible effect on HC(T) and the blocking temperature, TB, attributed to the
downshift of TC resulting from the finite size effect has been discussed also.Comment: 22 pages, 2 figures, 1 table, Accepted by Phys. Rev.
P-band in a rotating optical lattice
We investigate the effects of rotation on the excited bands of a tight
binding lattice, focusing particulary on the first excited (p-) band. Both the
on-site energies and the hopping between lattice sites are modified by the
effective magnetic field created by rotation, causing a non-trivial splitting
and magnetic fine structure of the p-band. We show that Peierls substitution
can be modified to describe p-band under rotation, and use this method to
derive an effective Hamiltonian. We compare the spectrum of the effective
Hamiltonian with a first principles calculation of the magnetic band structure
and find excellent agreement, confirming the validity of our approach. We also
discuss the on-site interaction terms for bosons and argue that many-particle
phenomena in a rotating p-band can be investigated starting from this effective
Hamiltonian.Comment: 7 pages, 4 figures, new discussion of effective Hamiltonian,
references adde
Probing molecular frame photoionization via laser generated high-order harmonics from aligned molecules
Present photoionization experiments cannot measure molecular frame
photoelectron angular distributions (MFPAD) from the outermost valence
electrons of molecules. We show that details of the MFPAD can be retrieved with
high-order harmonics generated by infrared lasers from aligned molecules. Using
accurately calculated photoionization transition dipole moments for
fixed-in-space molecules, we show that the dependence of the magnitude and
phase of the high-order harmonics on the alignment angle of the molecules
observed in recent experiments can be quantitatively reproduced. This result
provides the needed theoretical basis for ultrafast dynamic chemical imaging
using infrared laser pulses.Comment: 5 pages, 4 figure
- …