526 research outputs found
Exciton-exciton interaction and biexciton formation in bilayer systems
We report quantum Monte Carlo calculations of biexciton binding energies in
ideal two-dimensional bilayer systems with isotropic electron and hole masses.
We have also calculated exciton-exciton interaction potentials, and pair
distribution functions for electrons and holes in bound biexcitons. Comparing
our data with results obtained in a recent study using a model exciton-exciton
potential [C. Schindler and R. Zimmermann, Phys. Rev. B \textbf{78}, 045313
(2008)], we find a somewhat larger range of layer separations at which
biexcitons are stable. We find that individual excitons retain their identity
in bound biexcitons for large layer separations.Comment: 7 pages, 11 figures, 2 table
A variance-minimization scheme for optimizing Jastrow factors
We describe a new scheme for optimizing many-electron trial wave functions by
minimizing the unreweighted variance of the energy using stochastic integration
and correlated-sampling techniques. The scheme is restricted to parameters that
are linear in the exponent of a Jastrow correlation factor, which are the most
important parameters in the wave functions we use. The scheme is highly
efficient and allows us to investigate the parameter space more closely than
has been possible before. We search for multiple minima of the variance in the
parameter space and compare the wave functions obtained using reweighted and
unreweighted variance minimization.Comment: 19 pages; 12 figure
Norm-conserving Hartree-Fock pseudopotentials and their asymptotic behavior
We investigate the properties of norm-conserving pseudopotentials (effective
core potentials) generated by inversion of the Hartree-Fock equations. In
particular we investigate the asymptotic behaviour as
and find that such pseudopotentials are non-local over all space, apart from a
few special special cases such H and He. Such extreme non-locality leads to a
lack of transferability and, within periodic boundary conditions, an undefined
total energy. The extreme non-locality must therefore be removed, and we argue
that the best way to accomplish this is a minor relaxation of the
norm-conservation condition. This is implemented, and pseudopotentials for the
atoms HAr are constructed and tested.Comment: 13 pages, 4 figure
Pressure-induced s-band ferromagnetism in alkali metals
First-principles density-functional-theory calculations show that compression
of alkali metals stabilizes open structures with localized interstitial
electrons which may exhibit a Stoner-type instability towards ferromagnetism.
We find ferromagnetic phases of the lithium-IV-type, simple cubic, and simple
hexagonal structures in the heavier alkali metals, which may be described as
s-band ferromagnets. We predict that the most stable phases of potassium at low
temperatures and pressures around 20 GPa are ferromagnets.Comment: 5 pages, 3 figure
Electron Emission from Diamondoids: A Diffusion Quantum Monte Carlo Study
We present density-functional theory (DFT) and quantum Monte Carlo (QMC)
calculations designed to resolve experimental and theoretical controversies
over the optical properties of H-terminated C nanoparticles (diamondoids). The
QMC results follow the trends of well-converged plane-wave DFT calculations for
the size dependence of the optical gap, but they predict gaps that are 1-2 eV
higher. They confirm that quantum confinement effects disappear in diamondoids
larger than 1 nm, which have gaps below that of bulk diamond. Our QMC
calculations predict a small exciton binding energy and a negative electron
affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized
nature of the lowest unoccupied molecular orbital. The NEA suggests a range of
possible applications of diamondoids as low-voltage electron emitters
Diffusion quantum Monte Carlo calculation of the quasiparticle effective mass of the two-dimensional homogeneous electron gas
The quasiparticle effective mass is a key quantity in the physics of electron
gases, describing the renormalization of the electron mass due to
electron-electron interactions. Two-dimensional electron gases are of
fundamental importance in semiconductor physics, and there have been numerous
experimental and theoretical attempts to determine the quasiparticle effective
mass in these systems. In this work we report quantum Monte Carlo results for
the quasiparticle effective mass of a two-dimensional homogeneous electron gas.
Our calculations differ from previous quantum Monte Carlo work in that much
smaller statistical error bars have been achieved, allowing for an improved
treatment of finite-size effects. In some cases we have also been able to use
larger system sizes than previous calculations
Quantum Monte Carlo calculation of the energy band and quasiparticle effective mass of the two-dimensional Fermi fluid
We have used the diffusion quantum Monte Carlo method to calculate the energy
band of the two-dimensional homogeneous electron gas (HEG), and hence we have
obtained the quasiparticle effective mass and the occupied bandwidth. We find
that the effective mass in the paramagnetic HEG increases significantly when
the density is lowered, whereas it decreases in the fully ferromagnetic HEG.
Our calculations therefore support the conclusions of recent experimental
studies [Y.-W. Tan et al., Phys. Rev. Lett. 94, 016405 (2005); M. Padmanabhan
et al., Phys. Rev. Lett. 101, 026402 (2008); T. Gokmen et al., Phys. Rev. B 79,
195311 (2009)]. We compare our calculated effective masses with other
theoretical results and experimental measurements in the literature
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