15,050 research outputs found
Ferromagnetism of a Repulsive Atomic Fermi Gas in an Optical Lattice: a Quantum Monte Carlo Study
Using continuous-space quantum Monte Carlo methods we investigate the
zero-temperature ferromagnetic behavior of a two-component repulsive Fermi gas
under the influence of periodic potentials that describe the effect of a
simple-cubic optical lattice. Simulations are performed with balanced and with
imbalanced components, including the case of a single impurity immersed in a
polarized Fermi sea (repulsive polaron). For an intermediate density below half
filling, we locate the transitions between the paramagnetic, and the partially
and the fully ferromagnetic phases. As the intensity of the optical lattice
increases, the ferromagnetic instability takes place at weaker interactions,
indicating a possible route to observe ferromagnetism in experiments performed
with ultracold atoms. We compare our findings with previous predictions based
on the standard computational method used in material science, namely density
functional theory, and with results based on tight-binding models.Comment: Published version with Supplemental Material. Added comparison with
Hubbard model result
Localization landscape theory of disorder in semiconductors. III. Application to carrier transport and recombination in light emitting diodes
This paper introduces a novel method to account for quantum disorder effects
into the classical drift-diffusion model of semiconductor transport through the
localization landscape theory. Quantum confinement and quantum tunneling in the
disordered system change dramatically the energy barriers acting on the
perpendicular transport of heterostructures. In addition they lead to
percolative transport through paths of minimal energy in the 2D landscape of
disordered energies of multiple 2D quantum wells. This model solves the carrier
dynamics with quantum effects self-consistently and provides a computationally
much faster solver when compared with the Schr\"odinger equation resolution.
The theory also provides a good approximation to the density of states for the
disordered system over the full range of energies required to account for
transport at room-temperature. The current-voltage characteristics modeled by
3-D simulation of a full nitride-based light-emitting diode (LED) structure
with compositional material fluctuations closely match the experimental
behavior of high quality blue LEDs. The model allows also a fine analysis of
the quantum effects involved in carrier transport through such complex
heterostructures. Finally, details of carrier population and recombination in
the different quantum wells are given.Comment: 14 pages, 16 figures, 6 table
Accuracy of Quantum Monte Carlo Methods for Point Defects in Solids
Quantum Monte Carlo approaches such as the diffusion Monte Carlo (DMC) method
are among the most accurate many-body methods for extended systems. Their
scaling makes them well suited for defect calculations in solids. We review the
various approximations needed for DMC calculations of solids and the results of
previous DMC calculations for point defects in solids. Finally, we present
estimates of how approximations affect the accuracy of calculations for
self-interstitial formation energies in silicon and predict DMC values of
4.4(1), 5.1(1) and 4.7(1) eV for the X, T and H interstitial defects,
respectively, in a 16(+1)-atom supercell
QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion Quantum Monte Carlo
We review recent advances in the capabilities of the open source ab initio
Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for
greater efficiency and reproducibility. The auxiliary field QMC (AFQMC)
implementation has been greatly expanded to include k-point symmetries,
tensor-hypercontraction, and accelerated graphical processing unit (GPU)
support. These scaling and memory reductions greatly increase the number of
orbitals that can practically be included in AFQMC calculations, increasing
accuracy. Advances in real space methods include techniques for accurate
computation of band gaps and for systematically improving the nodal surface of
ground state wavefunctions. Results of these calculations can be used to
validate application of more approximate electronic structure methods including
GW and density functional based techniques. To provide an improved foundation
for these calculations we utilize a new set of correlation-consistent effective
core potentials (pseudopotentials) that are more accurate than previous sets;
these can also be applied in quantum-chemical and other many-body applications,
not only QMC. These advances increase the efficiency, accuracy, and range of
properties that can be studied in both molecules and materials with QMC and
QMCPACK
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