66 research outputs found
Electron correlations, spontaneous magnetization and momentum density in quantum dots
The magnetization of quantum dots is discussed in terms of a relatively
simple but exactly solvable model Hamiltonian. The model predicts oscillations
in spin polarization as a function of dot radius for a fixed electron density.
These oscillations in magnetization are shown to yield distinct signature in
the momentum density of the electron gas, suggesting the usefulness of momentum
resolved spectroscopies for investigating the magnetization of dot systems. We
also present variational quantum Monte Carlo calculations on a square dot
containing 12 electrons in order to gain insight into correlation effects on
the interactions between like and unlike spins in a quantum dot.Comment: 6 pages, 4 figure
Physisorption of positronium on quartz surfaces
The possibility of having positronium (Ps) physisorbed at a material surface
is of great fundamental interest, since it can lead to new insight regarding
quantum sticking and is a necessary first step to try to obtain a Ps
molecule on a material host. Some experiments in the past have produced
evidence for physisorbed Ps on a quartz surface, but firm theoretical support
for such a conclusion was lacking. We present a first-principles
density-functional calculation of the key parameters determining the
interaction potential between Ps and an -quartz surface. We show that
there is indeed a bound state with an energy of 0.14 eV, a value which agrees
very well with the experimental estimate of eV. Further, a brief
energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of
physisorbed atoms shows that the formation and desorption of a Ps molecule
in that picture is consistent with the above results.Comment: 5 pages, 3 figures, submitte
First-principles study of possible shallow donors in ZnAl2O4 spinel
ZnAl2O4 (gahnite) is a ceramic which is considered a possible transparent conducting oxide (TCO) due to its wide band gap and transparency for UV. Defects play an important role in controlling the conductivity of a TCO material along with the dopant, which is the main source of conductivity in an otherwise insulating oxide. A comprehensive first-principles density functional theory study for point defects in ZnAl2O4 spinel is presented using the Heyd, Scuseria, and Ernzerhof hybrid functional (HSE06) to overcome the band gap problem. We have investigated the formation energies of intrinsic defects which include the Zn, Al, and O vacancy and the antisite defects: Zn at the Al site (Zn-Al) and Al at the Zn site (Al-Zn). The antisite defect Al-Zn has the lowest formation energy and acts as a shallow donor, indicating possible n-type conductivity in ZnAl2O4 spinel by Al doping
Attracting shallow donors: Hydrogen passivation in (Al,Ga,In)-doped ZnO
The hydrogen interstitial and the substitutional Al_Zn, Ga_Zn and In_Zn are
all shallow donors in ZnO and lead to n-type conductivity. Although shallow
donors are expected to repel each other, we show by first principles
calculations that in ZnO these shallow donor impurities attract and form a
complex, leading to a donor level deep in the band gap. This puts a limit on
the n-type conductivity of (Al,Ga,In)-doped ZnO in the presence of hydrogen.Comment: 4 pages, 5 figure
Orbital mixing and nesting in the bilayer manganites LaSrMnO
A first principles study of LaSrMnO compounds for
doping levels shows that the low energy electronic
structure of the majority spin carriers is determined by strong momentum
dependent interactions between the Mn and
orbitals, which in addition to an dependent Jahn-Teller distortion, differ
in the ferromagnetic and antiferromagnetic phases. The Fermi surface exhibits
nesting behavior that is reflected by peaks in the static susceptibility, whose
positions as a function of momentum have a non-trivial dependence on .Comment: 4 pages, 5 figures, publishe
Spontaneous Magnetization and Electron Momentum Density in 3D Quantum Dots
We discuss an exactly solvable model Hamiltonian for describing the
interacting electron gas in a quantum dot. Results for a spherical square well
confining potential are presented. The ground state is found to exhibit
striking oscillations in spin polarization with dot radius at a fixed electron
density. These oscillations are shown to induce characteristic signatures in
the momentum density of the electron gas, providing a novel route for direct
experimental observation of the dot magnetization via spectroscopies sensitive
to the electron momentum density.Comment: 5 pages (Revtex4), 4 (eps) figure
First-principles study of the optoelectronic properties and photovoltaic absorber layer efficiency of Cu-based chalcogenides
Cu-based chalcogenides are promising materials for thin-film solar cells with
more than 20% measured cell efficiency. Using first-principles calculations
based on density functional theory, the optoelectronic properties of a group of
Cu-based chalcogenides Cu-II-IV-VI is studied. They are then screened
with the aim of identifying potential absorber materials for photovoltaic
applications. The spectroscopic limited maximum efficiency (SLME) introduced by
Yu and Zunger is used as a metric for the screening. After constructing the
current-voltage curve, the maximum spectroscopy dependent power conversion
efficiency is calculated from the maximum power output. The role of the nature
of the band gap, direct or indirect, and also of the absorptivity of the
studied materials on the maximum theoretical power conversion efficiency is
studied. Our results show that Cu-II-GeSe with II=Cd and Hg, and
Cu-II-SnS with II=Cd and Zn have a higher theoretical efficiency
compared to the materials currently used as absorber layer
- …