7,945 research outputs found
Dielectric Breakdown of a Mott Insulator
We study the nonequilibrium steady state of a Mott insulator coupled to a
thermostat and driven by a constant electric field, starting from weak fields,
until the dielectric breakdown, and beyond. We find that the conventional Zener
picture does not describe the steady-state physics. In particular, the current
at weak field is found to be controlled by the dissipation. Moreover, in
connection with the electric-field-driven dimensional crossover, we find that
the dielectric breakdown occurs when the field strength is on the order of the
Mott gap of the corresponding lower-dimensional system. We also report a
resonance and the meltdown of the quasiparticle peak when the field strength is
half of this Mott gap.Comment: 5 pages, 5 figures. v2: references adde
Oxidation of GaN: An ab initio thermodynamic approach
GaN is a wide-bandgap semiconductor used in high-efficiency LEDs and solar
cells. The solid is produced industrially at high chemical purities by
deposition from a vapour phase, and oxygen may be included at this stage.
Oxidation represents a potential path for tuning its properties without
introducing more exotic elements or extreme processing conditions. In this
work, ab initio computational methods are used to examine the energy potentials
and electronic properties of different extents of oxidation in GaN. Solid-state
vibrational properties of Ga, GaN, Ga2O3 and a single substitutional oxygen
defect have been studied using the harmonic approximation with supercells. A
thermodynamic model is outlined which combines the results of ab initio
calculations with data from experimental literature. This model allows free
energies to be predicted for arbitrary reaction conditions within a wide
process envelope. It is shown that complete oxidation is favourable for all
industrially-relevant conditions, while the formation of defects can be opposed
by the use of high temperatures and a high N2:O2 ratio
Central-field intermolecular potentials from the differential elastic scattering of H2(D2) by other molecules
Differential elastic scattering cross sections for the systems H2+ O2, SF6, NH3, CO, and CH4 and for D2+ O2, SF6, and NH3 have been obtained from crossed beam studies. In all cases, rapid quantum oscillations have been resolved which permit the determination of intermolecular potential parameters if a central-field assumption is adopted. These potentials were found to be independent of both the isotopic form of the hydrogen molecule, and the relative collision energy. As a result of this, and the ability of these spherical potentials quantitatively to describe the measured scattering, it is concluded that anisotropy effects do not seem to be important in these H2(D2) systems
A universal chemical potential for sulfur vapours
The unusual chemistry of sulfur is illustrated by the tendency for
catenation. Sulfur forms a range of open and closed S species in the gas
phase, which has led to speculation on the composition of sulfur vapours as a
function of temperature and pressure for over a century. Unlike elemental gases
such as O and N, there is no widely accepted thermodynamic potential
for sulfur. Here we combine a first-principles global structure search for the
low energy clusters from S to S with a thermodynamic model for the
mixed-allotrope system, including the Gibbs free energy for all gas-phase
sulfur on an atomic basis. A strongly pressure-dependent transition from a
mixture dominant in S to S is identified. A universal chemical
potential function, , is proposed with wide utility in
modelling sulfurisation processes including the formation of metal chalcogenide
semiconductors.Comment: 12 pages, 9 figures. Supporting code and data is available at
https://github.com/WMD-Bath/sulfur-model [snapshot DOI:
10.5281/zenodo.28536]. Further data will be available from
DOI:10.6084/m9.figshare.1513736 and DOI:10.6084/m9.figshare.1513833 following
peer-revie
Differences in the Angular Dependencies of Spin- and Symmetry-Forbidden Excitation Cross Sections by Low-Energy Electron Impact Spectroscopy
Optically forbidden electronic transitions can be produced by low-energy electron impact. Recent experimental investigations of helium (1-3) have shown that the differential scattering cross sections for forbidden excitations are generally enhanced relative to those for allowed ones at low incident energies and large scattering angles.
We have now observed marked differences in the angular and energy dependencies of differential cross sections for various kinds of forbidden (spin, symmetry, or both) transitions in helium at low incident energies. Such differences may well provide a basis for determining the nature of optically forbidden transitions detected by electron-impact spectroscopy in other atoms and molecules
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