4,389 research outputs found
Analytical solution for the electric field in a half space conductor due to alternating current injected at the surface
An analytical expression for the electric field in a half space conductor, due to alternating current injected at the surface, is derived. Assuming that the injected current flows in wires perpendicular to the surface of the test piece, the problem can be formulated in terms of a single, transverse magnetic, potential. Considering at first one wire, the cylindrical symmetry permits simplification of the calculation by use of the Hankel transform. The final result for a system with two current-carrying wires is obtained by superposition
Influence of crystal structure on charge carrier effective masses in BiFeO
Ferroelectric-based photovoltaics have shown great promise as a source of
renewable energy, thanks to their in-built charge separation capability, yet
their efficiency is often limited by low charge carrier mobilities. In this
work, we compare the photovoltaic prospects of various phases of the
multiferroic material BiFeO by evaluating their charge carrier effective
masses using first-principles simulations. We identify a tetragonal phase with
the promising combination of a large spontaneous polarisation and relatively
light charge carriers. From a systematic study of the octahedral distortions
present in BiFeO, we explain the relationship between structure and
effective masses in terms of the changes to the orbital character and overlap
at the band edges that result from changes in the geometry. The findings in
this study provide some design principles to engineer desired effective masses
in BiFeO and similar materials through manipulation of their crystal
structures in experimentally accessible ways.Comment: 12 pages, 10 figure
Unveiling the nature of bright z ~ 7 galaxies with the Hubble Space Telescope
We present new Hubble Space Telescope/Wide Field Camera 3 imaging of 25
extremely luminous (-23.2 < M_ UV < -21.2) Lyman-break galaxies (LBGs) at z ~
7. The sample was initially selected from 1.65 deg^2 of ground-based imaging in
the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman-alpha
emitters, `Himiko' and `CR7'. A deconfusion analysis of the deep Spitzer
photometry available suggests that these galaxies exhibit strong rest-frame
optical nebular emission lines (EW_0(H_beta + [OIII]) > 600A). We find that
irregular, multiple-component morphologies suggestive of clumpy or merging
systems are common (f_multi > 0.4) in bright z ~ 7 galaxies, and ubiquitous at
the very bright end (M_UV < -22.5). The galaxies have half-light radii in the
range r_1/2 ~ 0.5-3 kpc. The size measurements provide the first determination
of the size-luminosity relation at z ~ 7 that extends to M_UV ~ -23. We find
the relation to be steep with r_1/2 ~ L^1/2. Excluding clumpy, multi-component
galaxies however, we find a shallower relation that implies an increased
star-formation rate surface density in bright LBGs. Using the new, independent,
HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ~ 7
favours a power-law decline at the bright-end, compared to an exponential
Schechter function drop-off. Finally, these results have important implications
for the Euclid mission, which we predict will detect > 1000 similarly bright
galaxies at z ~ 7. Our new HST imaging suggests that the vast majority of these
galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf
star contamination.Comment: 26 pages, 11 figures and 5 tables. Updated to match MNRAS accepted
versio
Understanding the role of ions and water molecules in the NaCl dissolution process
The dissolution of NaCl in water is one of the most common everyday
processes, yet it remains poorly understood at the molecular level. Here we
report the results of an extensive density functional theory study in which the
initial stages of NaCl dissolution have been examined at low water coverages.
Our specific approach is to study how the energetic cost of moving an ion or a
pair of ions to a less coordinated site at the surface of various NaCl crystals
varies with the number of water molecules adsorbed on the surface. This
"microsolvation" approach allows us to study the dependence of the defect
energies on the number of water molecules in the cluster and thus to establish
when and where dissolution becomes favorable. Moreover, this approach allows us
to understand the roles of the individual ions and water molecules in the
dissolution process. Consistent with previous work we identify a clear
preference for dissolution of Cl ions over Na ions. However, the detailed
information obtained here leads to the conclusion that the process is governed
by the higher affinity of the water molecules to Na ions than to Cl ions. The
Cl ions are released first as this exposes more Na ions at the surface creating
favorable adsorption sites for water. We discuss how this mechanism is likely
to be effective for other alkali halides
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