9,165 research outputs found
Predicted band structures of III-V semiconductors in wurtzite phase
While non-nitride III-V semiconductors typically have a zincblende structure,
they may also form wurtzite crystals under pressure or when grown as
nanowhiskers. This makes electronic structure calculation difficult since the
band structures of wurtzite III-V semiconductors are poorly characterized. We
have calculated the electronic band structure for nine III-V semiconductors in
the wurtzite phase using transferable empirical pseudopotentials including
spin-orbit coupling. We find that all the materials have direct gaps. Our
results differ significantly from earlier {\it ab initio} calculations, and
where experimental results are available (InP, InAs and GaAs) our calculated
band gaps are in good agreement. We tabulate energies, effective masses, and
linear and cubic Dresselhaus zero-field spin-splitting coefficients for the
zone-center states. The large zero-field spin-splitting coefficients we find
may lead to new functionalities for designing devices that manipulate spin
degrees of freedom
Tight binding formulation of the dielectric response in semiconductor nanocrystals
We report on a theoretical derivation of the electronic dielectric response
of semiconductor nanocrystals using a tight-binding framework. Extending to the
nanoscale the Hanke and Sham approach [Phys. Rev. B 12, 4501 (1975)] developed
for bulk semiconductors, we show how local field effects can be included in the
study of confined systems. A great advantage of this scheme is that of being
formulated in terms of localized orbitals and thus it requires very few
computational resources and times. Applications to the optical and screening
properties of semiconductor nanocrystals are presented here and discussed.
Results concerning the absorption cross section, the static polarizability and
the screening function of InAs (direct gap) and Si (indirect gap) nanocrystals
compare well to both first principles results and experimental data. We also
show that the present scheme allows us to easily go beyond the continuum
dielectric model, based on the Clausius-Mossotti equation, which is frequently
used to include the nanocrystal surface polarization. Our calculations indicate
that the continuum dielectric model, used in conjunction with a size dependent
dielectric constant, underestimates the nanocrystal polarizability, leading to
exceedingly strong surface polarization fields.Comment: 9 pages, 5 figures; corrected typos, added reference
Sensible and latent heating of the atmosphere as inferred from DST-6 data
The average distribution of convective latent heating, boundary layer sensible heat flux, and vertical velocity are determined for the winter 1976 DST period from GLAS model diagnostics. Key features are the regions of intense latent heating over Brazil, Central Africa, and Indonesia; and the regions of strong sensible heating due to air mass modification over the North Atlantic and North Pacific Oceans
The effect of clouds on the earth's radiation balance
The effect of global cloudiness on the radiation balance at the top of the atmosphere is studied in general circulation model experiments. Wintertime simulations were conducted with clouds that had realistic optical properties, and were compared with simulations in which the clouds were transparent to either solar or thermal radiation. Clouds increase the net balance by limiting longwave loss to space, but decrease it by reflecting solar radiation. It is found that the net result of cloudiness is to maintain net radiation which is less than would be realized under clear conditions: Clouds cause the net radiation at the top of the atmosphere to increase due to longwave absorption, but to decrease even more due to cloud reflectance of solar radiation
Cloud and ice in the planetary scale circulation and in climate
The roles of the cryosphere, and of cloud-radiative interactions are investigated. The effects clouds and ice have in the climate system are examined. The cloud radiation research attempts explain the modes of interaction (feedback) between raditive transfer, cloud formation, and atmospheric dynamics. The role of sea ice in weather and climate is also discussed. Models are used to describe the ice and atmospheric dynamics under study
Body_Machine? Encounters of the Human and the Mechanical in Education, Industry and Science
This paper unveils the body_machine as a key element of dynamic mental maps that have come to shape both educational praxis and research. It traces and analyses instances in which the human and the mechanical encountered each other in metaphorical, material and visual forms, thereby blurring to some extent the boundaries between them while capturing and mobilising specific forms of knowing and acting. The paper studies, first, how certain ‘orienting frames of reference’ and associated ‘experimental systems’ managed to materialise around the body_machine and penetrate theory and praxis; and, second, what visual and textual sources related to a vocational school may reveal about where and how the body_machine has come to operate in education, industry and science. The paper centres on early twentieth-century photographs and analyses these not only as media presenting, representing and interrogating common thought and practice but also as agents of meaning-making around the body_machine
Towards an optical potential for rare-earths through coupled channels
The coupled-channel theory is a natural way of treating nonelastic channels,
in particular those arising from collective excitations, defined by nuclear
deformations. Proper treatment of such excitations is often essential to the
accurate description of reaction experimental data. Previous works have applied
different models to specific nuclei with the purpose of determining
angular-integrated cross sections. In this work, we present an extensive study
of the effects of collective couplings and nuclear deformations on integrated
cross sections as well as on angular distributions in a consistent manner for
neutron-induced reactions on nuclei in the rare-earth region. This specific
subset of the nuclide chart was chosen precisely because of a clear static
deformation pattern. We analyze the convergence of the coupled-channel
calculations regarding the number of states being explicitly coupled. Inspired
by the work done by Dietrich \emph{et al.}, a model for deforming the spherical
Koning-Delaroche optical potential as function of quadrupole and hexadecupole
deformations is also proposed. We demonstrate that the obtained results of
calculations for total, elastic and inelastic cross sections, as well as
elastic and inelastic angular distributions correspond to a remarkably good
agreement with experimental data for scattering energies above around a few
MeV.Comment: 7 pages, 6 figures. Submitted to the proceedings of the XXXVI
Reuni\~ao de Trabalho de F\'{\i}sica Nuclear no Brasil (XXXVI Brazilian
Workshop on Nuclear Physics), held in Maresias, S\~ao Paulo, Brazil in
September 2013, which should be published on AIP Conference Proceeding
Series. arXiv admin note: substantial text overlap with arXiv:1311.1115,
arXiv:1311.042
Anomalous Suppression of Valley Splittings in Lead Salt Nanocrystals without Inversion Center
Atomistic sp3d5s* tight-binding theory of PbSe and PbS nanocrystals is
developed. It is demonstrated, that the valley splittings of confined electrons
and holes strongly and peculiarly depend on the geometry of a nanocrystal. When
the nanocrystal lacks a microscopic center of inversion and has T_d symmetry,
the splitting is strongly suppressed as compared to the more symmetric
nanocrystals with O_h symmetry, having an inversion center.Comment: 5 pages, 4 figures, 1 tabl
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