1,006 research outputs found
Diffusion of muonium and hydrogen in diamond
Jump rates of muonium and hydrogen in diamond are calculated by quantum
transition-state theory, based on the path-integral centroid formalism. This
technique allows us to study the influence of vibrational mode quantization on
the effective free-energy barriers Delta F for impurity diffusion, which are
renormalized respect to the zero-temperature classical calculation. For the
transition from a tetrahedral (T) site to a bond-center (BC) position, Delta F
is larger for hydrogen than for muonium, and the opposite happens for the
transition from BC to T. The calculated effective barriers decrease for rising
temperature, except for the muonium transition from T to BC sites. Calculated
jump rates are in good agreement to available muon spin rotation data.Comment: 4 pages, 3 figure
Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N
The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is
extensively studied over a vast range of Mn concentrations. We calculate the
electronic structures of these materials using density functional theory in
both the local spin density approximation and the LDA+U scheme, that we have
now implemented in the code SIESTA.
For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the
ferromagnetism, with an exchange constant =~ -2.8 eV. This is smaller
than that obtained with LSDA, which overestimates the exchange coupling between
Mn ions and the As holes.
In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the
double-exchange mechanism, since a hole of strong character is found at the
Fermi level in both the LSDA and the LDA+U approaches. In this case the
coupling between the Mn ions decays rapidly with the Mn-Mn separation. This
suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a
robust ferromagnetic phase at large Mn concentration coexisting with a diluted
weak ferromagnetic phase.Comment: 12 pages, 11 figure
Modeling and simulation of bulk gallium nitride power semiconductor devices
Bulk gallium nitride (GaN) power semiconductor devices are gaining significant interest in recent years, creating the need for technology computer aided design (TCAD) simulation to accurately model and optimize these devices. This paper comprehensively reviews and compares different GaN physical models and model parameters in the literature, and discusses the appropriate selection of these models and parameters for TCAD simulation. 2-D drift-diffusion semi-classical simulation is carried out for 2.6 kV and 3.7 kV bulk GaN vertical PN diodes. The simulated forward current-voltage and reverse breakdown characteristics are in good agreement with the measurement data even over a wide temperature range
Carrier-mediated ferromagnetic ordering in Mn ion-implanted p+GaAs:C
Highly p-type GaAs:C was ion-implanted with Mn at differing doses to produce
Mn concentrations in the 1 - 5 at.% range. In comparison to LT-GaAs and
n+GaAs:Si samples implanted under the same conditions, transport and magnetic
properties show marked differences. Transport measurements show anomalies,
consistent with observed magnetic properties and with epi- LT-(Ga,Mn)As, as
well as the extraordinary Hall Effect up to the observed magnetic ordering
temperature (T_C). Mn ion-implanted p+GaAs:C with as-grown carrier
concentrations > 10^20 cm^-3 show remanent magnetization up to 280 K
The future of automated fare collection systems : a perspective from the eThekwini municipality
Papers presented virtually at the 41st International Southern African Transport Conference on 10-13 July 2023.In 2009, the National Department of Transport (NDoT) legislated, as per the National Land
Transport Act (NLTA), 2009 (Act No.5 of 2009), that fare payments must be made through
any bank-issued fare payment system and be interoperable through all participating banks.
Commuters with participating bank accounts should be able to use bank cards and
commuters without bank accounts should be able to use prepaid cards. NDoT defined an
Automated Fare Collection (AFC) data structure that must be loaded onto all the Smart
Cards and stipulated that the Europay, Mastercard, Visa (EMV) card is the standard of
card technology that must be used to enable the hosting of the NDoT data structure.
The eThekwini Municipality (herein after referred to as “the City”) went out to tender in
2010 for an AFC system that was compliant with the requirements of the Act and the
system was successfully implemented in 2012 and is still in use today.
The system did not meet all the objectives that it was originally intended to. There were
aspects of the system that worked well but there were also several challenges that were
experienced. One of the major issues with the system was related to the inoperability of
the banks with the EMV/NDoT pre-paid cards. This issue only surfaced once the City had
already implemented the first compliant system and was in the process of implementing a
new system. The technical constraints were identified with NDoT and the participating
banks at the time, but there was a lack of capacity by the banks to co-operate to resolve
the issue. In addition the system was costly and complex to manage requiring
cumbersome reconciliation processes and technical constraints imposed by the restriction
to utilise the EMV/NDoT compliant card.
The lessons learned from the AFC implementations conducted by the City to-date has
informed the City’s procurement approach for future AFC systems. Research has been
conducted through various literature and discussion with colleagues in the sector to
establish the latest trends globally for AFC systems, exploring changes that have been
made to the technology and implementation approach that is being adopted to cater for the
future needs of Public Transport in the City. The findings show that the future of AFC’s
should first and foremost take into consideration the needs of the commuters by providing
convenience, ease of use and cater for seamless travel between multiple modes of
———————————————————————————————
41st Southern African Transport Conference
ISBN: 978-0-6397-8659-9
Produced by: www.betaproducts.co.za
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10-13 July 2023
Pretoria, South Africa
Conference Proceedings
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41st Southern African Transport Conference
ISBN: 978-0-6397-8659-9
Produced by: www.betaproducts.co.za
———————————————————————————————
10-13 July 2023
Pretoria, South Africa
Conference Proceedings
transport. Consideration also needs to be given to the overall operating costs for the City
and sustainability in terms of the ratio of AFC system implementation costs to fare
revenue.
The NDoT is in the process of legislating the South African National Roads Agency’s
(SANRAL) Account Based Ticketing (ABT) system as the AFC solution to be adopted for
all Public Transport in the South Africa (herein after referred to as “the country”). The City
has engaged with other Cities regarding the benefits, pitfalls and views on the
recommended approach with regards to AFC systems for use in Public Transport across
the Country. Considering global trends, current limitations, and unique challenges within
the context of the country, the collective view is that AFC systems should not be restricted
to a “one-size-fits-all” approach and the objectives of customer centricity and minimising
operating costs are paramount
Lattice location and thermal stability of implanted Fe in ZnO
The emission channeling technique was applied to evaluate the lattice location of implanted Fe in single-crystalline ZnO. The angular distribution of - particles emitted by Fe was monitored with a position-sensitive electron detector, following 60-keV low dose ( cm) room-temperature implantation of the precursor isotope Mn. The emission patterns around the [0001], [1102], [1101] and [2113] directions revealed that following annealing at 800°C 95(8)% of the Fe atoms occupy ideal substitutional Zn sites with root mean square displacements of 0.06-0.09 Å
Hydrogen and muonium in diamond: A path-integral molecular dynamics simulation
Isolated hydrogen, deuterium, and muonium in diamond have been studied by
path-integral molecular dynamics simulations in the canonical ensemble.
Finite-temperature properties of these point defects were analyzed in the range
from 100 to 800 K. Interatomic interactions were modeled by a tight-binding
potential fitted to density-functional calculations. The most stable position
for these hydrogenic impurities is found at the C-C bond center. Vibrational
frequencies have been obtained from a linear-response approach, based on
correlations of atom displacements at finite temperatures. The results show a
large anharmonic effect in impurity vibrations at the bond center site, which
hardens the vibrational modes with respect to a harmonic approximation.
Zero-point motion causes an appreciable shift of the defect level in the
electronic gap, as a consequence of electron-phonon interaction. This defect
level goes down by 70 meV when replacing hydrogen by muonium.Comment: 11 pages, 8 figure
Ferromagnetism in Fe-substituted spinel semiconductor ZnGaO
Motivated by the recent experimental observation of long range ferromagnetic
order at a relatively high temperature of 200K in the Fe-doped ZnGaO
semiconducting spinel, we propose a possible mechanism for the observed
ferromagnetism in this system. We show, supported by band structure
calculations, how a model similar to the double exchange model can be written
down for this system and calculate the ground state phase diagram for the two
cases where Fe is doped either at the tetrahedral position or at the octahedral
position. We find that in both cases such a model can account for a stable
ferromagnetic phase in a wide range of parameter space. We also argue that in
the limit of high Fe concentration at the tetrahedral positions a
description in terms of a two band model is essential. The two orbitals
and the hopping between them play a crucial role in stabilizing the
ferromagnetic phase in this limit. The case when Fe is doped simultaneously at
both the tetrahedral and the octahedral position is also discussed.Comment: 10 pages, 9 figures, added text, J. Phys. Cond. Mat. (to appear
Proprotein convertase expression and localization in epidermis: evidence for multiple roles and substrates
Specific proteolysis plays an important role in the terminal differentiation of keratinocytes in the epidermis and several types of proteases have been implicated in this process. The proprotein convertases (PCs) are a family of Ca 2+ -dependent serine proteases involved in processing and activation of several types of substrates. In this study we examined the expression and some potential substrates of PCs in epidermis. Four PCs are expressed in epidermis: furin, PACE4, PC5/6 and PC7/8. Furin is detected in two forms, either with or without the transmembrane domain, suggesting occurrence of post-translational cleavage to produce a soluble enzyme. In addition the furin active site has differential accessibility in the granular layer of the epidermis relative to the basal layer, whereas antibodies to the transmembrane domain stain both layers. These findings suggest that furin has access to different types of substrates in granular cells as opposed to basal cells. PC7/8, in contrast, is detected throughout the epidermis with antibodies to both the transmembrane and active site and no soluble form observed. A peptide PC inhibitor (dec-RVKR-CMK) inhibits cleavage of Notch-1, a receptor important in cell fate determination that is found throughout the epidermis. Profilaggrin, found in the granular layer, is specifically cleaved by furin and PACE4 in vitro at a site between the amino terminus and the first filaggrin repeat. This work suggests that the PCs play multiple roles during epidermal differentiation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75749/1/j.1600-0625.2001.010003193.x.pd
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