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 $N\beta$ =~ -2.8 eV. This is smaller than that obtained with LSDA, which overestimates the exchange coupling between Mn ions and the As $p$ holes. In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the double-exchange mechanism, since a hole of strong $d$ 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 ——————————————————————————————— 10-13 July 2023 Pretoria, South Africa Conference Proceedings ——————————————————————————————— 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 $^{59}$Fe in single-crystalline ZnO. The angular distribution of $\beta^{-}$- particles emitted by $^{59}$Fe was monitored with a position-sensitive electron detector, following 60-keV low dose ($2.0 \times 10^{13}$ cm$^{-2}$) room-temperature implantation of the precursor isotope $^{59}$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 ZnGa2_2O4_4

Motivated by the recent experimental observation of long range ferromagnetic order at a relatively high temperature of 200K in the Fe-doped ZnGa$_2$O$_4$ 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$^{2+}$ concentration at the tetrahedral positions a description in terms of a two band model is essential. The two $e_g$ 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