761 research outputs found
Studies in Chlorination : Part I - Magnesite
Preparation of anhydrous magnesium chloride by dehydration from hydrated magnesium chloride is not straight forward after dihydrate stage due to hydrolysis. Magnesium oxide and oxy-chloride contaminate the final product in uncertain amounts
Numerical simulation of exciton dynamics in Cu2O at ultra low temperatures within a potential trap
We have studied theoretically the relaxation behaviour of excitons in cuprous
oxide (Cu2O) at ultra low temperatures when excitons are confined within a
potential trap by solving numerically the Boltzmann equation. As relaxation
processes, we have included in this paper deformation potential phonon
scattering, radiative and non-radiative decay and Auger decay. The relaxation
kinetics has been analysed for temperatures in the range between 0.3K and 5K.
Under the action of deformation potential phonon scattering only, we find for
temperatures above 0.5K that the excitons reach local equilibrium with the
lattice i.e. that the effective local temperature is coming down to bath
temperature, while below 0.5K a non-thermal energy distribution remains.
Interestingly, for all temperatures the global spatial distribution of excitons
does not reach the equilibrium distribution, but stays at a much higher
effective temperature. If we include further a finite lifetime of the excitons
and the two-particle Auger decay, we find that both the local and the global
effective temperature are not coming down to bath temperature. In the first
case we find a Bose-Einstein condensation (BEC) to occur for all temperatures
in the investigated range. Comparing our results with the thermal equilibrium
case, we find that BEC occurs for a significantly higher number of excitons in
the trap. This effect could be related to the higher global temperature, which
requires an increased number of excitons within the trap to observe the BEC. In
case of Auger decay, we do not find at any temperature a BEC due to the heating
of the exciton gas
Condensation of Excitons in Cu2O at Ultracold Temperatures: Experiment and Theory
We present experiments on the luminescence of excitons confined in a
potential trap at milli-Kelvin bath temperatures under cw-excitation. They
reveal several distinct features like a kink in the dependence of the total
integrated luminescence intensity on excitation laser power and a bimodal
distribution of the spatially resolved luminescence. Furthermore, we discuss
the present state of the theoretical description of Bose-Einstein condensation
of excitons with respect to signatures of a condensate in the luminescence. The
comparison of the experimental data with theoretical results with respect to
the spatially resolved as well as the integrated luminescence intensity shows
the necessity of taking into account a Bose-Einstein condensed excitonic phase
in order to understand the behaviour of the trapped excitons.Comment: 41 pages, 23 figure
Oxidation mechanism in metal nanoclusters: Zn nanoclusters to ZnO hollow nanoclusters
Zn nanoclusters (NCs) are deposited by Low-energy cluster beam deposition
technique. The mechanism of oxidation is studied by analysing their
compositional and morphological evolution over a long span of time (three
years) due to exposure to ambient atmosphere. It is concluded that the
mechanism proceeds in two steps. In the first step, the shell of ZnO forms over
Zn NCs rapidly up to certain limiting thickness: with in few days -- depending
upon the size -- Zn NCs are converted to Zn-ZnO (core-shell), Zn-void-ZnO, or
hollow ZnO type NCs. Bigger than ~15 nm become Zn-ZnO (core-shell) type: among
them, NCs above ~25 nm could able to retain their initial geometrical shapes
(namely triangular, hexagonal, rectangular and rhombohedral), but ~25 to 15 nm
size NCs become irregular or distorted geometrical shapes. NCs between ~15 to 5
nm become Zn-void-ZnO type, and smaller than ~5 nm become ZnO hollow sphere
type i.e. ZnO hollow NCs. In the second step, all Zn-void-ZnO and Zn-ZnO
(core-shell) structures are converted to hollow ZnO NCs in a slow and gradual
process, and the mechanism of conversion proceeds through expansion in size by
incorporating ZnO monomers inside the shell. The observed oxidation behaviour
of NCs is compared with theory of Cabrera - Mott on low-temperature oxidation
of metal.Comment: 9 pages, 8 figure
Benefits and barriers in the design of harmonized access agreements for international data sharing
In the past decade, there has been a surge in the number of sensitive human genomic and health datasets available to researchers via Data Access Agreements (DAAs) and managed by Data Access Committees (DACs). As this form of sharing increases, so do the challenges of achieving a reasonable level of data protection, particularly in the context of international data sharing. Here, we consider how excessive variation across DAAs can hinder these goals, and suggest a core set of clauses that could prove useful in future attempts to harmonize data governance
Pleomorphic adenoma arising from the tail of the parotid gland – value of preoperative multi planar imaging: a case report
MULTIVALUED FIXED POINT RESULTS AND STABILITY OF FIXED POINT SETS IN METRIC SPACES
In this paper we establish certain multivalued fixed point results for mappings satisfying rational type almost contractions involving a control function in the framework of metric spaces. The main result is supported with an example. We use Hausdorff distance in our theorems. We also study the stability of fixed point sets of above mentioned set valued contractions. By applications of the multivalued results we obtain certain fixed point theorems of singlevalued mappings
The role of growth atmosphere on the structural and optical quality of defect free ZnO films for strong ultraviolet emission
Highly c-axis oriented wurtzite structured ZnO thin films were deposited on silicon substrates
using pulsed laser deposition (PLD) by ablating a ZnO target in different atmospheres,
including vacuum, argon and oxygen in the deposition chamber. The stress in the films
was shown to vary from −3.83 to −0.03 GPa as a function of the chamber atmosphere. The
minimum compressive stress (−0.03 GPa) was observed for the oxygen atmosphere. X-ray
photoelectron spectroscopy data indicated that the O1s peak consists of three components
designated as O1 (due to ZnO), O2 (due to defects) and O3 (due to adsorbed species). A small
defect level emission was obtained in the luminescence spectra of the ZnO film deposited in
the oxygen atmosphere, while strong ultraviolet (UV) emission was observed for the ZnO
films deposited in the vacuum and argon atmosphere. These PLD grown ZnO thin films have
the potential to be used as sources of UV radiation in light emitting devices.The authors want to thank to the National Laser Centre, CSIR,
Pretoria for providing pulsed laser deposition technique. This
work is based on the research supported by the South African
Research Chairs Initiative of the Department of Science and
Technology, and the National Research Foundation of South
Africa. The PL system used in this study is supported both
technically and financially by the rental pool programme of
the National Laser Centre. The financial support from the
Cluster program of the University of the Free State is highly
recognized.Highly c-axis oriented wurtzite structured ZnO thin films were deposited on silicon substrates
using pulsed laser deposition (PLD) by ablating a ZnO target in different atmospheres,
including vacuum, argon and oxygen in the deposition chamber. The stress in the films
was shown to vary from −3.83 to −0.03 GPa as a function of the chamber atmosphere. The
minimum compressive stress (−0.03 GPa) was observed for the oxygen atmosphere. X-ray
photoelectron spectroscopy data indicated that the O1s peak consists of three components
designated as O1 (due to ZnO), O2 (due to defects) and O3 (due to adsorbed species). A small
defect level emission was obtained in the luminescence spectra of the ZnO film deposited in
the oxygen atmosphere, while strong ultraviolet (UV) emission was observed for the ZnO
films deposited in the vacuum and argon atmosphere. These PLD grown ZnO thin films have
the potential to be used as sources of UV radiation in light emitting devices
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