39 research outputs found
Magnetic field dependence of the energy of negatively charged excitons in semiconductor quantum wells
A variational calculation of the spin-singlet and spin-triplet state of a
negatively charged exciton (trion) confined to a single quantum well and in the
presence of a perpendicular magnetic field is presented. We calculated the
probability density and the pair correlation function of the singlet and
triplet trion states. The dependence of the energy levels and of the binding
energy on the well width and on the magnetic field strength was investigated.
We compared our results with the available experimental data on GaAs/AlGaAs
quantum wells and find that in the low magnetic field region (B<18 T) the
observed transition are those of the singlet and the dark triplet trion (with
angular momentum ), while for high magnetic fields (B>25 T) the dark
trion becomes optically inactive and possibly a transition to a bright triplet
trion (angular momentum ) state is observed.Comment: 9 pages, 10 figures submitted to Phys. Rev.
Excitons and charged excitons in semiconductor quantum wells
A variational calculation of the ground-state energy of neutral excitons and
of positively and negatively charged excitons (trions) confined in a
single-quantum well is presented. We study the dependence of the correlation
energy and of the binding energy on the well width and on the hole mass. The
conditional probability distribution for positively and negatively charged
excitons is obtained, providing information on the correlation and the charge
distribution in the system. A comparison is made with available experimental
data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well
structures, which indicates that trions become localized with decreasing
quantum well width.Comment: 9 pages, 11 figure
Reversal of the Charge Transfer between Host and Dopant Atoms in Semiconductor Nanocrystals
We present ab initio density functional calculations that show P (Al) dopant
atoms in small hydrogen-terminated Si crystals to be negatively (positively)
charged. These signs of the dopant charges are reversed relative to the same
dopants in bulk Si. We predict this novel reversal of the dopant charge (and
electronic character of the doping) to occur at crystal sizes of order 100 Si
atoms. We explain it as a result of competition between fundamental principles
governing charge transfer in bulk semiconductors and molecules and predict it
to occur in nanocrystals of most semiconductors.Comment: 4 pages, 4 figures (3 in color), 2 table
The opposite of Dante's hell? The transfer of ideas for social housing at international congresses in the 1850sâ1860s
With the advent of industrialization, the question of developing adequate housing for the emergent working classes became more pressing than before. Moreover, the problem of unhygienic houses in industrial cities did not stop at the borders of a particular nation-state; sometimes literally as pandemic diseases spread out 'transnationally'. It is not a coincidence that in the nineteenth century the number of international congresses on hygiene and social topics expanded substantially. However, the historiography about social policy in general and social housing in particular, has often focused on individual cases because of the different pace of industrial and urban development and is thus dominated by national perspectives. In this paper, I elaborate on transnational exchange processes and local adaptations and transformations. I focus on the transfer of the housing model of SOMCO in Mulhouse, (a French house building association) during social international congresses. I examine whether cross-national networking enabled and facilitated the implementation of ideas on the local scale. I will elaborate on the transmission and the local adaptation of the Mulhouse-model in Belgium. Convergences, divergences, and different factors that influenced the local transformations (personal choice, political situation, socioeconomic circumstances) will be taken into accoun
Coexistence of Two Kinds of Fluorinated Hydrogenated Micelles as Building Blocks for the Design of Bimodal Mesoporous Silica with Two Ordered Mesopore Networks
Relation between the Lower Consolute Boundary and the Structure of Mesoporous Silica Materials
International audienc
Design of Ordered Bimodal Mesoporous Silica Materials by Using a Mixed FluorinatedâHydrogenated Surfactant-Based System
International audienc
Hydrophilic model drug delivery from concentrated reverse emulsions
Highly concentrated water-in-oil gel-emulsions have been prepared from five components: fluorocarbon, hydrocarbon, fluorinated and hydrogenated surfactants, and water. Whatever the ratio of fluorinated to hydrogenated compounds, the stability of these mixed emulsions allows them to be used for the transport of drugs. A series of model molecules solubilized in water have been chosen because of the modulation of the hydrophobicity of the probe molecule by varying the alkyl substitute length: methyl, ethyl, and propyl paraben. The release of these molecules has been studied with an automatic setup. The experiments take about 48 h to reach equilibrium regardless of the emulsion composition. Nevertheless some variations have been observed with respect to either the chosen probe molecule or the amount of hydrogenated and fluorinated compounds in the gel emulsion. In light of the results, diffusion takes place in two possible steps: (i) transfer from big droplets to the continuous micellar phase and (ii) transfer from the continuous oily phase to the aqueous receptor solution. The influence of these two steps is related to the molecular structure of the model drug on one hand and to its solubility both in the micellar oily phase and in the aqueous phase constituting the gel emulsion in the other hand.Peer reviewe