Suppression of nuclear spin diffusion at a GaAs/AlGaAs interface measured with a single quantum dot nano-probe

Nuclear spin polarization dynamics are measured in optically pumped individual GaAs/AlGaAs interface quantum dots by detecting the time-dependence of the Overhauser shift in photoluminescence (PL) spectra. Long nuclear polarization decay times of ~ 1 minute have been found indicating inefficient nuclear spin diffusion from the GaAs dot into the surrounding AlGaAs matrix in externally applied magnetic field. A spin diffusion coefficient two orders lower than that previously found in bulk GaAs is deduced.Comment: 5 pages, 3 figures, submitted to Phys Rev

A lattice gas model of II-VI(001) semiconductor surfaces

We introduce an anisotropic two-dimensional lattice gas model of metal terminated II-IV(001) seminconductor surfaces. Important properties of this class of materials are represented by effective NN and NNN interactions, which result in the competition of two vacancy structures on the surface. We demonstrate that the experimentally observed c(2x2)-(2x1) transition of the CdTe(001) surface can be understood as a phase transition in thermal equilbrium. The model is studied by means of transfer matrix and Monte Carlo techniques. The analysis shows that the small energy difference of the competing reconstructions determines to a large extent the nature of the different phases. Possible implications for further experimental research are discussed.Comment: 7 pages, 2 figure

Toxoplasma gondii regulates recruitment and migration of human dendritic cells via different soluble secreted factors

We investigated in vitro the properties of soluble factors produced by Toxoplasma gondii on the recruitment, maturation and migration of human dendritic cells (DC) derived from CD34(+) progenitor cells. We used soluble factors including excreted secreted antigens (ESA) produced under various conditions by the virulent type I RH strain (ESA-RH) and the less virulent PRU type II strain (ESA-PRU). Soluble factors of both T. gondii strains appeared to possess a chemokine-like activity that attracted immature DC. This recruitment activity required the presence of functional CCR5 molecules on the cell membrane. Incubation of DC for 24 h with ESA triggered the migration of a large percentage of these cells towards the chemokine MIP-3β; ESA-PRU was more efficient than ESA-RH. ESA produced in absence of exogenous protein and crude extract did not induce DC migration but retained recruitment activity. These data indicate that recruitment activity and migration-inducing activity are not governed by the same factors. Moreover, incubation of DC for 48 h with ESA did not modify the expression of costimulation or maturation markers (CD83, CD40, CD80, CD86 or HLA-DR), but induced a decrease in CCR6 expression associated with an increased expression of CCR7. Taken together, these results suggest that T. gondii controls recruitment and migration of immature DC by different soluble factors and may induce a dysfunction in the host-specific immune response