Spectral diffusion of single semiconductor nanocrystals: the influence of the dielectric environment

We have explored the influence of different matrices on the emission line shape of individual homogeneously coated CdSe/CdS/ZnS nanocrystals. The results obtained corroborate previous observations of a correlation between blinking events and spectral diffusion but in addition we have found that the extent of spectral diffusion is almost independent of the dielectric environment of the NC. Additionally, we report the observation of a correlation between the line width and emission energy which is not expected to occur in the spherical - symmetric NCs employed in this work. The implications of these results are discussed.Comment: 3 pages, 3 figure

On a possible origin for the lack of old star clusters in the Small Magellanic Cloud

We model the dynamical interaction between the Small and Large Magellanic Clouds and their corresponding stellar cluster populations. Our goal is to explore whether the lack of old clusters ($\gtrsim 7$ Gyr) in the Small Magellanic Cloud (SMC) can be the result of the capture of clusters by the Large Magellanic Cloud (LMC), as well as their ejection due to the tidal interaction between the two galaxies. For this purpose we perform a suite of numerical simulations probing a wide range of parameters for the orbit of the SMC about the LMC. We find that, for orbital eccentricities $e \geq 0.4$, approximately 15 per cent of the SMC clusters are captured by the LMC. In addition, another 20 to 50 per cent of its clusters are ejected into the intergalactic medium. In general, the clusters lost by the SMC are the less tightly bound cluster population. The final LMC cluster distribution shows a spatial segregation between clusters that originally belonged to the LMC and those that were captured from the SMC. Clusters that originally belonged to the SMC are more likely to be found in the outskirts of the LMC. Within this scenario it is possible to interpret the difference observed between the star field and cluster SMC Age-Metallicity Relationships for ages $\gtrsim 7$ Gyr.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter

Scaling law for the heating of solar coronal loops

We report preliminary results from a series of numerical simulations of the reduced magnetohydrodynamic equations, used to describe the dynamics of magnetic loops in active regions of the solar corona. A stationary velocity field is applied at the photospheric boundaries to imitate the driving action of granule motions. A turbulent stationary regime is reached, characterized by a broadband power spectrum $E_k\simeq k^{-3/2}$ and heating rate levels compatible with the heating requirements of active region loops. A dimensional analysis of the equations indicates that their solutions are determined by two dimensionless parameters: the Reynolds number and the ratio between the Alfven time and the photospheric turnover time. From a series of simulations for different values of this ratio, we determine how the heating rate scales with the physical parameters of the problem, which might be useful for an observational test of this model.Comment: 12 pages, 4 figures. Astrophysical Journal Letters (in press

Composition of Kinetic Momenta: The U_q(sl(2)) case

The tensor products of (restricted and unrestricted) finite dimensional irreducible representations of \uq are considered for $q$ a root of unity. They are decomposed into direct sums of irreducible and/or indecomposable representations.Comment: 27 pages, harvmac and tables macros needed, minor TeXnical revision to allow automatic TeXin

Radiocarbon reservoir ages and hardwater effect for the northeastern coastal waters of Argentina

Accelerator mass spectrometry (AMS) radiocarbon dates were obtained for 18 mollusk shells collected alive along the Buenos Aires province coast, Argentina, over the period AD 1914-1935. Reservoir ages were estimated for all samples on the basis of the tree-ring calibration curve for the Southern Hemisphere (SHCal04, McCormac et al. 2004) and the marine ΔR values calculated as the difference between the conventional 14C age and the age deduced from the marine, mixed-layer model calculation (Marine04, Hughen et al. 2004). For most coastal locations, a great ΔR scatter was observed, ranging from 191 to 2482 yr, which is explained by the input of varying content of dissolved carbonate by rivers and groundwater ("hardwater effect") and indicates a serious limitation for shell-based 14C chronologies. Within the interior of Bahía Blanca estuary, ΔR values ranged from -40 to 50 ± 46 as a consequence of the local geological particularities of the environment. This suggests that, with some restrictions, the marine calibration curve with standard parameters (ΔR = 0) could be used at this location.Facultad de Ciencias Naturales y Muse

Mechanical behavior and microstructural changes in polyurethane exposed to high doses of X rays, gamma rays or neutron irradiation

Polymeric materials are widely used in applications where the environmental conditions entail their exposure to different sources of irradiation (in most cases ultraviolet or low dose of electromagnetic irradiation for sterilization). In contrast, in this study we have assessed the modifications undergone by a series of polyurethane joints exposed to high radioactive doses of either X-rays or gamma rays (with doses of 20.5, 100, 300 and 900?kGy) or neutron irradiation (with a fluence of 7.23·1010 n/cm2) which are typical of the environment of nuclear reactors. Tensile tests were carried out to assess the change in mechanical properties derived from the radioactive exposure. Three mechanical parameters were used to monitor the evolution of strength, ductility and toughness: the tensile strength (?max), the strain corresponding to ?max (??max) and the density of energy absorbed prior to maximum load (U?max). With regards to X and gamma rays, a negative impact of radiation on strength, ductility and toughness was observed. The detailed statistical analysis of the results has shown that a threshold dose of 300?kGy must be overcome to trigger the damage process. For the fluence employed in this study, neutron irradiation produced very little change in the mechanical properties. The SEM fractographic study has allowed the influence of irradiation on the material failure mechanisms to be identified. Thus, the fracture surface of unirradiated samples shows evidence of plastic deformation and ductile tearing. In contrast, the fracture surface of those samples exposed to a dose of 900?kGy corresponds to brittle fracture. In a consistent way, samples exposed to neutron irradiation have a fracture surface similar to that of the non-irradiated material. In summary, electromagnetic radiation for doses above the threshold leads to the embrittlement of polyurethane. Raman spectroscopy was employed to identify the microstructural changes induced by the different sources of radiation at the molecular level. The band corresponding to the vibration of the C-H bending bonds present in the polyurethane was measured as a function of the dose, finding a strong correlation between its vibration frequency and the dose of exposure to electromagnetic radiation. This shift is more sensitive than the mechanical material response since the frequency is affected at doses of 100?kGy, below the threshold previously identified for any of the mechanical properties. This correlation opens the door for the use of Raman spectroscopy as a novel non-destructive tool to characterize the microstructural effect of irradiation on polyurethane

Pulse quality analysis on soliton pulse compression and soliton self-frequency shift in a hollow-core photonic bandgap fiber

A numerical investigation of low-order soliton evolution in a proposed seven-cell hollow-core photonic bandgap fiber is reported. In the numerical simulation, we analyze the pulse quality evolution in soliton pulse compression and soliton self-frequency shift in three fiber structures with different cross-section sizes. In the simulation, we consider unchirped soliton pulses (of 400 fs) at the wavelength of 1060 nm. Our numerical results show that the seven-cell hollow-core photonic crystal fiber, with a cross-section size reduction of 2%, promotes the pulse quality on the soliton pulse compression and soliton self-frequency shift. For an input soliton pulse of order 3 (which corresponds to an energy of 1.69 μJ), the pulse gets compressed with a factor of up to 5.5 and a quality factor of 0.73, in a distance of 12 cm. It also experiences a soliton-self frequency shift of up to 28 nm, in a propagation length of 6 m, with a pulse shape quality of ≈ 0.80

When Does Fertility End? The Timing of Tubal Ligations and Hysterectomies, and the Meaning of Menopause

We applied a biocultural lens to examine the temporal order of biological, behavioral, and medical events related to fertility across the female reproductive lifespan in three sites, two in Mexico and one in the United States. Using a mixed-method design, we expanded our thinking about the end of fertility in order to examine the timing of hysterectomies and tubal ligations. We discovered that menopause is not the end of fertility for a surprisingly high number of women. Across the three sites, between 43% and 50% of women underwent tubal ligations at mean ages of 32 years (in Campeche, Mexico) and 33 years (Puebla, Mexico). In Puebla, 23% had a history of hysterectomy at a mean age of 42 years, similar to Hilo, Hawaii, where 20% had undergone a hysterectomy at a mean age of 40 years. We hypothesized that women who underwent tubal ligations would less frequently describe menopause as the end of fertility. This was true in Hilo, Hawaii, where women with a history of tubal ligation were almost half as likely to choose loss of fertility to describe menopause. However, in urban and rural Campeche, Mexico, there was no indication - from either quantitative or qualitative responses - that individuals with a history of tubal ligation or hysterectomy were less likely to describe menopause as the end of fertility