Free induction signal from biexcitons and bound excitons

A theory of the free induction signal from biexcitons and bound excitons is presented. The simultaneous existence of the exciton continuum and a bound state is shown to result in a new type of time dependence of the free induction. The optically detected signal increases in time and oscillates with increasing amplitude until damped by radiative or dephasing processes. Radiative decay is anomalously fast and can result in strong picosecond pulses. The expanding area of a coherent exciton polarization (inflating antenna), produced by the exciting pulse, is the underlying physical mechanism. The developed formalism can be applied to different biexciton transients.Comment: RevTeX, 20 p. + 2 ps fig. To appear in Phys. Rev. B1

Towards improved cover glasses for photovoltaic devices

For the solar energy industry to increase its competitiveness there is a global drive to lower the cost of solar generated electricity. Photovoltaic (PV) module assembly is material-demanding and the cover glass constitutes a significant proportion of the cost. Currently, 3 mm thick glass is the predominant cover material for PV modules, accounting for 10-25% of the total cost. Here we review the state-of-the-art of cover glasses for PV modules and present our recent results for improvement of the glass. These improvements were demonstrated in terms of mechanical, chemical and optical properties by optimizing the glass composition, including addition of novel dopants, to produce cover glasses that can provide: (i) enhanced UV protection of polymeric PV module components, potentially increasing module service lifetimes; (ii) re-emission of a proportion of the absorbed UV photon energy as visible photons capable of being absorbed by the solar cells, thereby increasing PV module efficiencies; (iii) Successful laboratory-scale demonstration of proof-of-concept, with increases of 1-6% in Isc and 1-8% Ipm. Improvements in both chemical and crack resistance of the cover glass were also achieved through modest chemical reformulation, highlighting what may be achievable within existing manufacturing technology constraints

The Relationship Between Maternal Child Feeding Practices and Child\u27s BMI and Child\u27s Dietary Restraint in Mexican-American Families of Grand Rapids, MI

BACKGROUND: Research in Caucasian girls has shown a relationship between mother’s restrictive feeding practices and girl’s eating in absence of hunger and greater body mass index (BMI). Hispanic children disproportionately suffer from obesity, yet there are limited studies on this relationship. PURPOSE: To examine the relationship between maternal feeding practices (MFP) and child’s BMI; to explore effect of demographics and culture and child’s BMI. The hypothesis that children’s BMI is positively correlated with dietary restriction and monitoring; and negatively correlated with pressure to eat was tested. SUBJECTS: First or second-generation Mexican-American women and their children 5-15 years old (one child:mother pair per family; n=40) were recruited from churches in Grand Rapids, MI. METHODS & MATERIALS: Questionnaires were administered to mothers for demographic, acculturation (BAS) and child-feeding (CFQ) information. Mothers’ and children’s BMI (kg/m2) were calculated from measured height and weight. ANALYSES: Pearson correlation analyses were used to explore BAS, child’s BMI and MFP associations. ANOVA was used to determine the association between MFP, mother’s BMI and child’s BMI. RESULTS: Acculturation was not significantly associated with child’s BMI (p= 0.391). Maternal restriction (p=0.040) and monitoring (p=0.022) were positively associated with child’s BMI, while pressure to eat (p=0.022) was negatively associated with child’s BMI. These relationships were attenuated with mothers’ BMI in the model. CONCLUSION: The level of mothers’ acculturation was not associated with child’s BMI. MFP was associated with child’s BMI, however mother’s BMI had a greater influence. These findings suggest interventions in MFP may not impact on the childhood obesity in Hispanics

Temperature dependence of diffusion length, lifetime and minority electron mobility in GaInP

The article of record as published may be found at http://dx.doi.org/10.1063/1.4847635The mobility of electrons in double heterostructures of p-type Ga{sub 0.50}In{sub 0.50}P has been determined by measuring minority carrier diffusion length and lifetime. The minority electron mobility increases monotonically from 300K to 5K, limited primarily by optical phonon and alloy scattering. Comparison to majority electron mobility over the same temperature range in comparably doped samples shows a significant reduction in ionized impurity scattering at lower temperatures, due to differences in interaction of repulsive versus attractive carriers with ionized dopant sites. These results should be useful in modeling and optimization for multi-junction solar cells and other optoelectronic devices