Nominally forbidden transitions in the interband optical spectrum of quantum dots

We calculate the excitonic optical absorption spectra of (In,Ga)As/GaAs self-assembled quantum dots by adopting an atomistic pseudopotential approach to the single-particle problem followed by a configuration-interaction approach to the many-body problem. We find three types of allowed transitions that would be naively expected to be forbidden. (i) Transitions that are parity forbidden in simple effective mass models with infinite confining wells (e.g. 1S-2S, 1P-2P) but are possible by finite band-offsets and orbital-mixing effects; (ii) light-hole--to--conduction transitions, enabled by the confinement of light-hole states; and (iii) transitions that show and enhanced intensity due to electron-hole configuration mixing with allowed transitions. We compare these predictions with results of 8-band k.p calculations as well as recent spectroscopic data. Transitions in (i) and (ii) explain recently observed satellites of the allowed P-P transitions.Comment: Version published in Phys. Rev.

Dependence of the electronic structure of self-assembled InGaAs/GaAs quantum dots on height and composition

While electronic and spectroscopic properties of self-assembled In_{1-x}Ga_{x}As/GaAs dots depend on their shape, height and alloy compositions, these characteristics are often not known accurately from experiment. This creates a difficulty in comparing measured electronic and spectroscopic properties with calculated ones. Since simplified theoretical models (effective mass, k.p, parabolic models) do not fully convey the effects of shape, size and composition on the electronic and spectroscopic properties, we offer to bridge the gap by providing accurately calculated results as a function of the dot height and composition. Prominent results are the following. (i) Regardless of height and composition, the electron levels form shells of nearly degenerate states. In contrast, the hole levels form shells only in flat dots and near the highest hole level (HOMO). (ii) In alloy dots, the electrons' ``s-p'' splitting depends weakly on height, while the ``p-p'' splitting depends non-monotonically. In non-alloyed InAs/GaAs dots, both these splittings depend weakly on height. For holes in alloy dots, the ``s-p'' splitting decreases with increasing height, whereas the ``p-p'' splitting remains nearly unchaged. Shallow, non-alloyed dots have a ``s-p'' splitting of nearly the same magnitude, whereas the ``p-p'' splitting is larger. (iii) As height increases, the ``s'' and ``p'' character of the wavefunction of the HOMO becomes mixed, and so does the heavy- and light-hole character. (iv) In alloy dots, low-lying hole states are localized inside the dot. Remarkably, in non-alloyed InAs/GaAs dots these states become localized at the interface as height increases. This localization is driven by the biaxial strain present in the nanostructure.Comment: 14 pages, 12 figure

Modeling the Dispersion of Eastern Oyster Larvae (\u3ci\u3eCrassostrea virginica\u3c/i\u3e) and its Effects on the Movement of Disease Resistant Genes in the Delaware Bay Estuary

This study combines several models to address two primary research questions. How does the interaction of larval biology and environmental variability determine the spatial distribution of oyster larvae in Delaware Bay? What is the role of larval dispersion in the transference of disease-resistant genes? The particle-tracking module in the Regional Ocean Modeling System (ROMS) was converted into an Individual-Based model representing Eastern oyster larvae that has growth and vertical migration. Exchange of larvae between natural oyster reefs was estimated and used in an Individual-Based genetic model that simulates the genetic structure of eastern oysters. Particles were released from a number of reefs at several times and tracked until they reached a competent settlement size (330 µm). The simulated dispersal patterns showed that oyster larvae tend to drift down-estuary during the spawning season. The net result is that mixing of oyster larvae throughout Delaware Bay is extensive. Larval success is strongly affected by variability of temperature and salinity. Low temperature and salinity increases development times, which decreases the larval success. A stronger influence in the larval success is driven by salinity. The permanent salinity gradient in the estuary maintains an along estuary gradient in larval success. Larvae released in the upper bay populations encounter lower salinity than larvae release in the middle-lower bay populations. River discharge and spring-neap tides are the main forcing of the residual circulation. salinity and stratification in the Delaware estuary, playing an important role in the larval success and dispersion. Years with low success are related to large events of river discharge within the spawning season. Large river discharge also enhances the down-estuary dispersal pattern. Larvae released during spring tides are transported down-estuary to high salinity areas increasing the larval success of upper and middle bay reefs. The dominant. inflows in the subsurface layer and over the shoals during neap tides reduced the larval success by transporting larvae to low salinity areas. Thus, neap tides could be important in sustaining upper bay populations by increasing the export of larvae from middle to upper estuary populations. Nevertheless. the low exchange rates suggest that. this mechanism by itself can not completely explain the survival of upper estuary populations. The well-mixed conditions over most of the estuary maintain larvae distributed throughout the water column and overcome the effects of larval swimming behavior. The genetics simulations show larval dispersal might be important in the movement of disease-resistant genes from high (middle-lower bay) to low (upper bay) disease-resistant populations. The transference of the resistant trait will occur in periods of 5 to 100 years. The results of this research confirm that biophysical processes influence the dispersion pattern of oyster larvae, and thereby, the pattern of recruitment and genetic dispersal throughout Delaware Bay

Mating Patterns and Post-Mating Isolation in Three Cryptic Species of the Engystomops Petersi Species Complex

Determining the extent of reproductive isolation in cryptic species with dynamic geographic ranges can yield important insights into the processes that generate and maintain genetic divergence in the absence of severe geographic barriers. We studied mating patterns, propensity to hybridize in nature and subsequent fertilization rates, as well as survival and development of hybrid F1 offspring for three nominal species of the Engystomops petersi species complex in Yasuní National Park, Ecuador. We found at least two species in four out of six locations sampled, and 14.3% of the wild pairs genotyped were mixed-species (heterospecific) crosses. We also found reduced fertilization rates in hybrid crosses between E. petersi females and E. “magnus” males, and between E. “magnus” females and E. “selva” males but not in the reciprocal crosses, suggesting asymmetric reproductive isolation for these species. Larval development times decreased in F1 hybrid crosses compared to same species (conspecific) crosses, but we did not find significant reduction in larval survival or early metamorph survival. Our results show evidence of post-mating isolation for at least two hybrid crosses of the cryptic species we studied. The general decrease in fertilization rates in heterospecific crosses suggests that sexual selection and reinforcement might have not only contributed to the pattern of call variation and behavioral isolation we see between species today, but they may also contribute to further signal divergence and behavioral evolution, especially in locations where hybridization is common and fertilization success is diminished

Mating Patterns and Post-Mating Isolation in Three Cryptic Species of the Engystomops Petersi Species Complex

Determining the extent of reproductive isolation in cryptic species with dynamic geographic ranges can yield important insights into the processes that generate and maintain genetic divergence in the absence of severe geographic barriers. We studied mating patterns, propensity to hybridize in nature and subsequent fertilization rates, as well as survival and development of hybrid F1 offspring for three nominal species of the Engystomops petersi species complex in Yasuní National Park, Ecuador. We found at least two species in four out of six locations sampled, and 14.3% of the wild pairs genotyped were mixed-species (heterospecific) crosses. We also found reduced fertilization rates in hybrid crosses between E. petersi females and E. “magnus” males, and between E. “magnus” females and E. “selva” males but not in the reciprocal crosses, suggesting asymmetric reproductive isolation for these species. Larval development times decreased in F1 hybrid crosses compared to same species (conspecific) crosses, but we did not find significant reduction in larval survival or early metamorph survival. Our results show evidence of post-mating isolation for at least two hybrid crosses of the cryptic species we studied. The general decrease in fertilization rates in heterospecific crosses suggests that sexual selection and reinforcement might have not only contributed to the pattern of call variation and behavioral isolation we see between species today, but they may also contribute to further signal divergence and behavioral evolution, especially in locations where hybridization is common and fertilization success is diminished

Electronic structure of self-assembled InAs/InP quantum dots: A Comparison with self-assembled InAs/GaAs quantum dots

We investigate the electronic structure of the InAs/InP quantum dots using an atomistic pseudopotential method and compare them to those of the InAs/GaAs QDs. We show that even though the InAs/InP and InAs/GaAs dots have the same dot material, their electronic structure differ significantly in certain aspects, especially for holes: (i) The hole levels have a much larger energy spacing in the InAs/InP dots than in the InAs/GaAs dots of corresponding size. (ii) Furthermore, in contrast with the InAs/GaAs dots, where the sizeable hole $p$, $d$ intra-shell level splitting smashes the energy level shell structure, the InAs/InP QDs have a well defined energy level shell structure with small $p$, $d$ level splitting, for holes. (iii) The fundamental exciton energies of the InAs/InP dots are calculated to be around 0.8 eV ($\sim$ 1.55 $\mu$m), about 200 meV lower than those of typical InAs/GaAs QDs, mainly due to the smaller lattice mismatch in the InAs/InP dots. (iii) The widths of the exciton $P$ shell and $D$ shell are much narrower in the InAs/InP dots than in the InAs/GaAs dots. (iv) The InAs/GaAs and InAs/InP dots have a reversed light polarization anisotropy along the [100] and [1$\bar{1}$0] directions

The architectural adaptation of urban economic life: Location, use and form of the commercial-residential building in Cardiff

Revisiting Jane Jacob's notion of locality knowledge, this paper argues that combining commercial space and dwelling fosters social, economic and architectural processes that come about by factors of local urban economies. The mixing of uses merges the relation of 'what one does' and 'where one lives' in a particular building whereby urban and architectural scale effects come into place. Comparisons of commercial-residential buildings in two local districts of contrasting morphologies in the city of Cardiff are studied in the context of their urban-architectural design scales. From an urban scale analysis, attention is given to the distribution of commercial-residential buildings in relation to spatial centrality; from an architectural perspective, it examines the way residential building adapts commercial additions, defining how different functions associate distinctive adaptable typologies depending on the building's urban location. By using syntactical and morphological approaches, the paper combines Depth Distance analysis with patterns of use and building form, drawing two reportable findings: The identification of corner shops located within one turn of direction from main high streets within gridiron urban forms, while activities combining retail or local office businesses with residential functions are located in corner blocks along streets within radial urban morphologies. These spatial attributes of location combine the adaptability of local property markets to mixed use with advantages in accessibility to produce an urban building that can flexibly accommodate innovation that is both a reflection of new skills and knowledge contributing to a local diversity