104 research outputs found
Spin interactions and switching in vertically tunnel-coupled quantum dots
We determine the spin exchange coupling J between two electrons located in
two vertically tunnel-coupled quantum dots, and its variation when magnetic (B)
and electric (E) fields (both in-plane and perpendicular) are applied. We
predict a strong decrease of J as the in-plane B field is increased, mainly due
to orbital compression. Combined with the Zeeman splitting, this leads to a
singlet-triplet crossing, which can be observed as a pronounced jump in the
magnetization at in-plane fields of a few Tesla, and perpendicular fields of
the order of 10 Tesla for typical self-assembled dots. We use harmonic
potentials to model the confining of electrons, and calculate the exchange J
using the Heitler-London and Hund-Mulliken technique, including the long-range
Coulomb interaction. With our results we provide experimental criteria for the
distinction of singlet and triplet states and therefore for microscopic spin
measurements. In the case where dots of different sizes are coupled, we present
a simple method to switch on and off the spin coupling with exponential
sensitivity using an in-plane electric field. Switching the spin coupling is
essential for quantum computation using electronic spins as qubits.Comment: 13 pages, 9 figure
The ecology of sex explains patterns of helping in arthropod societies
Authors thank the Natural Sciences and Engineering Research Council of Canada (NGD), the Clarendon Fund (NGD) and the Natural Environment Research Council (LR, NE/K009516/1; AG, NE/K009524/1) for funding.Across arthropod societies, sib-rearing (e.g. nursing or nest defence) may be provided by females, by males or by both sexes. According to Hamilton's âhaplodiploidy hypothesisâ, this diversity reflects the relatedness consequences of diploid vs. haplodiploid inheritance. However, an alternative âpreadaptation hypothesisâ instead emphasises an interplay of ecology and the co-option of ancestral, sexually dimorphic traits for sib-rearing. The preadaptation hypothesis has recently received empirical support, but remains to be formalised. Here, we mathematically model the coevolution of sex-specific helping and sex allocation, contrasting these hypotheses. We find that ploidy per se has little effect. Rather, the ecology of sex shapes patterns of helping: sex-specific preadaptation strongly influences who helps; a freely adjustable sex ratio magnifies sex biases and promotes helping; and sib-mating, promiscuity, and reproductive autonomy also modulate the sex and abundance of helpers. An empirical survey reveals that patterns of sex-specific helping in arthropod taxa are consistent with the preadaptation hypothesis.Publisher PDFPeer reviewe
Species abundance distributions: moving beyond single prediction theories to integration within an ecological framework
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75247/1/j.1461-0248.2007.01094.x.pd
On the verge of Umdeutung in Minnesota: Van Vleck and the correspondence principle (Part One)
In October 1924, the Physical Review, a relatively minor journal at the time,
published a remarkable two-part paper by John H. Van Vleck, working in virtual
isolation at the University of Minnesota. Van Vleck combined advanced
techniques of classical mechanics with Bohr's correspondence principle and
Einstein's quantum theory of radiation to find quantum analogues of classical
expressions for the emission, absorption, and dispersion of radiation. For
modern readers Van Vleck's paper is much easier to follow than the famous paper
by Kramers and Heisenberg on dispersion theory, which covers similar terrain
and is widely credited to have led directly to Heisenberg's "Umdeutung" paper.
This makes Van Vleck's paper extremely valuable for the reconstruction of the
genesis of matrix mechanics. It also makes it tempting to ask why Van Vleck did
not take the next step and develop matrix mechanics himself.Comment: 82 page
Genetic structure among Charadrius plovers on the African mainland and islands of Madagascar and St Helena
Colonisation of islands by long distance dispersers has great impact on genetic diversification among populations and may spearhead speciation events. We investigated intraâ and interspecific divergence in Charadrius plovers with populations on mainland Africa, Madagascar and St Helena. We analysed microsatellite loci and sequence data from four nuclear and two mitochondrial gene regions. Charadrius plovers are shorebirds with high dispersal and mobility. Our results confirmed genetic differentiation between Madagascar and mainland populations of three plover species (Whiteâfronted Plovers, C. marginatus; Kittlitz's Plovers, C. pecuarius; and based on sequence data only, Threeâbanded Plovers, C. tricollaris) but highlight substantial variation in levels of intraspecific divergence among the three species. Namely, the Kittlitz's Plover, a dispersive habitat generalist with a polygamous mating system, exhibited lower islandâmainland differentiation (0.05% COI sequence divergence) than two monogamous species, the Whiteâfronted Plover (0.6% COI divergence) and Threeâbanded Plover (1.6% COI divergence). In addition, past colonisation of the islands of St Helena and Madagascar by ancestors of today's Kittlitz's Plover led to the evolution of two endemic island species, the Madagascar Plover C. thoracicus and the more closelyârelated St Helena Plover C. sanctaehelenae. We discuss the factors driving species differences in islandâmainland divergence, and highlight the importance of conserving genetically unique island populations and island habitats to safeâguard future evolutionary potential
The ecogenetic link between demography and evolution : can we bridge the gap between theory and data?
Calls to understand the links between ecology and evolution have been common for decades. Population dynamics, i.e. the demographic changes in populations, arise from life history decisions of individuals and thus are a product of selection, and selection, on the contrary, can be modified by such dynamical properties of the population as density and stability. It follows that generating predictions and testing them correctly requires considering this ecogenetic feedback loop whenever traits have demographic consequences, mediated via density dependence (or frequency dependence). This is not an easy challenge, and arguably theory has advanced at a greater pace than empirical research. However, theory would benefit from more interaction between related fields, as is evident in the many near-synonymous names that the ecogenetic loop has attracted. We also list encouraging examples where empiricists have shown feasible ways of addressing the question, ranging from advanced data analysis to experiments and comparative analyses of phylogenetic data
Closed-Form transformation between geodetic and ellipsoidal coordinates
We present formulas for direct closed-form transformation between geodetic coordinates(Ί, λ, h) and ellipsoidal coordinates (ÎČ, λ, u) for any oblate ellipsoid of revolution.These will be useful for those dealing with ellipsoidal representations of the Earth's gravityfield or other oblate ellipsoidal figures. The numerical stability of the transformations for nearpolarand near-equatorial regions is also considered
Rural waste generation: a geographical survey at local scale
"The paper examines the per capita waste generation rates from from rural areas of NeamÈ County (Romania) using thematic cartography. Geographical approach of this issue is difficult because the lack of a geostatistic database at commune scale. Spatial analysis of waste indicators reveals several disparities between localities. Comparability of data between communes located in various geographical conditions must be carrefully made according to local waste management systems. Several dysfunctionalities are outlined in order to compare these results, on the one hand, between localities and on the one hand, between recent years. Geographical analysis of waste generation rates is imperative for a proper monitoring of this sector. Data from 2009, 2010 and 2012 shows that rural waste management is in a full process of change towards a more organized, stable and efficient system." (author's abstract
True substrates: The exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces
Abstract: Rock outcrops of the sedimentaryâstratigraphic record often reveal bedding planes that can be considered to be true substrates: preserved surfaces that demonstrably existed at the sedimentâwater or sedimentâair interface at the time of deposition. These surfaces have high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Some true substrates are notable for their sedimentary, palaeontological and ichnological signatures that provide windows into key intervals of Earth history, but countless others occur routinely throughout the sedimentaryâstratigraphic record. They frequently reveal patterns that are strikingly familiar from modern sedimentary environments, such as ripple marks, animal trackways, raindrop impressions or mudcracks: all phenomena that are apparently ephemeral in modern settings, and which form on recognizably human timescales. This paper sets out to explain why these shortâterm, transient, smallâscale features are counterâintuitively abundant within a 3.8 billion yearâlong sedimentaryâstratigraphic record that is known to be inherently timeâincomplete. True substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information. Further, the intersection between the timeâlength scale of their formative processes and outcrop expressions can be used to explain why they are so frequently encountered in outcrop investigations. Explaining true substrates as inevitable and unexceptional byâproducts of the accrual of the sedimentaryâstratigraphic record should shift perspectives on what can be understood about Earth history from field studies of the sedimentaryâstratigraphic record. They should be recognized as providing highâdefinition information about the mundane day to day operation of ancient environments, and critically assuage the argument that the incomplete sedimentaryâstratigraphic record is unrepresentative of the geological past
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