5,903 research outputs found
Simulating evolutionary responses of an introgressed insect resistance trait for ecological effect assessment of transgene flow: a model for supporting informed decisionmaking in environmental risk assessment
Predicting outcomes of transgene flow from arable crops requires a system perspective that considers ecological and evolutionary processes within a landscape context. In Europe, the arable weed Raphanus raphanistrum is a potential hybridization partner of oilseed rape, and the two species are ecologically linked through the common herbivores Meligethes spp. Observations in Switzerland show that high densities of Meligethes beetles maintained by oilseed rape crops can lead to considerable damage on R. raphanistrum. We asked how increased insect resistance in R. raphanistrum – as might be acquired through introgression from transgenic oilseed rape – would affect seed production under natural herbivore pressure. In simulation experiments, plants protected against Meligethes beetles produced about twice as many seeds as unprotected plants. All stages in the development of reproductive structures from buds to pods were negatively affected by the herbivore, with the transition from buds to flowers being the most vulnerable. We conclude that resistance to Meligethes beetles could confer a considerable selective advantage upon R. raphanistrum in regions where oilseed rape is widely grown
Observation of the ground-state-geometric phase in a Heisenberg XY model
Geometric phases play a central role in a variety of quantum phenomena,
especially in condensed matter physics. Recently, it was shown that this
fundamental concept exhibits a connection to quantum phase transitions where
the system undergoes a qualitative change in the ground state when a control
parameter in its Hamiltonian is varied. Here we report the first experimental
study using the geometric phase as a topological test of quantum transitions of
the ground state in a Heisenberg XY spin model. Using NMR interferometry, we
measure the geometric phase for different adiabatic circuits that do not pass
through points of degeneracy.Comment: manuscript (4 pages, 3 figures) + supporting online material (6 pages
+ 7 figures), to be published in Phys. Rev. Lett. (2010
Experimental Observation of a Topological Phase in the Maximally Entangled State of a Pair of Qubits
Quantum mechanical phase factors can be related to dynamical effects or to
the geometrical properties of a trajectory in a given space - either parameter
space or Hilbert space. Here, we experimentally investigate a quantum
mechanical phase factor that reflects the topology of the SO(3) group: since
rotations by around antiparallel axes are identical, this space is doubly
connected. Using pairs of nuclear spins in a maximally entangled state, we
subject one of the spins to a cyclic evolution. If the corresponding trajectory
in SO(3) can be smoothly deformed to a point, the quantum state at the end of
the trajectory is identical to the initial state. For all other trajectories
the quantum state changes sign
Aging Maxwell Constitutive Model for Concrete
Fully-hydrated concrete not involved in any reaction has been observed to exhibit agingcreep. Both the solidification theory and the dissolution-precipitation theory are incapable ofpredicting such a behaviour. The microprestress theory proposed for this purpose is based uponan ambiguous physical mechanism. In this paper, a constitutive model motivated by Drozdov'sadaptive link mechanism has been proposed. The model is capable of predicting aging creep,recovery and relaxation for linear elastic concrete subjected to diverse load histories andtemperatures. The theoretical significance of the proposed aging Maxwell model has been criticallyevaluated
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