285 research outputs found
Nonlocal Electrodynamics of Rotating Systems
The nonlocal electrodynamics of uniformly rotating systems is presented and
its predictions are discussed. In this case, due to paucity of experimental
data, the nonlocal theory cannot be directly confronted with observation at
present. The approach adopted here is therefore based on the correspondence
principle: the nonrelativistic quantum physics of electrons in circular
"orbits" is studied. The helicity dependence of the photoeffect from the
circular states of atomic hydrogen is explored as well as the resonant
absorption of a photon by an electron in a circular "orbit" about a uniform
magnetic field. Qualitative agreement of the predictions of the classical
nonlocal electrodynamics with quantum-mechanical results is demonstrated in the
correspondence regime.Comment: 23 pages, no figures, submitted for publicatio
Spontaneous emission from large quantum dots in nanostructures: exciton-photon interaction beyond the dipole approximation
We derive a rigorous theory of the interaction between photons and spatially
extended excitons confined in quantum dots in inhomogeneous photonic materials.
We show that, beyond the dipole approximation, the radiative decay rate is
proportional to a non-local interaction function, which describes the
interaction between light and spatially extended excitons. In this regime,
light and matter degrees of freedom cannot be separated and a complex interplay
between the nanostructured optical environment and the exciton envelope
function emerges. We illustrate this by specific examples and derive a series
of important analytical relations, which are useful for applying the formalism
to practical problems. In the dipole limit, the decay rate is proportional to
the projected local density of optical states and we obtain the strong and weak
confinement regimes as special cases.Comment: 14 pages, 4 figure
Mutual Coupling of two Semiconductor Quantum Dots via an Optical Nanocavity Mode
We present an experimental and theoretical study of a system consisting of
two spatially separated self-assembled InGaAs quantum dots strongly coupled to
a single optical nanocavity mode. Due to their different size and compositional
profiles, the two quantum dots exhibit markedly different DC Stark shifts. This
allows us to tune them into mutual resonance with each other and a photonic
crystal nanocavity mode as a bias voltage is varied. Photoluminescence
measurements show a characteristic triple peak during the double anticrossing,
which is a clear signature of a coherently coupled system of three quantum
states. We fit the entire set of emission spectra of the coupled system to
theory and are able to investigate the coupling between the two quantum dots
via the cavity mode, and the coupling between the two quantum dots when they
are detuned from the cavity mode. We suggest that the resulting quantum
V-system may be advantageous since dephasing due to incoherent losses from the
cavity mode can be avoided
Electrical control of spontaneous emission and strong coupling for a single quantum dot
We report the design, fabrication and optical investigation of electrically
tunable single quantum dot - photonic crystal defect nanocavities operating in
both the weak and strong coupling regimes of the light matter interaction.
Unlike previous studies where the dot-cavity spectral detuning was varied by
changing the lattice temperature, or by the adsorption of inert-gases at low
temperatures, we demonstrate that the quantum confined Stark effect can be
employed to quickly and reversibly switch the dot-cavity coupling simply by
varying a gate voltage. Our results show that exciton transitions from
individual dots can be tuned by ~4 meV relative to the nanocavity mode before
the emission quenches due to carrier tunneling escape. This range is much
larger than the typical linewidth of the high-Q cavity modes (~0.10 meV)
allowing us to explore and contrast regimes where the dots couple to the cavity
or decay by spontaneous emission into the 2D photonic bandgap. In the weak
coupling regime, we show that the dot spontaneous emission rate can be tuned
using a gate voltage, with Purcell factors >=7. New information is obtained on
the nature of the dot-cavity coupling in the weak coupling regime and
electrical control of zero dimensional polaritons is demonstrated for the
highest-Q cavities (Q>=12000). Vacuum Rabi splittings up to ~0.13 meV are
observed, much larger than the linewidths of either the decoupled exciton or
cavity mode. These observations represent a voltage switchable optical
non-linearity at the single photon level, paving the way towards on-chip dot
based nano-photonic devices that can be integrated with passive optical
components
Radiative recombination of bare Bi83+: Experiment versus theory
Electron-ion recombination of completely stripped Bi83+ was investigated at
the Experimental Storage Ring (ESR) of the GSI in Darmstadt. It was the first
experiment of this kind with a bare ion heavier than argon. Absolute
recombination rate coefficients have been measured for relative energies
between ions and electrons from 0 up to about 125 eV. In the energy range from
15 meV to 125 eV a very good agreement is found between the experimental result
and theory for radiative recombination (RR). However, below 15 meV the
experimental rate increasingly exceeds the RR calculation and at Erel = 0 eV it
is a factor of 5.2 above the expected value. For further investigation of this
enhancement phenomenon the electron density in the interaction region was set
to 1.6E6/cm3, 3.2E6/cm3 and 4.7E6/cm3. This variation had no significant
influence on the recombination rate. An additional variation of the magnetic
guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted
in periodic oscillations of the rate which are accompanied by considerable
changes of the transverse electron temperature.Comment: 12 pages, 14 figures, to be published in Phys. Rev. A, see also
http://www.gsi.de/ap/ and http://www.strz.uni-giessen.de/~k
Wacker-oxidation of Ethylene over Pillared Layered Material Catalysts
This paper concerns the Wacker oxidation of ethylene by oxygen in the presence of water over supported Pd/VOx catalysts. High surface area porous supports were obtained from layer-structured materials, such as, montmorillonite (MT), laponite (LT) (smectites), and hydrotalcite (layered double hydroxide, LDH) by pillaring. Before introduction of Pd, supports MT and LDH were pillared by vanadia. The laponite was used in titania-pillared form (TiO2-LAP) as support of Pd/VOx active component. Acetaldehyde (AcH), acetic acid (AcOH) and CO2 were the products with yields and selectivities, depending on the reaction conditions and the properties of the applied catalyst. Under comparable conditions the pillared smectite catalysts gave higher AcH yield than the pillared LDH catalyst. UV vis spectroscopic examination suggested that the pillared smectites contained polymeric chains of VO4, whereas only isolated monomeric VO4 species were present in the pillared LDH. The higher catalytic activity in the Wacker oxidation was attributed to the more favorable redox property of the polymeric than of the monomeric vanadia. The V3+ ions in the polymeric species can reduce O2 to O2- ions, whereas the obtained V5+ ions are ready to pass over O to Pd0 to generate PdO whereon the oxidation of the ethylene proceeds
Buried alive: Aquatic plants survive in âghost pondsâ under agricultural fields
The widespread loss of wetlands due to agricultural intensification has been highlighted as a major threat to aquatic biodiversity. However, all is not lost as we reveal that the propagules of some aquatic species could survive burial under agricultural fields in the sediments of âghost pondsâ - ponds in-filled during agricultural land consolidation. Our experiments showed at least eight aquatic macrophyte species to germinate from seeds and oospores, following 50â150 years of dormancy in the sediments of ghost ponds. This represents a significant proportion of the expected macrophyte diversity for local farmland ponds, which typically support between 6 and 14 macrophyte species. The rapid (< 6 months) re-colonisation of resurrected ghost ponds by a diverse aquatic vegetation similarly suggests a strong seed-bank influence. Ghost ponds represent abundant, dormant time capsules for aquatic species in agricultural landscapes around the globe, affording opportunities for enhancing landscape-scale aquatic biodiversity and connectivity. While reports of biodiversity loss through agricultural intensification dominate conservation narratives, our study offers a rare positive message, demonstrating that aquatic organisms survive prolonged burial under intensively managed agricultural fields. We urge conservationists and policy makers to consider utilizing and restoring these valuable resources in biodiversity conservation schemes and in agri-environmental approaches and policies
Sparse canonical correlation analysis for identifying, connecting and completing gene-expression networks
<p>Abstract</p> <p>Background</p> <p>We generalized penalized canonical correlation analysis for analyzing microarray gene-expression measurements for checking completeness of known metabolic pathways and identifying candidate genes for incorporation in the pathway. We used Wold's method for calculation of the canonical variates, and we applied ridge penalization to the regression of pathway genes on canonical variates of the non-pathway genes, and the elastic net to the regression of non-pathway genes on the canonical variates of the pathway genes.</p> <p>Results</p> <p>We performed a small simulation to illustrate the model's capability to identify new candidate genes to incorporate in the pathway: in our simulations it appeared that a gene was correctly identified if the correlation with the pathway genes was 0.3 or more. We applied the methods to a gene-expression microarray data set of 12, 209 genes measured in 45 patients with glioblastoma, and we considered genes to incorporate in the glioma-pathway: we identified more than 25 genes that correlated > 0.9 with canonical variates of the pathway genes.</p> <p>Conclusion</p> <p>We concluded that penalized canonical correlation analysis is a powerful tool to identify candidate genes in pathway analysis.</p
New AMS 14C dates track the arrival and spread of broomcorn millet cultivation and agricultural change in prehistoric Europe
Broomcorn millet (Panicum miliaceum L.) is not one of the founder crops domesticated in Southwest Asia in the early Holocene, but was domesticated in northeast China by 6000 bc. In Europe, millet was reported in Early Neolithic contexts formed by 6000 bc, but recent radiocarbon dating of a dozen 'early' grains cast doubt on these claims. Archaeobotanical evidence reveals that millet was common in Europe from the 2nd millennium bc, when major societal and economic transformations took place in the Bronze Age. We conducted an extensive programme of AMS-dating of charred broomcorn millet grains from 75 prehistoric sites in Europe. Our Bayesian model reveals that millet cultivation began in Europe at the earliest during the sixteenth century bc, and spread rapidly during the fifteenth/fourteenth centuries bc. Broomcorn millet succeeds in exceptionally wide range of growing conditions and completes its lifecycle in less than three summer months. Offering an additional harvest and thus surplus food/fodder, it likely was a transformative innovation in European prehistoric agriculture previously based mainly on (winter) cropping of wheat and barley. We provide a new, high-resolution chronological framework for this key agricultural development that likely contributed to far-reaching changes in lifestyle in late 2nd millennium bc Europe
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