51 research outputs found
Optical Control of Field-Emission Sites by Femtosecond Laser Pulses
We have investigated field emission patterns from a clean tungsten tip apex
induced by femtosecond laser pulses. Strongly asymmetric modulations of the
field emission intensity distributions are observed depending on the
polarization of the light and the laser incidence direction relative to the
azimuthal orientation of tip apex. In effect, we have realized an ultrafast
pulsed field-emission source with site selectivity on the 10 nm scale.
Simulations of local fields on the tip apex and of electron emission patterns
based on photo-excited nonequilibrium electron distributions explain our
observations quantitatively.Comment: 4 pages, submitted to Physical Review Letter
On the Nature of Black Holes in Loop Quantum Gravity
A genuine notion of black holes can only be obtained in the fundamental
framework of quantum gravity resolving the curvature singularities and giving
an account of the statistical mechanical, microscopic degrees of freedom able
to explain the black hole thermodynamical properties. As for all quantum
systems, a quantum realization of black holes requires an operator algebra of
the fundamental observables of the theory which is introduced in this study
based on aspects of loop quantum gravity. From the eigenvalue spectra of the
quantum operators for the black hole area, charge and angular momentum, it is
demonstrated that a strict bound on the extensive parameters, different from
the relation arising in classical general relativity, holds, implying that the
extremal black hole state can neither be measured nor can its existence be
proven. This is, as turns out, a result of the specific form of the chosen
angular momentum operator and the corresponding eigenvalue spectrum, or rather
the quantum measurement process of angular momentum. Quantum mechanical
considerations and the lowest, non-zero eigenvalue of the loop quantum gravity
black hole mass spectrum indicate, on the one hand, a physical Planck scale
cutoff of the Hawking temperature law and, on the other hand, give upper and
lower bounds on the numerical value of the Immirzi parameter. This analysis
provides an approximative description of the behavior and the nature of quantum
black holes
Polyhedra in loop quantum gravity
Interwiners are the building blocks of spin-network states. The space of
intertwiners is the quantization of a classical symplectic manifold introduced
by Kapovich and Millson. Here we show that a theorem by Minkowski allows us to
interpret generic configurations in this space as bounded convex polyhedra in
Euclidean space: a polyhedron is uniquely described by the areas and normals to
its faces. We provide a reconstruction of the geometry of the polyhedron: we
give formulas for the edge lengths, the volume and the adjacency of its faces.
At the quantum level, this correspondence allows us to identify an intertwiner
with the state of a quantum polyhedron, thus generalizing the notion of quantum
tetrahedron familiar in the loop quantum gravity literature. Moreover, coherent
intertwiners result to be peaked on the classical geometry of polyhedra. We
discuss the relevance of this result for loop quantum gravity. In particular,
coherent spin-network states with nodes of arbitrary valence represent a
collection of semiclassical polyhedra. Furthermore, we introduce an operator
that measures the volume of a quantum polyhedron and examine its relation with
the standard volume operator of loop quantum gravity. We also comment on the
semiclassical limit of spinfoams with non-simplicial graphs.Comment: 32 pages, many figures. v2 minor correction
Measurements of the reaction of antiproton annihilation at rest at three hydrogen target densities
The proton-antiproton annihilation at rest into the final state
was measured for three different target densities: liquid hydrogen, gaseous
hydrogen at NTP and at a low pressure of 5 mbar. The yield of this reaction in
the liquid hydrogen target is smaller than in the low-pressure gas target. The
branching ratios of the channel were calculated on the basis of
simultaneous analysis of the three data samples. The branching ratio for
annihilation into from the protonium state turns out to be
about ten times smaller as compared to the one from the state.Comment: 10 pages, 3 Postscript figures. Accepted by Physics Letters
Correlações entre ambientes baseadas na produtividade de híbridos de milho avaliados na safra e safrinha.
Asymptotic safety guaranteed
We study the ultraviolet behaviour of four-dimensional quantum field theories involving non-abelian gauge fields, fermions and scalars in the Veneziano limit. In a regime where asymptotic freedom is lost, we explain how the three types of fields cooperate to develop fully interacting ultraviolet fixed points, strictly controlled by perturbation theory. Extensions towards strong coupling and beyond the large-N limit are discussed
Depressão endogâmica e heterose de híbridos de populações F2 de milho no estado de São Paulo
Experiência de treinamento com Nintendo Wii sobre a funcionalidade, equilíbrio e qualidade de vida de idosas
Continuous catalyst-free production of esters from Jatropha curcas L. oil under supercritical ethanol
Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles
A continuing goal in catalysis is the transformation of processes from homogeneous to heterogeneous. To this end, nanoparticles represent a new frontier in heterogeneous catalysis, where this conversion is supplemented by the ability to obtain new or divergent reactivity and selectivity. We report a novel method for applying heterogeneous catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles. These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl{sub 2}, and catalyze a range of {pi}-bond activation reactions previously only homogeneously catalyzed. Multiple experimental methods are utilized to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously inaccessible in heterogeneous catalysis. Furthermore, our size and capping agent study revealed that Pt PAMAM dendrimer-capped nanoparticles demonstrate superior activity and recyclability compared to larger, polymer-capped analogues
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