825 research outputs found
Testing Ecological Theory with Lianas
Lianas constitute a diverse polyphyletic plant group that is advancing our understanding of ecological theory. Specifically, lianas are providing new insights into the mechanisms that control plant distribution and diversity maintenance. For example, there is now evidence that a single, scalable mechanism may explain local, regional, and pan‐tropical distribution of lianas, as well as the maintenance of liana species diversity. The ability to outcompete trees under dry, stressful conditions in seasonal forests provides lianas a growth advantage that, over time, results in relatively high abundance in seasonal forests and low abundance in aseasonal forests. Lianas may also gain a similar growth advantage following disturbance, thus explaining why liana density and diversity peak following disturbance at the local, forest scale. The study of ecology, however, is more than the effect of the environment on organisms; it also includes the effects of organisms on the environment. Considerable empirical evidence now indicates that lianas substantially alter their environment by consuming resources, suppressing tree performance, and influencing emergent properties of forests, such as ecosystem functioning, plant and animal diversity, and community composition. These recent studies using lianas are transcending classical tropical ecology research and are now providing novel insights into fundamental ecological theory
Physical realization of coupled Hilbert-space mirrors for quantum-state engineering
Manipulation of superpositions of discrete quantum states has a mathematical
counterpart in the motion of a unit-length statevector in an N-dimensional
Hilbert space. Any such statevector motion can be regarded as a succession of
two-dimensional rotations. But the desired statevector change can also be
treated as a succession of reflections, the generalization of Householder
transformations. In multidimensional Hilbert space such reflection sequences
offer more efficient procedures for statevector manipulation than do sequences
of rotations. We here show how such reflections can be designed for a system
with two degenerate levels - a generalization of the traditional two-state atom
- that allows the construction of propagators for angular momentum states. We
use the Morris-Shore transformation to express the propagator in terms of
Morris-Shore basis states and Cayley-Klein parameters, which allows us to
connect properties of laser pulses to Hilbert-space motion. Under suitable
conditions on the couplings and the common detuning, the propagators within
each set of degenerate states represent products of generalized Householder
reflections, with orthogonal vectors. We propose physical realizations of this
novel geometrical object with resonant, near-resonant and far-off-resonant
laser pulses. We give several examples of implementations in real atoms or
molecules.Comment: 15 pages, 6 figure
On Glauber modes in Soft-Collinear Effective Theory
Gluon interactions involving spectator partons in collisions at hadronic
machines are investigated. We find a class of examples in which a mode, called
Glauber gluons, must be introduced to the effective theory for consistency.Comment: 19 pages, three figures. Uses JHEP3.cl
Looking back at superfluid helium
A few years after the discovery of Bose Einstein condensation in several
gases, it is interesting to look back at some properties of superfluid helium.
After a short historical review, I comment shortly on boiling and evaporation,
then on the role of rotons and vortices in the existence of a critical velocity
in superfluid helium. I finally discuss the existence of a condensate in a
liquid with strong interactions, and the pressure variation of its superfluid
transition temperature.Comment: Conference "Bose Einstein Condensation", Institut henri Poincare,
Paris, 29 march 200
Discovery of the Acoustic Faraday Effect in Superfluid 3He-B
We report the discovery of the acoustic Faraday effect in superfluid 3He-B.
The observation of this effect provides the first direct evidence for
propagating transverse acoustic waves in liquid 3He, a mode first predicted by
Landau in 1957. The Faraday rotation is large and observable because of
spontaneously broken spin-orbit symmetry in 3He-B. We compare the experimental
observations with a simulation of the transverse acoustic impedance that
includes the field-induced circular birefringence of transverse waves.Comment: 4 pages in RevTex plus 3 postscript figures; new version includes:
minor corrections to the text and an updated of list of reference
Type Ia Supernova Explosion Models
Because calibrated light curves of Type Ia supernovae have become a major
tool to determine the local expansion rate of the Universe and also its
geometrical structure, considerable attention has been given to models of these
events over the past couple of years. There are good reasons to believe that
perhaps most Type Ia supernovae are the explosions of white dwarfs that have
approached the Chandrasekhar mass, M_ch ~ 1.39 M_sun, and are disrupted by
thermonuclear fusion of carbon and oxygen. However, the mechanism whereby such
accreting carbon-oxygen white dwarfs explode continues to be uncertain. Recent
progress in modeling Type Ia supernovae as well as several of the still open
questions are addressed in this review. Although the main emphasis will be on
studies of the explosion mechanism itself and on the related physical
processes, including the physics of turbulent nuclear combustion in degenerate
stars, we also discuss observational constraints.Comment: 38 pages, 4 figures, Annual Review of Astronomy and Astrophysics, in
pres
Chern-Simons theory on L(p,q) lens spaces and Gopakumar-Vafa duality
We consider aspects of Chern-Simons theory on L(p,q) lens spaces and its
relation with matrix models and topological string theory on Calabi-Yau
threefolds, searching for possible new large N dualities via geometric
transition for non-SU(2) cyclic quotients of the conifold. To this aim we find,
on one hand, some novel matrix integral representations of the SU(N) CS
partition function in a generic flat background for the whole L(p,q) family and
provide a solution for its large N dynamics; on the other, we perform in full
detail the construction of a family of would-be dual closed string backgrounds
via conifold geometric transition from T^*L(p,q). We can then explicitly prove
that Gopakumar-Vafa duality in a fixed vacuum fails in the case q>1, and
briefly discuss how it could be restored in a non-perturbative setting.Comment: 17 pages, 6 figures; references adde
Vortices in (2+1)d Conformal Fluids
We study isolated, stationary, axially symmetric vortex solutions in
(2+1)-dimensional viscous conformal fluids. The equations describing them can
be brought to the form of three coupled first order ODEs for the radial and
rotational velocities and the temperature. They have a rich space of solutions
characterized by the radial energy and angular momentum fluxes. We do a
detailed study of the phases in the one-parameter family of solutions with no
energy flux. This parameter is the product of the asymptotic vorticity and
temperature. When it is large, the radial fluid velocity reaches the speed of
light at a finite inner radius. When it is below a critical value, the velocity
is everywhere bounded, but at the origin there is a discontinuity. We comment
on turbulence, potential gravity duals, non-viscous limits and non-relativistic
limits.Comment: 39 pages, 10 eps figures, v2: Minor changes, refs, preprint numbe
Combustion in thermonuclear supernova explosions
Type Ia supernovae are associated with thermonuclear explosions of white
dwarf stars. Combustion processes convert material in nuclear reactions and
release the energy required to explode the stars. At the same time, they
produce the radioactive species that power radiation and give rise to the
formation of the observables. Therefore, the physical mechanism of the
combustion processes, as reviewed here, is the key to understand these
astrophysical events. Theory establishes two distinct modes of propagation for
combustion fronts: subsonic deflagrations and supersonic detonations. Both are
assumed to play an important role in thermonuclear supernovae. The physical
nature and theoretical models of deflagrations and detonations are discussed
together with numerical implementations. A particular challenge arises due to
the wide range of spatial scales involved in these phenomena. Neither the
combustion waves nor their interaction with fluid flow and instabilities can be
directly resolved in simulations. Substantial modeling effort is required to
consistently capture such effects and the corresponding techniques are
discussed in detail. They form the basis of modern multidimensional
hydrodynamical simulations of thermonuclear supernova explosions. The problem
of deflagration-to-detonation transitions in thermonuclear supernova explosions
is briefly mentioned.Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 24 pages, 4 figure
Wall roughness induces asymptotic ultimate turbulence
Turbulence is omnipresent in Nature and technology, governing the transport
of heat, mass, and momentum on multiple scales. For real-world applications of
wall-bounded turbulence, the underlying surfaces are virtually always rough;
yet characterizing and understanding the effects of wall roughness for
turbulence remains a challenge, especially for rotating and thermally driven
turbulence. By combining extensive experiments and numerical simulations, here,
taking as example the paradigmatic Taylor-Couette system (the closed flow
between two independently rotating coaxial cylinders), we show how wall
roughness greatly enhances the overall transport properties and the
corresponding scaling exponents. If only one of the walls is rough, we reveal
that the bulk velocity is slaved to the rough side, due to the much stronger
coupling to that wall by the detaching flow structures. If both walls are
rough, the viscosity dependence is thoroughly eliminated in the boundary layers
and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of
transport, whose existence had been predicted by Robert Kraichnan in 1962
(Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be
extrapolated to arbitrarily large Reynolds numbers
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