8,050 research outputs found
Sand ripple volume generator for underwater acoustic models, a cellular automaton Monte-Carlo approach
Cellular automata have been successfully used to model the sand dynamics of aeolian dunes and ripples. The cellular automata Monte-Carlo model proposed in this paper expands the capabilities of cellular automata models to under water ripple formation introducing not a two dimensional matrix but two three dimensional volumes, being a sand volume and a water volume. The proposed model has the capability to generate optimal environmental data to input in other mathematical models in need of environmental data. The following enhancements were implemented: optional abstraction levels of the hydrodynamic behavior, morphological formation of underwater ripples under unilateral currents in any direction as well as morphological formation of underwater ripples under wave current interaction, grain size distribution of the sand in every time step in the entire volume and compaction distribution in every time step in the entire sediment volume. The proposed cellular automata model is a closed toroidal system. The toroidal approach of the model enables to build up infinite rippled surfaces by using the generated sediment volumes as tiles; this solves boundary problems in for example acoustic models. Using the fractal properties of the sand ripples, infinite surfaces containing rippled dunes can be generated
Coherent Control of Photocurrents in Graphene and Carbon Nanotubes
Coherent one photon () and two photon () electronic
excitations are studied for graphene sheets and for carbon nanotubes using a
long wavelength theory for the low energy electronic states. For graphene
sheets we find that coherent superposition of these excitations produces a
polar asymmetry in the momentum space distribution of the excited carriers with
an angular dependence which depends on the relative polarization and phases of
the incident fields. For semiconducting nanotubes we find a similar effect
which depends on the square of the semiconducting gap, and we calculate its
frequency dependence.
We find that the third order nonlinearity which controls the direction of the
photocurrent is robust for semiconducting t ubes and vanishes in the continuum
theory for conducting tubes. We calculate corrections to these results arising
from higher order crystal field effects on the band structure and briefly
discuss some applications of the theory.Comment: 12 pages in RevTex, 6 epsf figure
Unraveling of free carrier absorption for terahertz radiation in heterostructures
The relation between free carrier absorption and intersubband transitions in
semiconductor heterostructures is resolved by comparing a sequence of
structures. Our numerical and analytical results show how free carrier
absorption evolves from the intersubband transitions in the limit of an
infinite number of wells with vanishing barrier width. It is explicitly shown
that the integral of the absorption over frequency matches the value obtained
by the f-sum rule. This shows that a proper treatment of intersubband
transitions is fully sufficient to simulate the entire electronic absorption in
heterostructure THz devices.Comment: 6 pages, accepted by Physical Review
Remarks on the tight-binding model of graphene
We address a simple but fundamental issue arising in the study of graphene,
as well as of other systems that have a crystalline structure with more than
one atom per unit cell. For these systems, the choice of the tight-binding
basis is not unique. For monolayer graphene two bases are widely used in the
literature. While the expectation values of operators describing physical
quantities should be independent of basis, the form of the operators may depend
on the basis, especially in the presence of disorder or of an applied magnetic
field. Using the inappropriate form of certain operators may lead to erroneous
physical predictions. We discuss the two bases used to describe monolayer
graphene, as well as the form of the most commonly used operators in the two
bases. We repeat our analysis for the case of bilayer graphene.Comment: 15 pages, 4 figure
Optimal produktion af økologisk græsprotein til bioraffinering
JØRGEN ERIKSEN, professor, Institut for Agroøkologi på Aarhus Universitet, går i dybden med, om optimal produktion af græs til proteinfremstilling og om græsprotein er det mest bæredygtige valg
An algebraic proof of Bogomolov-Tian-Todorov theorem
We give a completely algebraic proof of the Bogomolov-Tian-Todorov theorem.
More precisely, we shall prove that if X is a smooth projective variety with
trivial canonical bundle defined over an algebraically closed field of
characteristic 0, then the L-infinity algebra governing infinitesimal
deformations of X is quasi-isomorphic to an abelian differential graded Lie
algebra.Comment: 20 pages, amspro
Paleoseismology of the Xorxol Segment of the Central Altyn Tagh Fault, Xinjiang, China
Although the Altyn Tagh Fault (ATF) is thought to play a key role in accommodating India-Eurasian convergence,
little is known about its earthquake history. Studies of this strike-slip fault are important for interpretation
of the role of faulting versus distributed deformation in the accommodation of the India- Eurasia collision.
In addition, the > 1200 km long fault represents one of the most important and exemplary intracontinental
strike-slip faults in the world. We mapped fault trace geometry and interpreted paleoseismic trench exposures to
characterize the seismogenic behavior of the ATF. We identified 2 geometric segment boundaries in a 270 km
long reach of the central ATF. These boundaries define the westernmost Wuzhunxiao, the Central Pingding, and
the easternmost Xorxol (also written as Suekuli or Suo erkuli) segments. In this paper, we present the results
from the Camel paleoseismic site along the Xorxol Segment at 91.759°E, 38.919°N. There evidence for the last
two earthquakes is clear and 14C dates from layers exposed in the excavation bracket their ages. The most recent
earthquake occurred between 1456 and 1775 cal A.D. and the penultimate event was between 60 and 980 cal
A.D. Combining the Camel interpretations with our published results for the central ATF, we conclude that multiple
earthquakes with shorter rupture lengths (?? 50 km) rather than complete rupture of the Xorxol Segment better
explain the paleoseismic data. We found 2-3 earthquakes in the last 2-3 kyr. When coupled with typical
amounts of slip per event (5-10 m), the recurrence times are tentatively consistent with 1-2 cm/yr slip rates. This
result favors models that consider the broader distribution of collisional deformation, rather than those with
northward motion of India into Asia absorbed along a few faults bounding rigid blocks
Dual channel self-oscillating optical magnetometer
We report on a two-channel magnetometer based on nonlinear magneto-optical
rotation in a Cs glass cell with buffer gas. The Cs atoms are optically pumped
and probed by free running diode lasers tuned to the D line. A wide
frequency modulation of the pump laser is used to produce both synchronous
Zeeman optical pumping and hyperfine repumping. The magnetometer works in an
unshielded environment and spurious signal from distant magnetic sources is
rejected by means of differential measurement. In this regime the magnetometer
simultaneously gives the magnetic field modulus and the field difference.
Rejection of the common-mode noise allows for high-resolution magnetometry
with a sensitivity of \pthz{2}. This sensitivity, in conjunction with long-term
stability and a large bandwidth, makes possible to detect water proton
magnetization and its free induction decay in a measurement volume of 5 cmComment: 13 pages, 9 figures. Improved version (v2). Accepted for publicatio
Coastal Management: A guide to using archaeological, palaeoenvironmental, historical and artistic resources
This ‘Guide’ has been produced as part of the project ‘Archaeology, art and coastal heritage: tools to support coastal management and climate change planning across the Channel Regional Sea’ (Arch-Manche). It details how data sources have been identified, ranked and analysed together to provide evidence of coastal change. Experiences of deploying a range of field investigation techniques to gather scientific data supporting understanding of past coastal change are detailed. The importance of this work in relation to coastal management is presented through a range of results from case studies within areas exhibiting different physical and geomorphological characteristics. The results demonstrate the asyet unrealised potential within archaeological, paleoenvironmental, historical and artistic resources to inform on the scale and pace of coastal change
Arch-Manche: Archaeology, Art and Coastal Heritage - tools to support coastal management and climate change planning across the Channel Regional Sea: Technical Report
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