20,840 research outputs found
An alternative explanation for the density depletions observed by Freja and Viking satellites
In this paper, we have studied the linear and nonlinear propagation of ion acoustic waves in the presence of electrons that follow the generalized (r,q) distribution. It has been shown that for positive values of r, which correspond to a flat-topped electron velocity distribution, the nonlinear ion acoustic waves admit rarefactive solitary structures or density depletions. It has been shown that the generalized (r,q) distribution function provides another way to explicate the density depletions observed by Freja and Viking satellites previously explained by proposing Cairns distribution function.In this paper, we have studied the linear and nonlinear propagation of ion acoustic waves in the presence of electrons that follow the generalized (r,q) distribution. It has been shown that for positive values of r, which correspond to a flat-topped electron velocity distribution, the nonlinear ion acoustic waves admit rarefactive solitary structures or density depletions. It has been shown that the generalized (r,q) distribution function provides another way to explicate the density depletions observed by Freja and Viking satellites previously explained by proposing Cairns distribution function
Development of a three-dimensional time-dependent flow field model
A three-dimensional, time-dependent mathematical model to represent Mobile Bay was developed. Computer programs were developed which numerically solve the appropriate conservation equations for predicting bay and estuary flow fields. The model is useful for analyzing the dispersion of sea water into fresh water and the transport of sediment, and for relating field and physical model data
Decellularised cartilage ECM culture coatings drive rapid and robust chondrogenic differentiation of human periosteal cells
The control of cell behaviour in an effort to create highly homogeneous cultures is becoming an area of intense research, both to elucidate fundamental biology and for regenerative applications. The extracellular matrix (ECM) controls many cellular processes in vivo, and as such is a rich source of cues that may be translated in vitro. Herein, we describe the creation of cell culture coatings from porcine decellularised hyaline cartilage through enzymatic digestion. Surprisingly, heat-mediated sterilisation created a coating with the capacity to rapidly and robustly induce chondrogenic differentiation of human periosteal cells. This differentiation was validated through the alteration of cell phenotype from a fibroblastic to a cuboidal/cobblestone chondrocyte-like appearance. Moreover, chondrogenic gene expression further supported this observation, where cells cultured on heat sterilised ECM-coated plastic displayed higher expression of COL2A1, ACAN and PRG4 (p 0.05) compared to non-coated plastic cultures. Interestingly, COL2A1 and ACAN expression in this context were sensitive to initial cell density; however, SOX9 expression appeared to be mainly driven by the coating independent of seeding density. The creation of a highly chondrogenic coating may provide a cost-effective solution for the differentiation and/or expansion of human chondrocytes aimed towards cartilage repair strategies
Scaling and Formulary cross sections for ion-atom impact ionization
The values of ion-atom ionization cross sections are frequently needed for
many applications that utilize the propagation of fast ions through matter.
When experimental data and theoretical calculations are not available,
approximate formulas are frequently used. This paper briefly summarizes the
most important theoretical results and approaches to cross section calculations
in order to place the discussion in historical perspective and offer a concise
introduction to the topic. Based on experimental data and theoretical
predictions, a new fit for ionization cross sections is proposed. The range of
validity and accuracy of several frequently used approximations (classical
trajectory, the Born approximation, and so forth) are discussed using, as
examples, the ionization cross sections of hydrogen and helium atoms by various
fully stripped ions.Comment: 46 pages, 8 figure
Green Gasification Technology for Wet Biomass
The world now is facing two energy related threats which are lack of sustainable, secure and affordable energy supplies and the environmental damage acquired in producing and consuming ever-increasing amount of energy. In the first decade of the twenty-first century, increasing energy prices reminds us that an affordable energy plays an important role in economic growth and human development. To overcome the abovementioned problem, we cannot continue much longer to consume finite reserves of fossil fuels, the use of which contributes to global warming. Preferably, the world should move towards more sustainable energy sources such as wind energy, solar energy and biomass. However, the abovementioned challenges may not be met solely by introduction of sustainable energy forms. We also need to use energy more efficiently. Developing and introducing more efficient energy conversion technologies is therefore important, for fossil fuels as well as renewable fuels. This assignment addresses the question how biomass may be used more efficiently and economically than it is being used today. Wider use of biomass, a clean and renewable feedstock may extend the lifetime of our fossil fuels resources and alleviate global warming problems. Another advantage of using of biomass as a source of energy is to make developed countries less interdependent on oil-exporting countries, and thereby reduce political tension. Furthermore, the economies of agricultural regions growing energy crops benefit as new jobs are created.Keywords: energy, gasification, sustainable, wet biomas
Interplay of the exciton and electron-hole plasma recombination on the photoluminescence dynamics in bulk GaAs
We present a systematic study of the exciton/electron-hole plasma
photoluminescence dynamics in bulk GaAs for various lattice temperatures and
excitation densities. The competition between the exciton and electron-hole
pair recombination dominates the onset of the luminescence. We show that the
metal-to-insulator transition, induced by temperature and/or excitation
density, can be directly monitored by the carrier dynamics and the
time-resolved spectral characteristics of the light emission. The dependence on
carrier density of the photoluminescence rise time is strongly modified around
a lattice temperature of 49 K, corresponding to the exciton binding energy (4.2
meV). In a similar way, the rise-time dependence on lattice temperature
undergoes a relatively abrupt change at an excitation density of 120-180x10^15
cm^-3, which is about five times greater than the calculated Mott density in
GaAs taking into account many body corrections.Comment: 15 pages, 7 figures, submitted to Phys. Rev.
Kinetics of four-wave mixing for a 2D magneto-plasma in strong magnetic fields
We investigate the femtosecond kinetics of an optically excited 2D
magneto-plasma at intermediate and high densities under a strong magnetic field
perpendicular to the quantum well (QW). We assume an additional weak lateral
confinement which lifts the degeneracy of the Landau levels partially. We
calculate the femtosecond dephasing and relaxation kinetics of the laser pulse
excited magneto-plasma due to bare Coulomb potential scattering, because
screening is under these conditions of minor importance. In particular the
time-resolved and time-integrated four-wave mixing (FWM) signals are calculated
by taking into account three Landau subbands in both the valance and the
conduction band assuming an electron-hole symmetry. The FWM signals exhibit
quantum beats mainly with twice the cyclotron frequency. Contrary to general
expectations, we find no pronounced slowing down of the dephasing with
increasing magnetic field. On the contrary, one obtains a decreasing dephasing
time because of the increase of the Coulomb matrix elements and the number of
states in a given Landau subband. In the situation when the loss of scattering
channels exceeds these increasing effects, one gets a slight increase at the
dephasing time. However, details of the strongly modulated scattering kinetics
depend sensitively on the detuning, the plasma density, and the spectral pulse
width relative to the cyclotron frequency.Comment: 13 pages, in RevTex format, 10 figures, Phys. Rev B in pres
Biomimetic strategies for fracture repair: engineering the cell microenvironment for directed tissue formation
Complications resulting from impaired fracture healing have major clinical implications on fracture management strategies. Novel concepts taken from developmental biology have driven research strategies towards the elaboration of regenerative approaches that can truly harness the complex cellular events involved in tissue formation and repair. Advances in polymer technology and a better understanding of naturally derived scaffolds have given rise to novel biomaterials with an increasing ability to recapitulate native tissue environments. This coupled with advances in the understanding of stem cell biology and technology has opened new avenues for regenerative strategies with true clinical translatability. These advances have provided the impetus to develop alternative approaches to enhance the fracture repair process. We provide an update on these advances, with a focus on the development of novel biomimetic approaches for bone regeneration and their translational potential
Non-degenerate four-wave mixing in rubidium vapor: transient regime
We investigate the transient response of the generated light from Four-Wave
Mixing (FWM) in the diamond configuration using a step-down field excitation.
The transients show fast decay times and oscillations that depend on the
detunings and intensities of the fields. A simplified model taking into account
the thermal motion of the atoms, propagation, absorption and dispersion effects
shows qualitative agreement with the experimental observations with the energy
levels in rubidium (5S1/2, 5P1/2, 5P3/2 and 6S1/2). The atomic polarization
comes from all the contributions of different velocity classes of atoms in the
ensemble modifying dramatically the total transient behavior of the light from
FWM.Comment: 11 pages, 11 figures, to be published in Physical Review
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