699 research outputs found
Morphometric Analysis of Martian Valley Network Basins Using a Circularity Function
This paper employs a circularity function to quantify the internal morphology of
Martian watershed basins in Margaritifer Sinus region and to infer the primary erosional
processes that led to their current geomorphologic characteristics and possible climatic
conditions under which these processes operated. The circularity function describes the
elongation of a watershed basin at different elevations. We have used the circularity
functions of terrestrial basins that were interpreted as having been modified by (1) erosion
related to primarily groundwater sapping and (2) erosion related to primarily rainfall and
surface run-off, as well as the circularity functions of cratering basins on the Moon, in
order to formulate discriminant functions that are able to separate the three types of
landforms. The spatial pattern of the classification of Martian basins based on discriminant
functions shows that basins that look morphologically similar to terrestrial fluvial basins
are mostly clustered near the mainstream at low elevation, while those that look
morphologically similar to terrestrial basins interpreted as groundwater sapping origin are
located near the tributaries and at higher elevation. There are more of the latter than
the former. This spatial distribution is inconsistent with a continuous Earth-like warm and
wet climate for early Mars. Instead, it is more aligned with an overall early dry climate
punctuated with episodic wet periods. Alternatively, the concentrated erosion in the
mainstream could also be caused by a change of water source from rainfall to snowfall or
erosion cut through a duricrust layer
Computer simulation of the role of groundwater seepage in forming Martian valley networks
The role of groundwater in forming Martian valley networks is simulated in a
computer model as seepage erosion by contributing to surface runoff and as seepage
weathering by causing accelerated weathering of bedrock, which makes its subsequent
erosion and removal easier. Simulation results show that seepage erosion cannot mobilize
large grain size sediment and is marginally effective at generating integrated valley
networks with realistic rates of aquifer recharge. On the other hand, seepage weathering
may play a major role in forming Martian valley networks. Seepage weathering combined
with fluvial runoff creates stubby deep canyons with abrupt headwalls that are similar
in morphology to terrestrial and Martian valley systems attributed to erosion by
groundwater. Depending on the relative contribution of groundwater weathering to surface
runoff erosion, a continuum of valley network morphology can be generated. Eolian
modification masks the original differences in fluvial landforms, making different
scenarios visually more similar. Martian valley networks may have developed through a
range of combinations of runoff erosion and seepage weathering, which can complicate
the interpretation of the processes based on final landform morphology. Unequivocal
identification of seepage involvement of valley incision on Mars may not be possible
without knowledge of subsurface properties (hydraulic conductivity, layering, degree of
cementation, etc.) and the grain sizes of sediment transported through the valley systems
Effects of Br and Cl on organometallic vapor phase epitaxial growth and ordering in GaInP
Journal ArticleCuPt ordering in GaInP has significant effects on the electrical and optical properties. In fact, band gap reductions as large as 160 meV are potentially useful for devices. Thus, control of ordering is important. This has led to the investigation of surfactants that affect the surface processes during growth with little incorporation into the solid. A potentially interesting class of surfactant elements are those from group VII, that are known not to be incorporated during growth, but to change the dynamics of surface processes
Effects of surfactants Sb and Bi on the incorporation of zinc and carbon in III/V materials grown by organometallic vapor-phase epitaxy
Journal ArticleThe incorporation of both dopants and background impurities during the organometallic vapor phase epitaxial _x0002_OMVPE_x0003_ growth of GaAs, GaInP, and GaP has been significantly altered by the use of the surfactants Sb and Bi. Sb and Bi are isoelectronic with the group V host elements, and so produce no independent doping. This paper demonstrates that the incorporation of Zn can be increased by an order of magnitude in GaP to a value of approximately 1020 cm−3, the highest value reported to date. Additionally, these same surfactants lead to significant decreases in carbon contamination during growth. At high growth temperatures the reduction can be as large as 100_x0002_ in GaP. The surfactants also result in marked decreases in autodoping due to S and Si from the substrate. A marker that may help identify the mechanism for these effects is H incorporation. Both Sb and Bi surfactants give rise to increased concentrations of H in the GaP layers. Similar effects are observed in GaInP. However, in GaAs, no H is detected in the layers. One possible explanation for these phenomena is that Sb or Bi increases the surface H concentration. The increased H would have two possible effects on the incorporation of dopants and impurities. _x0002_1_x0003_ Passivation of the Zn acceptor by formation of a neutral Zn-H complex would lead to increased incorporation for thermodynamic reasons. _x0002_2_x0003_ Allowing increased desorption of C, S, and Si from the surface by increased formation of volatile hydrides leads to decreased incorporation levels. These results suggest a simple and effective method of controlling the incorporation of dopant and impurity atoms by adding a minute amount of surfactant during OMVPE growth
Note on the pseudo-Nambu-Goldstone Boson of Meta-stable SUSY Violation
Many models of meta-stable supersymmetry (SUSY) breaking lead to a very light
scalar pseudo-Nambu Goldstone boson (PNGB), P, associated with spontaneous
breakdown of a baryon number like symmetry in the hidden sector. Current
particle physics data provide no useful constraints on the existence of P. For
example, the predicted decay rates for both K --> pi + P, b--> s + P and
Upsilon --> photon + P are many orders of magnitude below the present
experimental bounds. We also consider astrophysical implications of the PNGB
and find a significant constraint from its effect on the evolution of red
giants. This constraint either rules out models with a hidden sector gauge
group larger than SU(4), or requires a new intermediate scale, of order at most
10^{10} GeV, at which the hidden sector baryon number is explicitly broken.Comment: 17 pages, 3 figures. Version 2: minor typographical errors fixed.
Version 3: a more reliable estimate for the decay rate of K-->pi+PNGB is
provided, and the predicted rate for b-->s+PNGB is now include
Cellular Models for River Networks
A cellular model introduced for the evolution of the fluvial landscape is
revisited using extensive numerical and scaling analyses. The basic network
shapes and their recurrence especially in the aggregation structure are then
addressed. The roles of boundary and initial conditions are carefully analyzed
as well as the key effect of quenched disorder embedded in random pinning of
the landscape surface. It is found that the above features strongly affect the
scaling behavior of key morphological quantities. In particular, we conclude
that randomly pinned regions (whose structural disorder bears much physical
meaning mimicking uneven landscape-forming rainfall events, geological
diversity or heterogeneity in surficial properties like vegetation, soil cover
or type) play a key role for the robust emergence of aggregation patterns
bearing much resemblance to real river networks.Comment: 7 pages, revtex style, 14 figure
Random Walks in Logarithmic and Power-Law Potentials, Nonuniversal Persistence, and Vortex Dynamics in the Two-Dimensional XY Model
The Langevin equation for a particle (`random walker') moving in
d-dimensional space under an attractive central force, and driven by a Gaussian
white noise, is considered for the case of a power-law force, F(r) = -
Ar^{-sigma}. The `persistence probability', P_0(t), that the particle has not
visited the origin up to time t, is calculated. For sigma > 1, the force is
asymptotically irrelevant (with respect to the noise), and the asymptotics of
P_0(t) are those of a free random walker. For sigma < 1, the noise is
(dangerously) irrelevant and the asymptotics of P_0(t) can be extracted from a
weak noise limit within a path-integral formalism. For the case sigma=1,
corresponding to a logarithmic potential, the noise is exactly marginal. In
this case, P_0(t) decays as a power-law, P_0(t) \sim t^{-theta}, with an
exponent theta that depends continuously on the ratio of the strength of the
potential to the strength of the noise. This case, with d=2, is relevant to the
annihilation dynamics of a vortex-antivortex pair in the two-dimensional XY
model. Although the noise is multiplicative in the latter case, the relevant
Langevin equation can be transformed to the standard form discussed in the
first part of the paper. The mean annihilation time for a pair initially
separated by r is given by t(r) \sim r^2 ln(r/a) where a is a microscopic
cut-off (the vortex core size). Implications for the nonequilibrium critical
dynamics of the system are discussed and compared to numerical simulation
results.Comment: 10 pages, 1 figur
Operator-free HPLC automated method development guided by Bayesian optimization
The need to efficiently develop high performance liquid chromatography (HPLC) methods, whilst adhering to quality by design principles is of paramount importance when it comes to impurity detection in the synthesis of active pharmaceutical ingredients. This study highlights a novel approach that fully automates HPLC method development using black-box single and multi-objective Bayesian optimization algorithms. Three continuous variables including the initial isocratic hold time, initial organic modifier concentration and the gradient time were adjusted to simultaneously optimize the number of peaks detected, the resolution between peaks and the method length. Two mixtures of analytes, one with seven compounds and one with eleven compounds, were investigated. The system explored the design space to find a global optimum in chromatogram quality without human assistance, and methods that gave baseline resolution were identified. Optimal operating conditions were typically reached within just 13 experiments. The single and multi-objective Bayesian optimization algorithms were compared to show that multi-objective optimization was more suitable for HPLC method development. This allowed for multiple chromatogram acceptance criteria to be selected without having to repeat the entire optimization, making it a useful tool for robustness testing. Work in this paper presents a fully “operator-free” and closed loop HPLC method optimization process that can find optimal methods quickly when compared to other modern HPLC optimization techniques such as design of experiments, linear solvent strength models or quantitative structure retention relationships
Persistence of a Continuous Stochastic Process with Discrete-Time Sampling: Non-Markov Processes
We consider the problem of `discrete-time persistence', which deals with the
zero-crossings of a continuous stochastic process, X(T), measured at discrete
times, T = n(\Delta T). For a Gaussian Stationary Process the persistence (no
crossing) probability decays as exp(-\theta_D T) = [\rho(a)]^n for large n,
where a = \exp[-(\Delta T)/2], and the discrete persistence exponent, \theta_D,
is given by \theta_D = \ln(\rho)/2\ln(a). Using the `Independent Interval
Approximation', we show how \theta_D varies with (\Delta T) for small (\Delta
T) and conclude that experimental measurements of persistence for smooth
processes, such as diffusion, are less sensitive to the effects of discrete
sampling than measurements of a randomly accelerated particle or random walker.
We extend the matrix method developed by us previously [Phys. Rev. E 64,
015151(R) (2001)] to determine \rho(a) for a two-dimensional random walk and
the one-dimensional random acceleration problem. We also consider `alternating
persistence', which corresponds to a < 0, and calculate \rho(a) for this case.Comment: 14 pages plus 8 figure
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