847 research outputs found
Facile technique for the removal of metal contamination from graphene
Metal contamination deposited on few-layer graphene (3 ± 1 monolayers) grown on SiC(0001) was successfully removed from the surface, using low cost adhesive tape. More than 99% of deposited silver contamination was removed from the surface via peeling, causing minimal damage to the graphene. A small change in the adhesion of graphene to the SiC(0001) substrate was indicated by changes observed in pleat defects on the surface; however, atomic resolution images show the graphene lattice remains pristine. Thin layers of contamination deposited via an electron gun during Auger electron spectroscopy/low energy electron diffraction measurements were also found to be removable by this technique. This contamination showed similarities to “roughened” graphene previously reported in the literature
Globally Anisotropic High Porosity Silica Aerogels
We discuss two methods by which high porosity silica aerogels can be
engineered to exhibit global anisotropy. First, anisotropy can be introduced
with axial strain. In addition, intrinsic anisotropy can result during growth
and drying stages and, suitably controlled, it can be correlated with
preferential radial shrinkage in cylindrical samples. We have performed small
angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We
show that global anisotropy originating from either strain or shrinkage leads
to optical birefringence and that optical cross-polarization studies are a
useful characterization of the uniformity of the imposed global anisotropy.Comment: 18 pages, 14 figures, submitted to Journal of Non-Crystalline Solid
Sodium atoms and clusters on graphite: a density functional study
Sodium atoms and clusters (N<5) on graphite (0001) are studied using density
functional theory, pseudopotentials and periodic boundary conditions. A single
Na atom is observed to bind at a hollow site 2.45 A above the surface with an
adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates
a flat potential energy surface. Increased Na coverage results in a weak
adsorbate-substrate interaction, which is evident in the larger separation from
the surface in the cases of Na_3, Na_4, Na_5, and the (2x2) Na overlayer. The
binding is weak for Na_2, which has a full valence electron shell. The presence
of substrate modifies the structures of Na_3, Na_4, and Na_5 significantly, and
both Na_4 and Na_5 are distorted from planarity. The calculated formation
energies suggest that clustering of atoms is energetically favorable, and that
the open shell clusters (e.g. Na_3 and Na_5) can be more abundant on graphite
than in the gas phase. Analysis of the lateral charge density distributions of
Na and Na_3 shows a charge transfer of about 0.5 electrons in both cases.Comment: 20 pages, 6 figure
Minimal model for aeolian sand dunes
We present a minimal model for the formation and migration of aeolian sand
dunes. It combines a perturbative description of the turbulent wind velocity
field above the dune with a continuum saltation model that allows for
saturation transients in the sand flux. The latter are shown to provide the
characteristic length scale. The model can explain the origin of important
features of dunes, such as the formation of a slip face, the broken scale
invariance, and the existence of a minimum dune size. It also predicts the
longitudinal shape and aspect ratio of dunes and heaps, their migration
velocity and shape relaxation dynamics. Although the minimal model employs
non-local expressions for the wind shear stress as well as for the sand flux,
it is simple enough to serve as a very efficient tool for analytical and
numerical investigations and to open up the way to simulations of large scale
desert topographies.Comment: 19 pages, 22 figure
The fictitious force method for efficient calculation of vibration from a tunnel embedded in a multi-layered half-space
This paper presents an extension of the Pipe-in-Pipe (PiP) model for calculating vibrations from underground railways that allows for the incorporation of a multi-layered half-space geometry. The model is based on the assumption that the tunnel displacement is not influenced by the existence of a free surface or ground layers. The displacement at the tunnel–soil interface is calculated using a model of a tunnel embedded in a full space with soil properties corresponding to the soil in contact with the tunnel. Next, a full space model is used to determine the equivalent loads that produce the same displacements at the tunnel–soil interface. The soil displacements are calculated by multiplying these equivalent loads by Green׳s functions for a layered half-space. The results and the computation time of the proposed model are compared with those of an alternative coupled finite element–boundary element model that accounts for a tunnel embedded in a multi-layered half-space. While the overall response of the multi-layered half-space is well predicted, spatial shifts in the interference patterns are observed that result from the superposition of direct waves and waves reflected on the free surface and layer interfaces. The proposed model is much faster and can be run on a personal computer with much less use of memory. Therefore, it is a promising design tool to predict vibration from underground tunnels and to assess the performance of vibration countermeasures in an early design stage.Engineering and Physical Sciences Research Council - grant # [EP/K006665/1]
Kelyphite textures experimentally reproduced through garnet breakdown in the presence of a melt phase
Complex multiphase reaction rims that form during garnet breakdown are known as kelyphite coronae and are common amongst exhumed mantle xenoliths. It has long been established that a reaction of garnet and olivine produces kelyphite corona consisting of spinel and pyroxenes, and that preservation of high-pressure garnet cores requires sufficiently rapid uplift of material through the spinel lherzolite stability field from depths of at least 60 km.We present new high-pressure, high-temperature experiments of garnet breakdown in the spinel-lherzolite stability field demonstrating that a series of cascading reactions can reproduce the multilayer, multiphase kelyphites seen in nature. In all experiments where breakdown occurred, a melt appears to have moderated the reactions towards equilibrium; we believe this to be the first experimental confirmation of the importance of such melts in garnet breakdown reactions. In our experiments at least three distinct zones of concentric kelyphite growth can occur at a single pressure, temperature condition; we suggest, therefore, that such kelyphites seen in natural samples do not have to be caused by a multistage uplift path as is often assumed.Kelyphitic coronae surrounding garnet have previously been used to estimate uplift rates, however, the lack of kinetic data for relevant exhumation reactions has limited their use for PTt pathway estimations and the understanding of emplacement mechanisms. In order to constrain accurate PTt pathways we use reaction rim thickness as a proxy for reaction progress and present preliminary results for the kinetics of garnet breakdown
Relationship between dynamical heterogeneities and stretched exponential relaxation
We identify the dynamical heterogeneities as an essential prerequisite for
stretched exponential relaxation in dynamically frustrated systems. This
heterogeneity takes the form of ordered domains of finite but diverging
lifetime for particles in atomic or molecular systems, or spin states in
magnetic materials. At the onset of the dynamical heterogeneity, the
distribution of time intervals spent in such domains or traps becomes stretched
exponential at long time. We rigorously show that once this is the case, the
autocorrelation function of the renewal process formed by these time intervals
is also stretched exponential at long time.Comment: 8 pages, 4 figures, submitted to PR
Archaeology and Desertification in the Wadi Faynan: the Fourth (1999) Season of the Wadi Faynan Landscape Survey
Reproduced with permission of the publisher. © 2000 Council for British Research in the Levant. Details of the publication are available at: http://www.cbrl.org.uk/Publications/publications_default.shtmThis report describes the fourth season of fieldwork by an interdisciplinary team of archaeologists and geographers working together to reconstruct the landscape history of the Wadi Faynan in southern Jordan. The particular focus of the project is the long-term history of inter-relationships between landscape and people, as a contribution to the study of processes of desertification and environmental degradation. The 1999 fieldwork contributed significantly towards the five
Objectives defined for the final two field seasons of the project in 1999 and 2000: to map the archaeology outside the ancient field systems flooring the wadi that have formed the principal focus of the archaeological survey in the previous seasons; to use ethnoarchaeological studies both to reconstruct modern and recent land use and also to yield archaeological signatures of land use to
inform the analysis of the survey data; to complete the survey of ancient field systems and refine understanding of when and how they functioned; to complete the programme of geomorphological and palaeoecological fieldwork, and in particular to refine the chronology of climatic change and human impacts; and to complete the recording and classification of finds
Eutectic colony formation: A phase field study
Eutectic two-phase cells, also known as eutectic colonies, are commonly
observed during the solidification of ternary alloys when the composition is
close to a binary eutectic valley. In analogy with the solidification cells
formed in dilute binary alloys, colony formation is triggered by a
morphological instability of a macroscopically planar eutectic solidification
front due to the rejection by both solid phases of a ternary impurity that
diffuses in the liquid. Here we develop a phase-field model of a binary
eutectic with a dilute ternary impurity and we investigate by dynamical
simulations both the initial linear regime of this instability, and the
subsequent highly nonlinear evolution of the interface that leads to fully
developed two-phase cells with a spacing much larger than the lamellar spacing.
We find a good overall agreement with our recent linear stability analysis [M.
Plapp and A. Karma, Phys. Rev. E 60, 6865 (1999)], which predicts a
destabilization of the front by long-wavelength modes that may be stationary or
oscillatory. A fine comparison, however, reveals that the assumption commonly
attributed to Cahn that lamella grow perpendicular to the envelope of the
solidification front is weakly violated in the phase-field simulations. We show
that, even though weak, this violation has an important quantitative effect on
the stability properties of the eutectic front. We also investigate the
dynamics of fully developed colonies and find that the large-scale envelope of
the composite eutectic front does not converge to a steady state, but exhibits
cell elimination and tip-splitting events up to the largest times simulated.Comment: 18 pages, 18 EPS figures, RevTeX twocolumn, submitted to Phys. Rev.
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