72 research outputs found
Growth of (110) Diamond using pure Dicarbon
We use a density-functional based tight-binding method to study diamond
growth steps by depositing dicarbon species onto a hydrogen-free diamond (110)
surface. Subsequent C_2 molecules are deposited on an initially clean surface,
in the vicinity of a growing adsorbate cluster, and finally, near vacancies
just before completion of a full new monolayer. The preferred growth stages
arise from C_2n clusters in near ideal lattice positions forming zigzag chains
running along the [-110] direction parallel to the surface. The adsorption
energies are consistently exothermic by 8--10 eV per C_2, depending on the size
of the cluster. The deposition barriers for these processes are in the range of
0.0--0.6 eV. For deposition sites above C_2n clusters the adsorption energies
are smaller by 3 eV, but diffusion to more stable positions is feasible. We
also perform simulations of the diffusion of C_2 molecules on the surface in
the vicinity of existing adsorbate clusters using an augmented Lagrangian
penalty method. We find migration barriers in excess of 3 eV on the clean
surface, and 0.6--1.0 eV on top of graphene-like adsorbates. The barrier
heights and pathways indicate that the growth from gaseous dicarbons proceeds
either by direct adsorption onto clean sites or after migration on top of the
existing C_2n chains.Comment: 8 Pages, 7 figure
User-friendly tail bounds for sums of random matrices
This paper presents new probability inequalities for sums of independent,
random, self-adjoint matrices. These results place simple and easily verifiable
hypotheses on the summands, and they deliver strong conclusions about the
large-deviation behavior of the maximum eigenvalue of the sum. Tail bounds for
the norm of a sum of random rectangular matrices follow as an immediate
corollary. The proof techniques also yield some information about matrix-valued
martingales.
In other words, this paper provides noncommutative generalizations of the
classical bounds associated with the names Azuma, Bennett, Bernstein, Chernoff,
Hoeffding, and McDiarmid. The matrix inequalities promise the same diversity of
application, ease of use, and strength of conclusion that have made the scalar
inequalities so valuable.Comment: Current paper is the version of record. The material on Freedman's
inequality has been moved to a separate note; other martingale bounds are
described in Caltech ACM Report 2011-0
Anisotropic optical response of the diamond (111)-2x1 surface
The optical properties of the 21 reconstruction of the diamond (111)
surface are investigated. The electronic structure and optical properties of
the surface are studied using a microscopic tight-binding approach. We
calculate the dielectric response describing the surface region and investigate
the origin of the electronic transitions involving surface and bulk states. A
large anisotropy in the surface dielectric response appears as a consequence of
the asymmetric reconstruction on the surface plane, which gives rise to the
zigzag Pandey chains. The results are presented in terms of the reflectance
anisotropy and electron energy loss spectra. While our results are in good
agreement with available experimental data, additional experiments are proposed
in order to unambiguously determine the surface electronic structure of this
interesting surface.Comment: REVTEX manuscript with 6 postscript figures, all included in uu file.
Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.html Submitted to
Phys. Rev.
Influência do tratamento térmico no comportamento mecânico não-linear elástico da liga Ti-13Nb-13Zr
Suriname: Reconciling agricultural development and conservation of unique natural wealth
National and transboundary adverse effects of competition for land are being increasingly recognized by researchers and decision-makers, however the consideration of these impacts within national planning strategies is not yet commonplace. To estimate how increasing agricultural production can be conciliated with protection of natural resources at the national scale, we analyzed current land use in Suriname, and investigated opportunities for, and constraints to developing a sustainable agricultural sector. Suriname is a remarkable case study. To date, Suriname has retained most of its natural resources with forest areas covering over 90% of the country. Surinamese forests combine extremely high levels of both biodiversity and carbon, making them top priority from a global ecosystem services perspective. Among other national and international pressures from increased demand for agricultural products, the country is also considering significant expansion of agricultural output to both diminish imports and become a ‘bread basket’ for the Caribbean region, which collectively may pose risks to natural resources. In this study, combining locally-obtained primary data, expert consultation and secondary data from the Food and Agriculture Organization we analyzed a range of scenarios, we show the complexities associated with current land management and we discuss alternatives for developing a sustainable agricultural sector in Suriname. We show that Suriname can increase the production of rice, which is the most important agricultural activity in the country, without expanding rice area. Rather, future increase in rice production could be promoted through an increase in rice productivity, and the employment of more environmentally-favorable management methods, in order to both diminish pollution and avoid encroachment of the agriculture into pristine areas. Further, we show a potential to both contribute to greening of the agricultural sector and to higher economic returns through expanding the production of ‘safe food’ and through possible development of organic agriculture in Suriname. If Suriname develops a ‘greener’ agricultural sector, it may both increase economic returns from the agricultural sector and benefit from continuing protection of natural resources. Because most of Suriname forests present top levels of carbon and biodiversity, the country could benefit from so-called ‘early-action’ Reducing Emissions from Deforestation and Forest Degradation (REDD) finance, which is already being paid mostly through bilateral agreements. Further, by adopting land-use planning that protects natural resources, Suriname may be in extraordinary position to benefit from both improved-quality agricultural production and from incentives to conserve forest carbon and biodiversity, such as payments for ecosystem services. Given the high stakes and the severe lack of both primary data and applied analyses in Suriname, further research focused on better informing land-use policies would be a valuable investment for the country. Although this analysis was performed for Suriname, conclusions drawn here are transferrable and may assist formulation of policy recommendations for land use elsewhere
The micromechanics of fatigue crack growth at 25 °C in Ti-6Al-4V reinforced with SCS-6 fibers
Computational Modeling and Analysis of the Role of Physical Activity in Mood Regulation and Depression
Abstract. Physical activity is often considered an important factor in handling mood regulation and depression. This paper presents a computational model of this role of physical activity in mood regulation. It is shown on the one hand how a developing depression can go hand in hand with a low level of physical activity, and on the other hand, how Exercise Therapy is able to reverse this pattern and make the depression disappear. Simulation results are presented, and properties are formally verified against these simulation runs
Detection and Utilization of Emotional State for Disabled Users
In this paper, we present an experimental approach to design systems sensitive to emotion. We describe a system for the detection of emotional states based on physiological signals and an application use case utilizing the detected emotional state. The application is an emotion management system to be used for the support in the improvement of life conditions of users suffering from cerebral palsy (CP). The system presented here combines effectively biofeedback sensors and a set of software algorithms to detect the current emotional state of the user and to react to them appropriately
Bridging Mechanisms in a Tiβ21s/SCS-6 Composite: In-Situ Scanning Electron Microscopy Study
- …