1,462 research outputs found
Zero-temperature phase diagram of the second layer of He adsorbed on graphene
The phase diagram at zero temperature of He adsorbed on an helium
incommensurate triangular solid on top of a single graphene sheet has been
obtained using the diffusion Monte Carlo method. We have found that, in
accordance with previous experimental and simulation results for graphite, the
ground state of He on this setup is a liquid that, upon compression,
transforms into a triangular solid. To define the stability limits of both
liquid and solid phases, we considered not only the adsorption energies of the
atoms located on the second layer but the average energy of the atoms in both
layers. Our results show that the lower density limit for a stable liquid in
the second layer is 0.163 0.005 \AA and that the lower limit for
the existence of an incommensurate solid on the second layer is 0.186
0.003 \AA. Both values are in overall agreement with the results of
torsional oscillator experiments and heat capacity measurements on graphite.
The 4/7 and 7/12 registered solids are found to be metastable with respect to
triangular incommensurate arrangements of the same density.Comment: 7 pages, accepted for publication in Phys. Rev.
4He adsorbed outside a single carbon nanotube
The phase diagrams of He adsorbed on the external surfaces of single
armchair carbon nanotubes with radii in the range 3.42 -- 10.85 \AA \ are
calculated using the diffusion Monte Carlo method. For nanotubes narrower than
a (10,10) one, the ground state is an incommensurate solid similar to the one
found for H on the same substrates. For wider nanotubes, the phase with the
minimum energy per particle is a liquid layer. Curved
registered solids similar to the ones found on graphene and graphite were
unstable for all the tubes considered.Comment: 6 pages, accepted for publication in Phys. Rev.
Liquid and solid phases of 3He on graphite
Recent heat-capacity experiments show quite unambiguously the existence of a
liquid He phase adsorbed on graphite. This liquid is stable at an extremely
low density, possibly one of the lowest found in Nature. Previous theoretical
calculations of the same system, and in strictly two dimensions, agree with the
result that this liquid phase is not stable and the system is in the gas phase.
We calculated the phase diagram of normal He adsorbed on graphite at
using quantum Monte Carlo methods. Considering a fully corrugated substrate we
observe that at densities lower that 0.006 \AA the system is a very
dilute gas, that at that density is in equilibrium with a liquid of density
0.014 \AA. Our prediction matches very well the recent experimental
findings on the same system. On the contrary, when a flat substrate is
considered, no gas-liquid coexistence is found, in agreement with previous
calculations. We also report results on the different solid structures, and the
corresponding phase transitions that appear at higher densities.Comment: 5 page
Isotopic effects of hydrogen adsorption in carbon nanotubes
We present diffusion Monte Carlo calculations of D adsorbed inside a
narrow carbon nanotube. The 1D D equation of state is reported, and the
one-dimensional character of the adsorbed D is analyzed. The isotopic
dependence of the constitutive properties of the quantum fluid are studied by
comparing D and H. Quantum effects due to their different masses are
observed both in the energetic and the structural properties. The influence of
the interatomic potential in one-dimensional systems is also studied by
comparing the properties of D and He which have nearly the same mass
but a sizeably different potential. The physics of molecular hydrogen adsorbed
in the interstitial channels of a bundle of nanotubes is analyzed by means of
both a diffusion Monte Carlo calculation and an approximate mean field method.Comment: 17 pages, revtex, 9 ps figures, to be appear in Phys. Rev.
Phase diagram of H2 adsorbed on graphene
The phase diagram of the first layer of H adsorbed on top of a single
graphene sheet has been calculated by means of a series of diffusion Monte
Carlo (DMC) simulations. We have found that, as in the case of He, the
ground state of molecular hydrogen is a commensurate
structure, followed, upon a pressure increase, by an incommensurate triangular
solid. A striped phase of intermediate density was also considered, and found
lying on top of the equilibrium curve separating both commensurate and
incommensurate solids.Comment: 5 pages, 3 figure
Rural Livelihood Strategies and Social Capital in Latin America: Implications for Rural Development Projects
Evidence from studies across Latin America show that rural households in the region are increasingly employing a diverse set of activities to maintain and improve livelihoods suggesting that households use multiple paths to get out of poverty. Of particular importance in household livelihood strategies are the assets households own and the context in which they operate. Recent development literature has highlighted the important role of social capital in development and particular emphasis is place on the role of this asset in this paper. The objective of this paper is to examine these trends in order to inform project design and implementation in the region. Based on the conceptual framework and evidence from Latin America, the paper offers a number of suggestions for improving project design and implementation in Latin America.rural livelihoods, social capital, Latin America, rural development projects, Community/Rural/Urban Development, Labor and Human Capital,
Food Insecurity and Vulnerability in Latin America and the Caribbean
This paper examines how Latin American and Caribbean (LAC) nations have proceeded in addressing the issues of food insecurity and vulnerability in response to the 1996 Rome Declaration on World Food Security. The data suggests that while the food insecurity situation has improved in LAC over the past few years, many nations are worse off in terms of food insecurity than they were at the beginning of the 1990s. Nations that reduced food insecurity tend to be those who have effectively reduced poverty and have increased social spending. While some regional initiatives have occurred to address regional food insecurity issues, additional national and international policy responses are required in the Latin American Region.food insecurity, vulnerability, Latin America and the Caribbean, World Food Summit, Food Security and Poverty,
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