10,051 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
Catalysts for regional development: putting territorial coordination in practice
Lack of coordination among agencies at project level and scarce promotion of contracts at micro-scale are critical gaps widely spread in many Latin American regions. We discuss some specific and feasible mechanisms, namely: (i) alliances for rural development and (ii) contract promoters; that may play a catalytic role to deal with the mentioned problems. Based on a continuous improving integral strategy and an effective operative framework, these alliances would be prone to unveil areas for interventions and to channel them into the pipelines of the ministries, financing agencies or private investor initiatives. These alliances can assist in solving tradeoffs between enough economies of scale for enhancing capabilities and sufficient local knowledge. Also, they might reduce capture problems. Contract promoters on the other hand, can be viewed as facilitators for startup businesses. They evolve as enterprise incubators, with expertise for rural areas, projects and marketing; combined with a vision for development. Both catalysts have a synergetic effect for coordinating regional development and should be prominent in rural modernization agendas.Rural development, Catalyst, Experimental, Promoter, Local Governance, Applied Political Economy
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