648 research outputs found
Combining Molecular Dynamics with Lattice-Boltzmann: A Hybrid Method for the Simulation of (Charged) Colloidal Systems
We present a hybrid method for the simulation of colloidal systems, that
combines molecular dynamics (MD) with the Lattice-Boltzmann (LB) scheme. The LB
method is used as a model for the solvent in order to take into account the
hydrodynamic mass and momentum transport through the solvent. The colloidal
particles are propagated via MD and they are coupled to the LB fluid by viscous
forces. With respect to the LB fluid, the colloids are represented by uniformly
distributed points on a sphere. Each such point (with a velocity V(r) at any
off-lattice position r is interacting with the neighboring eight LB nodes by a
frictional force F=\xi_0(V(r)-u(r)) with \xi_0 being a friction force and u(r)
being the velocity of the fluid at the position r. Thermal fluctuations are
introduced in the framework of fluctuating hydrodynamics. This coupling scheme
has been proposed recently for polymer systems by Ahlrichs and D"unweg [J.
Chem. Phys. 111, 8225 (1999)]. We investigate several properties of a single
colloidal particle in a LB fluid, namely the effective Stokes friction and long
time tails in the autocorrelation functions for the translational and
rotational velocity. Moreover, a charged colloidal system is considered
consisting of a macroion, counterions and coions that are coupled to a LB
fluid. We study the behavior of the ions in a constant electric field. In
particular, an estimate of the effective charge of the macroion is yielded from
the number of counterions that move with the macroion in the direction of the
electric field.Comment: 37 pages, 12 figure
Exploring Planets with Directed Aerial Robot Explorers
Global Aerospace Corporation (GAC) is developing a revolutionary system architecture for exploration of planetary atmospheres and surfaces from atmospheric altitudes. The work is supported by the NASA Institute for Advanced Concepts (NIAC). The innovative system architecture relies upon the use of Directed Aerial Robot Explorers (DAREs), which essentially are long-duration-flight autonomous balloons with trajectory control capabilities that can deploy swarms of miniature probes over multiple target areas. Balloon guidance capabilities will offer unprecedented opportunities in high-resolution, targeted observations of both atmospheric and surface phenomena. Multifunctional microprobes will be deployed from the balloons once over the target areas, and perform a multitude of functions, such as atmospheric profiling or surface exploration, relaying data back to the balloons or an orbiter. This architecture will enable low-cost, low-energy, long-term global exploration of planetary atmospheres and surfaces. This paper focuses on a conceptual analysis of the DARE architecture capabilities and science applications for Venus, Titan and Jupiter. Preliminary simulations with simplified atmospheric models show that a relatively small trajectory control wing can enable global coverage of the atmospheres of Venus and Titan by a single balloon over a 100-day mission. This presents unique opportunities for global in situ sampling of the atmospheric composition and dynamics, atmospheric profiling over multiple sites with small dropsondes and targeted deployment of surface microprobes. At Jupiter, path guidance capabilities of the DARE platforms permits targeting localized regions of interest, such as "hot spots" or the Great Red Spot. A single DARE platform at Jupiter can sample major types of the atmospheric flows (zones and belts) over a 100-day mission. Observations by deployable probes would reveal if the differences exist in radiative, dynamic and compositional environments at these sites
Outsourcing or efficiency? Investigating the decline in final energy consumption in the UK productive sectors
Over the past two decades reductions in the final energy consumption of the productive sectors (industry, public administration, commercial services and agriculture), have made important contributions to overall reductions in UK final energy consumption. This study investigates the drivers of the reductions in final energy consumption in the UK productive sectors between 1997 and 2013 using a decomposition analysis that incorporates two novel approaches. Firstly, it uses results from a multi-regional input-output model to investigate how much of the structural change in the economy has been driven by outsourcing production overseas. Secondly, it utilises energy conversion chain analysis to determine how much increases in the conversion efficiency from final energy to useful exergy have contributed to improvements in final energy intensity. In aggregate all energy savings from structural change are attributed to outsourcing. Improvements in the conversion efficiency produced savings of a similar size. However energy savings from both factors have stalled since 2009. Improvements in useful exergy intensity, the useful exergy used per unit of monetary output, provided the biggest share of energy savings, but these savings are concentrated in a few sectors and rarely lead to absolute reductions in final energy use. All of this suggests that a return to the rates of energy reduction seen between 2001 and 2009 should not be taken for granted and that active policy interventions might be required to achieve further reductions
Three-dimensional magnetic flux-closure patterns in mesoscopic Fe islands
We have investigated three-dimensional magnetization structures in numerous
mesoscopic Fe/Mo(110) islands by means of x-ray magnetic circular dichroism
combined with photoemission electron microscopy (XMCD-PEEM). The particles are
epitaxial islands with an elongated hexagonal shape with length of up to 2.5
micrometer and thickness of up to 250 nm. The XMCD-PEEM studies reveal
asymmetric magnetization distributions at the surface of these particles.
Micromagnetic simulations are in excellent agreement with the observed magnetic
structures and provide information on the internal structure of the
magnetization which is not accessible in the experiment. It is shown that the
magnetization is influenced mostly by the particle size and thickness rather
than by the details of its shape. Hence, these hexagonal samples can be
regarded as model systems for the study of the magnetization in thick,
mesoscopic ferromagnets.Comment: 12 pages, 11 figure
The Contributions of Muscle and Machine Work to Land and Labor Productivity in World Agriculture Since 1800
Since 1800, there have been enormous changes in mechanical technologies farmers use and in the relative contributions of human and animal muscles and machines to farm work. We develop a database from 1800 to 2012 of on-farm physical work in world agriculture from muscles and machines. We do so to analyze how on-farm physical work has contributed to changes in land and human labor productivities. We find two distinct periods. First, from 1800 to around 1950, land productivity (measured as kcal food supply per hectare of cropland) was relatively stagnant at about 1.7 million kcal/ha, in part due to a scarcity of on-farm physical work. During this period, physical work was scarce because most of on-farm physical work (approximately 80% in 1950) was being powered by low power, low energy efficiency muscle work provided by humans and draft animals. From 1950 to 2012, land productivity nearly tripled as more machine-based work inputs became available. The additional machine-based work inputs have contributed to the growth in land and labor productivities, as they have enabled farmers to control more physical work enabling more irrigation and agrochemical applications. However, the tripling of land productivity has required a near 4.5-fold increase in physical work per hectare, suggesting diminishing returns. Farmers accomplished this extra work with less final energy because they transitioned from low-efficiency muscle work to high-efficiency machines which drove farm-wide energy conversion efficiency up fourfold from 1950 to 2012. By 1990, machine conversion efficiencies started to plateau. Given diminishing returns and plateauing efficiencies, we predict that fuel and electricity usage on farms will increase to continue raising land productivity
Gate-controlled supercurrent in ballistic InSb nanoflag Josephson junctions
High-quality III-V narrow bandgap semiconductor materials with strong spin-orbit coupling and large Landé g-factor provide a promising platform for next-generation applications in the field of high-speed electronics, spintronics, and quantum computing. Indium antimonide (InSb) offers a narrow bandgap, high carrier mobility, and small effective mass and, thus, is very appealing in this context. In fact, this material has attracted tremendous attention in recent years for the implementation of topological superconducting states supporting Majorana zero modes. However, high-quality heteroepitaxial two-dimensional (2D) InSb layers are very difficult to realize owing to the large lattice mismatch with all commonly available semiconductor substrates. An alternative pathway is the growth of free-standing single-crystalline 2D InSb nanostructures, the so-called nanoflags. Here, we demonstrate fabrication of ballistic Josephson-junction devices based on InSb nanoflags with Ti/Nb contacts that show a gate-tunable proximity-induced supercurrent up to 50 nA at 250 mK and a sizable excess current. The devices show clear signatures of subharmonic gap structures, indicating phase-coherent transport in the junction and a high transparency of the interfaces. This places InSb nanoflags in the spotlight as a versatile and convenient 2D platform for advanced quantum technologies
Mars Exploration with Directed Aerial Robot Explorers
Global Aerospace Corporation (GAC) is developing a revolutionary system architecture for exploration of planetary atmospheres and surfaces from atmospheric altitudes. The work is supported by the NASA Institute for Advanced Concepts (NIAC). The innovative system architecture relies upon the use of Directed Aerial Robot Explorers (DAREs), which essentially are longâdurationâflight autonomous balloons with trajectory control capabilities that can deploy swarms of miniature probes over multiple target areas. Balloon guidance capabilities will offer unprecedented opportunities in highâresolution, targeted observations of both atmospheric and surface phenomena. Multifunctional microprobes will be deployed from the balloons when over the target areas, and perform a multitude of functions, such as atmospheric profiling or surface exploration, relaying data back to the balloons or an orbiter. This architecture will enable lowâcost, lowâenergy, longâterm global exploration of planetary atmospheres and surfaces. A conceptual analysis of DARE capabilities and science applications for Mars is presented. Initial results of simulations indicate that a relatively small trajectory control wing can significantly change planetary balloon flight paths, especially during summer seasons in Polar Regions. This opens new possibilities for highâresolution observations of crustal magnetic anomalies, polar layered terrain, polar clouds, dust storms at the edges of the Polar caps and of seasonal variability of volatiles in the atmosphere
The Ctf18 RFC-like complex positions yeast telomeres but does not specify their replication time
Peer reviewedPreprin
Kalb-Ramond excitations in a thick-brane scenario with dilaton
We compute the full spectrum and eigenstates of the Kalb-Ramond field in a
warped non-compact Randall-Sundrum -type five-dimensional spacetime in which
the ordinary four-dimensional braneworld is represented by a sine-Gordon
soliton. This 3-brane solution is fully consistent with both the warped
gravitational field and bulk dilaton configurations. In such a background we
embed a bulk antisymmetric tensor field and obtain, after reduction, an
infinite tower of normalizable Kaluza-Klein massive components along with a
zero-mode. The low lying mass eigenstates of the Kalb-Ramond field may be
related to the axion pseudoscalar. This yields phenomenological implications on
the space of parameters, particularly on the dilaton coupling constant. Both
analytical and numerical results are given.Comment: 10 pages, 13 figures, and 2 tables. Final version to appear in The
European Physical Journal
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