14,916 research outputs found
Mechanics of bio–hybrid systems
Bio–hybrid system are morphing structures whose shaping can be electrically driven and strongly depends on the geometrical and mechanical characteristics of the system. The estimation of those characteristics which allow for getting target shapes is a great challenge. We present and discuss an approximate model for narrow bio–hybrid strips which works well in plane bending. A generalization towards three–layers bio–hybrid system is presented
Uniformization, Unipotent Flows and the Riemann Hypothesis
We prove equidistribution of certain multidimensional unipotent flows in the
moduli space of genus principally polarized abelian varieties (ppav). This
is done by studying asymptotics of -automorphic forms averaged along unipotent flows, toward the
codimension-one component of the boundary of the ppav moduli space. We prove a
link between the error estimate and the Riemann hypothesis. Further, we prove
modularity of the function obtained by iterating the
unipotent average process times. This shows uniformization of modular
integrals of automorphic functions via unipotent flows
Electromagnetic Formation Flying with Eccentric Reference Orbits
Over the last decade, a considerable amount of research work has been done in the area of spacecraft formation flight, with particular emphasis on control techniques using thruster-based systems. Nevertheless, thrusters require propellant to work and this limit the lifetime of the mission. Electromagnetic Formation Flight (EMFF) is presented in this paper as a fuel-less strategy to control spacecraft formations by means of electromagnets. In EMFF, spacecraft can be equipped with one or more coils and reactions wheels which could be arranged in several combinations according to mission requirements. An electric current flows through the coils in order to produce a magnetic dipole in a specific direction. The magnetic field of a spacecraft reacts against the magnetic dipoles of the others, generating forces and torques which in turn could be used as control inputs. The main objective of this paper is to provide a formulation for EMFF when a formation is moving in eccentric reference orbits and for this purpose, the Tschauner and Hempel model will be used. Results are presented after analysing different formation scenarios providing the necessary magnetic requirements for station keeping and resolving which cases are suitable to be controlled by this technology. High-Temperature Semiconductor (HTS) plays an important role in EMFF and for that reason the paper also investigates the correlation of the magnetic force and the coil mass, which in turn affects the total mass of the spacecraft
The pole-sitter mission concept : an overview of recent developments and possible future applications
The paper provides a survey of novel mission concepts for continuous, hemispheric polar observation and direct-link polar telecommunications. It is well known that these services cannot be provided by traditional platforms: geostationary satellites do not cover high-latitude regions, while low- and medium-orbit Sun-synchronous spacecraft only cover a narrow swath of the Earth at each passage. Concepts that are proposed in the literature are described, including the pole-sitter concept (in which a spacecraft is stationary above the pole), spacecraft in artificial equilibrium points in the Sun-Earth system and non-Keplerian polar Molniya orbits. Additionally, novel displaced eight-shaped orbits at Lagrangian points are presented. For many of these concepts, a continuous acceleration is required and propulsion systems include solar electric propulsion, solar sail and a hybridisation of the two. Advantages and drawbacks of each mission concept are assessed, and a comparison in terms of high-latitude coverage and distance, spacecraft mass, payload and lifetime is presented. Finally, the paper will describe a number of potential applications enabled by these concepts, focusing on polar Earth observation and telecommunications
Structure and energetics of solvated ferrous and ferric ions: Car-Parrinello molecular dynamics in the DFT+U formalism
We implemented a rotationally-invariant Hubbard U extension to
density-functional theory in the Car-Parrinello molecular dynamics framework,
with the goal of bringing the accuracy of the DFT+U approach to
finite-temperature simulations, especially for liquids or solids containing
transition-metal ions. First, we studied the effects on the Hubbard U on the
static equilibrium structure of the hexa-aqua ferrous and ferric ions, and the
inner-sphere reorganization energy for the electron-transfer reaction between
aqueous ferrous and ferric ions. It is found that the reorganization energy is
increased, mostly as a result of the Fe-O distance elongation in the hexa-aqua
ferrous ion. Second, we performed a first-principles molecular dynamics study
of the solvation structure of the two aqueous ferrous and ferric ions. The
Hubbard term is found to change the Fe-O radial distribution function for the
ferrous ion, while having a negligible effect on the aqueous ferric ion.
Moreover, the frequencies of vibrations between Fe and oxygen atoms in the
first-solvation shell are shown to be unaffected by the Hubbard corrections for
both ferrous and ferric ions.Comment: 13 pages, 2 figures, 1 table. Submitted to Journal of
Electroanalytical Chemistr
Optimal cloning of coherent states by linear optics
We describe an optical scheme for optimal Gaussian n to m cloning of coherent
states. The scheme, which generalizes a recently demonstrated scheme for 1 to 2
cloning, involves only linear optical components and homodyne detection.Comment: 5 pages, 4 figures, presented at the 13th Central European Workshop
on Quantum Optics, May 23-27 2006, Vienna, Austria (Proceedings will be
published in "Acta Physica Hungarica"); reference added, Eq. (8) correcte
Measuring the Temperature of a Mesoscopic Quantum Electron System by means of Single Electron Statistics
We measure the temperature of a mesoscopic system consisting of an
ultra-dilute two dimensional electron gas at the interface in a
metal-oxide-semiconductor field effect transistor (MOSFET) quantum dot by means
of the capture and emission of an electron in a point defect close to the
interface. Contrarily to previous reports, we show that the capture and
emission by point defects in Si n-MOSFETs can be temperature dependent down to
800 mK. As the finite quantum grand canonical ensemble model applies, the time
domain charge fluctuation in the defect is used to determine the temperature of
the few electron gas in the channel.Comment: 4 Figures (color
Old stellar counter-rotating components in early-type galaxies from elliptical-spiral mergers
We investigate, by means of numerical simulations, the possibility of forming
counter-rotating old stellar components by major mergers between an elliptical
and a spiral galaxy. We show that counter-rotation can appear both in
dissipative and dissipationless retrograde mergers, and it is mostly associated
to the presence of a disk component, which preserves part of its initial spin.
In turn, the external regions of the two interacting galaxies acquire part of
the orbital angular momentum, due to the action of tidal forces exerted on each
galaxy by the companion.Comment: 6 pages, 15 figures. Accepted on Astronomy & Astrophysic
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