4,107 research outputs found
Stellar collisions in accreting protoclusters: a Monte Carlo dynamical study
We explore the behaviour of accreting protoclusters with a Monte Carlo
dynamical code in order to evaluate the relative roles of accretion, two body
relaxation and stellar collisions in the cluster evolution. We corroborate the
suggestion of Clarke & Bonnell that the number of stellar collisions should
scale as (independent of other cluster parameters, where
N is the number of stars in the cluster and the rate of mass
accretion) and thus strengthen the argument that stellar collisions are more
likely in populous (large N) clusters. We however find that the estimates of
Clarke & Bonnell were pessimistic in the sense that we find that more than 99 %
of the stellar collisions occur within the post-adiabatic regime as the cluster
evolves towards core collapse, driven by a combination of accretion and
two-body relaxation. We discuss how the inclusion of binaries may reduce the
number of collisions through the reversal of core collapse but also note that
it opens up another collisional channel involving the merger of stars within
hard binaries; future Nbody simulations are however required in order to
explore this issue.Comment: 9 pages, 9 figures; accepted for publication in MNRAS. This version
contains minor revisions after referee's comments
Role of Single Defects in Electronic Transport through Carbon Nanotube Field-Effect Transistors
The influence of defects on electron transport in single-wall carbon nanotube
field effect transistors (CNFETs) is probed by combined scanning gate
microscopy (SGM) and scanning impedance microscopy (SIM). SGM reveals a
localized field effect at discrete defects along the CNFET length. The
depletion surface potential of individual defects is quantified from the
SGM-imaged radius of the defect as a function of tip bias voltage. This
provides a measure of the Fermi level at the defect with zero tip voltage,
which is as small as 20 meV for the strongest defects. The effect of defects on
transport is probed by SIM as a function of backgate and tip-gate voltage. When
the backgate voltage is set so the CNFET is "on" (conducting), SIM reveals a
uniform potential drop along its length, consistent with diffusive transport.
In contrast, when the CNFET is "off", potential steps develop at the position
of depleted defects. Finally, high-resolution imaging of a second set of weak
defects is achieved in a new "tip-gated" SIM mode.Comment: to appear in Physical Review Letter
Device modeling of long-channel nanotube electro-optical emitter
We present a simple analytic model of nanotube electro-optical emitters,
along with improved experimental measurements using PMMA-passivated devices
with reduced hysteresis. Both the ambipolar electrical characteristics and the
motion of the infrared-emission spot are well described. The model indicates
that the electric field is strongly enhanced at the emission spot, and that
device performance can be greatly improved by the use of thinner gate oxides
Performance evaluation of wheels for lunar vehicles
Performance evaluation of wheels for lunar vehicle
Energy-efficient Internet of Things monitoring with low-capacity devices
The Internet of Things (IoT) allows users to gather data from the physical environment. While sensors in public spaces are already widely used, users are reluctant to deploy sensors for shared data at their homes. The deployment of IoT nodes at the users premises presents privacy issues regarding who can access to their data once it is sent to the Cloud which the users cannot control. In this paper we present an energy-efficient and low cost solution for environmental monitoring at the users home. Our system is built completely with open source components and is easy to reproduce. We leverage the infrastructure and trust of a community network to store and control the access to the monitored data. We tested our solution during several months on different low-capacity single board computers (SBC) and it showed to be stable. Our results suggest that this solution could become a permanently running service in SBCs at the users homes.Peer ReviewedPostprint (author's final draft
Numerical modelling of nonlinear electromechanical coupling of an atomic force microscope with finite element method
In this contribution, an atomic force microscope is modelled and in this context, a non-linear coupled 3-D-boundary value problem is solved numerically using the finite element method. The coupling of this system is done by using the Maxwell stress tensor. In general, an iterative weak coupling is used, where the two physical problems are solved separately. However, this method does not necessarily guarantee convergence of the nonlinear computation. Hence, this contribution shows the possibility of solving the multiphysical problem by a strong coupling, which is also referred to as monolithic approach. The electrostatic field and the mechanical displacements are calculated simultaneously by solving only one system of equation. Since the Maxwell stress tensor depends nonlinearly on the potential, the solution is solved iteratively by the Newton method. © 2010 Author(s).DF
Permalloy-based carbon nanotube spin-valve
In this Letter we demonstrate that Permalloy (Py), a widely used Ni/Fe alloy,
forms contacts to carbon nanotubes (CNTs) that meet the requirements for the
injection and detection of spin-polarized currents in carbon-based spintronic
devices. We establish the material quality and magnetization properties of Py
strips in the shape of suitable electrical contacts and find a sharp
magnetization switching tunable by geometry in the anisotropic
magnetoresistance (AMR) of a single strip at cryogenic temperatures. In
addition, we show that Py contacts couple strongly to CNTs, comparable to Pd
contacts, thereby forming CNT quantum dots at low temperatures. These results
form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance
switchings in the tunneling magnetoresistance, which directly correspond to the
magnetization reversals in the individual contacts observed in AMR experiments.Comment: 3 page
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