772 research outputs found
Quantitative measurement of the surface charge density
We present a method of measuring the charge density on dielectric surfaces.
Similar to electrostatic force microscopy we record the electrostatic
interaction between the probe and the sample surface, but at large tip-sample
distances. For calibration we use a pyroelectric sample which allows us to
alter the surface charge density by a known amount via a controlled temperature
change. For proof of principle we determined the surface charge density under
ambient conditions of ferroelectric lithium niobate
ANALYSIS OF TRENDS AND FORECASTS IN COFFEE PRICES AND CONSUMER CONSUMPTION IN THE NORTHEAST AND UNITED STATES
Demand and Price Analysis, Food Consumption/Nutrition/Food Safety,
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
Controlling Interface Properties for Advanced Energy Applications
Internal interfaces in materials play an important role in the performance of many devices used in energy applications including solar cells, LEDs, passive electronics, and fuel cells. Efficiencies in energy and power consumption may be realized by optimizing and often miniaturizing these devices. Our studies show that internal boundaries and biomaterial interfaces cause local property variations. These effects will dominate device performance as the systems become smaller. A fundamental understanding of the effect of atomic structure on local properties is a prerequisite to device optimization. Developing this understanding requires new probes that access local properties, controlled interface structure, atomic resolution electron microscopy and first principles calculations of geometric and electronic structure
Dynamical Friction on Star Clusters near the Galactic Center
Numerical simulations of the dynamical friction suffered by a star cluster
near the Galactic center have been performed with a parallelized tree code.
Gerhard (2001) has suggested that dynamical friction, which causes a cluster to
lose orbital energy and spiral in towards the galactic center, may explain the
presence of a cluster of very young stars in the central parsec, where star
formation might be prohibitively difficult owing to strong tidal forces. The
clusters modeled in our simulations have an initial total mass of 10^5-10^6
Msun and initial galactocentric radii of 2.5-30 pc. We have identified a few
simulations in which dynamical friction indeed brings a cluster to the central
parsec, although this is only possible if the cluster is either very massive
(~10^6 Msun), or is formed near the central parsec (<~ 5 pc). In both cases,
the cluster should have an initially very dense core (> 10^6 Msun pc-3). The
initial core collapse and segregation of massive stars into the cluster core,
which typically happens on a much shorter time scale than that characterizing
the dynamical inspiral of the cluster toward the Galactic center, can provide
the requisite high density. Furthermore, because it is the cluster core which
is most likely to survive the cluster disintegration during its journey
inwards, this can help account for the observed distribution of presumably
massive HeI stars in the central parsec.Comment: Accepted for publication in Ap
The effect of magnetic fields on star cluster formation
We examine the effect of magnetic fields on star cluster formation by
performing simulations following the self-gravitating collapse of a turbulent
molecular cloud to form stars in ideal MHD. The collapse of the cloud is
computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is
using both weak and strong magnetic fields. Whilst even at very low strengths
the magnetic field is able to significantly influence the star formation
process, for magnetic fields with plasma beta < 1 the results are substantially
different to the hydrodynamic case. In these cases we find large-scale
magnetically-supported voids imprinted in the cloud structure; anisotropic
turbulent motions and column density structure aligned with the magnetic field
lines, both of which have recently been observed in the Taurus molecular cloud.
We also find strongly suppressed accretion in the magnetised runs, leading to
up to a 75% reduction in the amount of mass converted into stars over the
course of the calculations and a more quiescent mode of star formation. There
is also some indication that the relative formation efficiency of brown dwarfs
is lower in the strongly magnetised runs due to the reduction in the importance
of protostellar ejections.Comment: 16 pages, 9 figures, 8 very pretty movies, MNRAS, accepted. Version
with high-res figures + movies available from
http://www.astro.ex.ac.uk/people/dprice/pubs/mcluster/index.htm
Observational Implications of Precessing Protostellar Discs and Jets
We consider the dynamics of a protostellar disc in a binary system where the
disc is misaligned with the orbital plane of the binary, with the aim of
determining the observational consequences for such systems. The disc wobbles
with a period approximately equal to half the binary's orbital period and
precesses on a longer timescale. We determine the characteristic timescale for
realignment of the disc with the orbital plane due to dissipation. If the
dissipation is determined by a simple isotropic viscosity then we find, in line
with previous studies, that the alignment timescale is of order the viscous
evolution timescale. However, for typical protostellar disc parameters, if the
disc tilt exceeds the opening angle of the disc, then tidally induced shearing
within the disc is transonic. In general, hydrodynamic instabilities associated
with the internally driven shear result in extra dissipation which is expected
to drastically reduce the alignment timescale. For large disc tilts the
alignment timescale is then comparable to the precession timescale, while for
smaller tilt angles , the alignment timescale varies as . We discuss the consequences of the wobbling, precession and
rapid realignment for observations of protostellar jets and the implications
for binary star formation mechanisms.Comment: MNRAS, in press. 10 pages. Also available at
http://www.ast.cam.ac.uk/~mbat
Polarization reorientation in ferroelectric lead zirconate titanate thin films with electron beams
Ferroelectric domain patterning with an electron beam is demonstrated. Polarization of lead zirconate titanate thin films is shown to be reoriented in both positive and negative directions using piezoresponse force and scanning surface potential microscopy. Reorientation of the ferroelectric domains is a response to the electric field generated by an imbalance of electron emission and trapping at the surface. A threshold of 500 ”C/cm2 and a saturation of 1500 ”C/cm2 were identified. Regardless of beam energy, the polarization is reoriented negatively for beam currents less than 50 pA and positively for beam currents greater than 1 nA
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