327 research outputs found
Dynamical electron transport through a nanoelectromechanical wire in a magnetic field
We investigate dynamical transport properties of interacting electrons moving
in a vibrating nanoelectromechanical wire in a magnetic field. We have built an
exactly solvable model in which electric current and mechanical oscillation are
treated fully quantum mechanically on an equal footing. Quantum mechanically
fluctuating Aharonov-Bohm phases obtained by the electrons cause nontrivial
contribution to mechanical vibration and electrical conduction of the wire. We
demonstrate our theory by calculating the admittance of the wire which are
influenced by the multiple interplay between the mechanical and the electrical
energy scales, magnetic field strength, and the electron-electron interaction
On thermalization of magnetic nano-arrays at fabrication
We propose a model to predict and control the statistical ensemble of
magnetic degrees of freedom in Artificial Spin Ice (ASI) during thermalized
adiabatic growth. We predict that as-grown arrays are controlled by the
temperature at fabrication and by their lattice constant, and that they can be
described by an effective temperature. If the geometry is conducive to a phase
transition, then the lowest temperature phase is accessed in arrays of lattice
constant smaller than a critical value, which depends on the temperature at
deposition. Alternatively, for arrays of equal lattice constant, there is a
temperature threshold at deposition and the lowest temperature phase is
accessed for fabrication temperatures {\it larger rather than smaller} than
this temperature threshold. Finally we show how to define and control the
effective temperature of the as-grown array and how to measure critical
exponents directly. We discuss the role of kinetics at the critical point, and
applications to experiments, in particular to as-grown thermalized square ASI,
and to magnetic monopole crystallization in as-grown honeycomb ASI.Comment: 14 pages, 2 figures. A theoretical approach to experimental results
reported in: Morgan J P, Stein A, Langridge S and Marrows C (2010) Nature
Physics 7 7
Landau Level Crossings and Extended-State Mapping in Magnetic Two-dimensional Electron Gases
We present longitudinal and Hall magneto-resistance measurements of a
``magnetic'' two-dimensional electron gas (2DEG) formed in modulation-doped
ZnCdMnSe quantum wells. The electron spin splitting is
temperature and magnetic field dependent, resulting in striking features as
Landau levels of opposite spin cross near the Fermi level. Magnetization
measurements on the same sample probe the total density of states and Fermi
energy, allowing us to fit the transport data using a model involving extended
states centered at each Landau level and two-channel conduction for spin-up and
spin-down electrons. A mapping of the extended states over the whole quantum
Hall effect regime shows no floating of extended states as Landau levels cross
near the Fermi level.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Determinants of health seeking behaviour following rabies exposure in Ethiopia
The objective of this study was to identify factors that determine medical treatment seeking behaviour following potential rabies exposure after being bitten by a suspected dog and the likelihood of compliance to receive sufficient doses of post-exposure prophylaxis after the visit to a health centre visit. A detailed survey based on case investigation was conducted on suspected rabid dog bite cases in three areas of Ethiopia. Two multivariable logistic regression models were created with a set of putative variables to explain treatment seeking and compliance outcomes. Based on the registered bite cases at each health centre and the set of unregistered bite cases derived by contact tracing, 655 bite victim cases were identified to have occurred between September 2013 and August 2014. Of these evaluated bite incidences, 465 cases were considered to have been caused by a potentially rabid dog. About 77% of these suspected rabid dog bite victims visited a health centre, while 57% received sufficient doses of PEP. The overall likelihood of seeking medical services following rabies exposure was higher for people bitten by dogs of unknown ownership, where the bite was severe, being bitten on the leg, spend of more than 100 USD per month and where the victim lived close to the nearest health centre, while the likelihood of receiving sufficient doses of PEP was sensitive to monthly spending and distance to health centre. However, the evaluated factors did only explain a part of the variation among the three districts. The district in which victims lived appeared to have a relevant influence on the likelihood of seeking medical treatment but did not improve the prediction on the likelihood of treatment compliance. Given the insights obtained from this study, improvements in the rural districts with regard to accessibility of post-exposure prophylaxis delivering health centres in shorter distance could improve health seeking behaviour. In addition, in rural districts, majority of exposed persons who seek medical treatment tend to comply with treatment regimen, indicating that the promotion of medical treatment through awareness creation campaigns could be beneficial
Magnetic properties and giant magnetoresistance in melt-spun CoCu alloys
Magnetic, structural, and transport properties of as quenched and annealed Co10Cu90 samples have been investigated using x¿ray diffraction and a SQUID magnetometer. The largest value of MR change was observed for the as¿quenched sample annealed at 450°C for 30 min. The magnetic and transport properties closely correlate with the microstructure, mainly with Co magnetic particle size and its distribution. For thermal annealing the as quenched samples below 600°C, the Co particle diameters increase from 4.0 to 6.0 nm with a magnetoresistance (MR) drop from 33.0% to 5.0% at 10 K. Comparison with the theory indicates that the interfacial electron spin¿dependent scattering mechanism correlates with GMR for Co particle diameters up to about 6.0 nm
The role of the alloy structure in the magnetic behavior of granular systems
The effect of grain size, easy magnetization axis and anisotropy constant
distributions in the irreversible magnetic behavior of granular alloys is
considered. A simulated granular alloy is used to provide a realistic grain
structure for the Monte Carlo simulation of the ZFC-FC curves. The effect of
annealing and external field is also studied. The simulation curves are in good
agreement with the FC and ZFC magnetization curves measured on melt spun Cu-Co
ribbons.Comment: 13 pages, 10 figures, submitted to PR
Testing for a large local void by investigating the Near-Infrared Galaxy Luminosity Function
Recent cosmological modeling efforts have shown that a local underdensity on
scales of a few hundred Mpc (out to z ~ 0.1), could produce the apparent
acceleration of the expansion of the universe observed via type Ia supernovae.
Several studies of galaxy counts in the near-infrared (NIR) have found that the
local universe appears under-dense by ~25-50% compared with regions a few
hundred Mpc distant. Galaxy counts at low redshifts sample primarily L ~ L*
galaxies. Thus, if the local universe is under-dense, then the normalization of
the NIR galaxy luminosity function (LF) at z>0.1 should be higher than that
measured for z 90%) spectroscopic
sample of 1436 galaxies selected in the H-band to study the normalization of
the NIR LF at 0.1<z<0.3 and address the question of whether or not we reside in
a large local underdensity. We find that for the combination of our six fields,
the product phi* L* at 0.1 < z < 0.3 is ~ 30% higher than that measured at
lower redshifts. While our statistical errors in this measurement are on the
~10% level, we find the systematics due to cosmic variance may be larger still.
We investigate the effects of cosmic variance on our measurement using the
COSMOS cone mock catalogs from the Millennium simulation and recent empirical
estimates. We find that our survey is subject to systematic uncertainties due
to cosmic variance at the 15% level ($1 sigma), representing an improvement by
a factor of ~ 2 over previous studies in this redshift range. We conclude that
observations cannot yet rule out the possibility that the local universe is
under-dense at z<0.1.Comment: Accepted for publication in Ap
Two fossil groups of galaxies at z~0.4 in the COSMOS: accelerated stellar-mass build-up, different progenitors
We report on 2 fossil groups of galaxies at z=0.425 and 0.372 discovered in
the Cosmic Evolution Survey (COSMOS) area. Selected as X-ray extended sources,
they have total masses (M_200) of 1.9(+/-0.41)E13 and 9.5(+/-0.42)E13 M_sun,
respectively, as obtained from a recent X-ray luminosity-mass scaling relation.
The lower mass system appears isolated, whereas the other sits in a well-known
large-scale structure (LSS) populated by 27 other X-ray emitting groups. The
identification as fossil is based on the i-band photometry of all the galaxies
with a photo-z consistent with that of the group at the 2-sigma confidence
level and within a projected group-centric distance equal to 0.5R_200, and
i_AB<=22.5-mag limited spectroscopy. Both fossil groups exhibit high
stellar-to-total mass ratios compared to all the X-ray selected groups of
similar mass at 0.3<=z<=0.5 in the COSMOS. At variance with the composite
galaxy stellar mass functions (GSMFs) of similarly massive systems, both fossil
group GSMFs are dominated by passively evolving galaxies down to M^stars~1E10
M_sun (according to the galaxy broad-band spectral energy distributions). The
relative lack of star-forming galaxies with 1E10<=M^stars<=1E11 M_sun is
confirmed by the galaxy distribution in the b-r vs i color-magnitude diagram.
Hence, the 2 fossil groups appear as more mature than the coeval, similarly
massive groups. Their overall star formation activity ended rapidly after an
accelerated build up of the total stellar mass; no significant infall of
galaxies with M^stars>=1E10 M_sun took place in the last 3 to 6 Gyr. This
similarity holds although the 2 fossil groups are embedded in two very
different density environments of the LSS, which suggests that their galaxy
populations were shaped by processes that do not depend on the LSS. However,
their progenitors may do so. ...Comment: 12 pages, 5 color figures, 1 table; to be published in the MNRA
Franck-Condon blockade in suspended carbon nanotube quantum dots
Understanding the influence of vibrational motion of the atoms on electronic
transitions in molecules constitutes a cornerstone of quantum physics, as
epitomized by the Franck-Condon principle of spectroscopy. Recent advances in
building molecular-electronics devices and nanoelectromechanical systems open a
new arena for studying the interaction between mechanical and electronic
degrees of freedom in transport at the single-molecule level. The tunneling of
electrons through molecules or suspended quantum dots has been shown to excite
vibrational modes, or vibrons. Beyond this effect, theory predicts that strong
electron-vibron coupling dramatically suppresses the current flow at low
biases, a collective behaviour known as Franck-Condon blockade. Here we show
measurements on quantum dots formed in suspended single-wall carbon nanotubes
revealing a remarkably large electron-vibron coupling and, due to the high
quality and unprecedented tunability of our samples, admit a quantitative
analysis of vibron-mediated electronic transport in the regime of strong
electron-vibron coupling. This allows us to unambiguously demonstrate the
Franck-Condon blockade in a suspended nanostructure. The large observed
electron-vibron coupling could ultimately be a key ingredient for the detection
of quantized mechanical motion. It also emphasizes the unique potential for
nanoelectromechanical device applications based on suspended graphene sheets
and carbon nanotubes.Comment: 7 pages, 3 figure
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