547 research outputs found
Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes
Monolayer graphene exhibits exceptional electronic and mechanical properties,
making it a very promising material for nanoelectromechanical (NEMS) devices.
Here, we conclusively demonstrate the piezoresistive effect in graphene in a
nano-electromechanical membrane configuration that provides direct electrical
readout of pressure to strain transduction. This makes it highly relevant for
an important class of nano-electromechanical system (NEMS) transducers. This
demonstration is consistent with our simulations and previously reported gauge
factors and simulation values. The membrane in our experiment acts as a strain
gauge independent of crystallographic orientation and allows for aggressive
size scalability. When compared with conventional pressure sensors, the sensors
have orders of magnitude higher sensitivity per unit area.Comment: 20 pages, 3 figure
A Survey for Infall Motions toward Starless Cores. II. and Mapping Observations
We present the results of an extensive mapping survey of 53 `starless' cores
in the optically thick line of CS 2-1 and the optically thin lines of N2H+ 1-0
and C18O 1-0. The purpose of this survey was to search for signatures of
extended inward motions.
This study finds 10 `strong' and 9 `probable' infall candidates, based on
analysis and on the spectral shapes of CS lines.
From our analysis of the blue-skewed CS spectra and the
parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple
two layer radiative transfer model to fit the profiles, we derive
one-dimensional infall speeds, half of whose values lie in the range of
0.05-0.09 km s. These values are similar to those found in L1544 by
Tafalla et al., and this result confirms that infall speeds in starless cores
are generally faster than expected from ambipolar diffusion in a strongly
sub-critical core. In addition, the observed infall regions are too extended to
be consistent with the `inside-out' collapse model applied to a very low-mass
star. In the largest cores, the spatial extent of the CS spectra with infall
asymmetry is larger than the extent of the core by a factor of
2-3. All these results suggest that extended inward motions are a common
feature in starless cores, and that they could represent a necessary stage in
the condensation of a star-forming dense core.Comment: Two tex files for manuscript and tables, and 38 figures. To appear in
ApJ
Transmission through a biased graphene bilayer barrier
We study the electronic transmission through a graphene bilayer in the
presence of an applied bias between layers. We consider different geometries
involving interfaces between both a monolayer and a bilayer and between two
bilayers. The applied bias opens a sizable gap in the spectrum inside the
bilayer barrier region, thus leading to large changes in the transmission
probability and electronic conductance that are controlled by the applied bias.Comment: 10 pages, 8 figures, extended versio
Edge-functionalized and substitutional doped graphene nanoribbons: electronic and spin properties
Graphene nanoribbons are the counterpart of carbon nanotubes in
graphene-based nanoelectronics. We investigate the electronic properties of
chemically modified ribbons by means of density functional theory. We observe
that chemical modifications of zigzag ribbons can break the spin degeneracy.
This promotes the onset of a semiconducting-metal transition, or of an
half-semiconducting state, with the two spin channels having a different
bandgap, or of a spin-polarized half-semiconducting state -where the spins in
the valence and conduction bands are oppositely polarized. Edge
functionalization of armchair ribbons gives electronic states a few eV away
from the Fermi level, and does not significantly affect their bandgap. N and B
produce different effects, depending on the position of the substitutional
site. In particular, edge substitutions at low density do not significantly
alter the bandgap, while bulk substitution promotes the onset of
semiconducting-metal transitions. Pyridine-like defects induce a
semiconducting-metal transition.Comment: 12 pages, 5 figure
Ground Improvement for Increasing Lateral Pile Group Resistance
Lateral load tests were performed on a full-scale pile cap in untreated clay along with pile groups involving (a) excavation and replacement with sand backfill, (b) a soilcrete wall along the side of the pile group, and (c) a jet grouted zone below the pile cap. The average compressive strength of the soft, plastic clay increased from an average of 50 kPa to an average of about 1000 kPa with soil mixing (10% cement) and to 3000 kPa with jet grouting (20% cement). Excavation and replacement only increased resistance by about 20%; however, the soil mixed wall increased resistance by 60%, and jet grouting increased resistance by 160%. For the soil mixed wall, essentially all of the increased resistance can be explained due to passive pressure and side/base shear against the soil mixed wall. However, for the jet grout treatment, additional resistance can also be attributed to increased structural resistance of the composite soilcrete volume under the cap. Soil mixing and jet grouting provide a means to significantly increase the lateral resistance of existing pile group foundations with relatively little investment of time, effort, and expense relative to the addition of more piles
HIV Infection among Young People in Northwest Tanzania: The Role of Biological, Behavioural and Socio-Demographic Risk Factors.
BACKGROUND: Young people are at high risk of HIV and developing appropriate prevention programmes requires an understanding of the risk factors for HIV in this age group. We investigated factors associated with HIV among participants aged 15-30 years in a 2007-8 cross-sectional survey nested within a community-randomised trial of the MEMA kwa Vijana intervention in 20 rural communities in northwest Tanzania. METHODS: We analysed data for 7259(53%) males and 6476(47%) females. Using a proximate-determinant conceptual framework and conditional logistic regression, we obtained sex-specific Odds Ratios (ORs) for the association of HIV infection with socio-demographic, knowledge, behavioural and biological factors. RESULTS: HSV-2 infection was strongly associated with HIV infection (females: adjOR 4.4, 95%CI 3.2-6.1; males: adjOR 4.2, 95%CI 2.8-6.2). Several socio-demographic factors (such as age, marital status and mobility), behavioural factors (condom use, number and type of sexual partnerships) and biological factors (blood transfusion, lifetime pregnancies, genital ulcers, Neisseria gonorrhoeae) were also associated with HIV infection. Among females, lifetime sexual partners (linear trend, p<0.001), â„2 partners in the past year (adjOR 2.0, 95%CI 1.4-2.8), â„2 new partners in the past year (adjOR 1.9 95%CI 1.2, 3.3) and concurrent partners in the past year (adjOR 1.6 95%CI 1.1, 2.4) were all associated with HIV infection. CONCLUSIONS: Efforts must be intensified to find effective interventions to reduce HSV-2. Effective behavioural interventions focusing on reducing the number of sexual partnerships and risk behaviour within partnerships are also needed. An increase in risky sexual behaviour may occur following marriage dissolution or when a young woman travels outside of her community and interventions addressing the needs of these subgroups of vulnerable women may be important. TRIAL REGISTRATION: ClinicalTrial.gov NCT00248469
SIMPLIFIED PARAMETERS FOR THE EVALUATION OF SITE EFFECTS IN THE
After the 2002 earthquakes, that struck the southern Italy provinces of Campobasso (Molise) and Foggia
(Apulia), the safety assessment of buildings has allowed to evaluate, for different building typologies, the
damage level caused by the seismic event; in particular, the survey of the damaged cultural heritage was carried
out by the Task Cultural Heritage of the Mixed Operating Centre (COM) in Larino (a village in the epicentral
area), coordinated by the âWorking Group for the Safeguard of the Cultural Heritage from Natural Risksâ
(Italyâs Gazzetta Ufficiale n. 116, 21 May 2001 - PCM-DPC Decree, 3 May 2001)â.
For this survey activity, the churches were investigated with a specific form proposed by the same Working
Group (Ministerial Decree n. 133, 23th January 2001), adopting different forms for other kinds of buildings. The
surveyors were organized in teams, called NOPSA (Nuclei Operativi Patrimonio Storico Artistico), composed by
officials of the Regional board of the Ministry of Cultural Heritage and Environmental Conservation (architects
and art historians), an engineer and a Fire Brigade technician. The first phase of the safety assessment has been developed on the basis of the warnings received by the Larino COM, and most of the surveyed buildings were
churches. The damage assessment of public or private buildings began, initially, near the earthquake epicentre
(area 1 - IMCS between 6 and 8.5) and subsequently was carried out for all the villages from which inspection
requests arrived. At the same time a vulnerability survey was carried out for the churches, using the II level form
(Lagomarsino et al., 2004) proposed in the SAVE Project (âUpdated Tools for the Seismic Vulnerability
Evaluation of the Italian Real Estate and of Urban Systemsâ â http://gndt.ingv.it). The damage and vulnerability
survey has allowed to verify and optimize the procedures for damage assessment immediately after a seismic
event (emergency phase) and to obtain useful indications for the reconstruction phase. On the whole 379
monumental buildings in 101 Molise villages and 207 monumental buildings in 44 Apulia villages were
investigated. Figure 1 shows the number of monumental buildings surveyed after the earthquakes for each
Molise village, subdividing the region according to the seismic classification proposed by the recent Italian
Seismic Code (OPCM n. 3274, 2003 â http://zonesismiche.mi.ingv.it)
Large Scale Integration of Graphene Transistors for Potential Applications in the Back End of the Line
A chip to wafer scale, CMOS compatible method of graphene device fabrication
has been established, which can be integrated into the back end of the line
(BEOL) of conventional semiconductor process flows. In this paper, we present
experimental results of graphene field effect transistors (GFETs) which were
fabricated using this wafer scalable method. The carrier mobilities in these
transistors reach up to several hundred cmVs. Further, these
devices exhibit current saturation regions similar to graphene devices
fabricated using mechanical exfoliation. The overall performance of the GFETs
can not yet compete with record values reported for devices based on
mechanically exfoliated material. Nevertheless, this large scale approach is an
important step towards reliability and variability studies as well as
optimization of device aspects such as electrical contacts and dielectric
interfaces with statistically relevant numbers of devices. It is also an
important milestone towards introducing graphene into wafer scale process
lines
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
- âŠ