977 research outputs found
Inelastic Scattering and Current Saturation in Graphene
We present a study of transport in graphene devices on polar insulating
substrates by solving the Bolzmann transport equation in the presence of
graphene phonon, surface polar phonon, and Coulomb charged impurity scattering.
The value of the saturated velocity shows very weak dependence on the carrier
density, the nature of the insulating substrate, and the low-field mobility,
varied by the charged impurity concentration. The saturated velocity of 4 - 8 x
10^7 cm/s calculated at room temperature is significantly larger than reported
experimental values. The discrepancy is due to the self-heating effect which
lowers substantially the value of the saturated velocity. We predict that by
reducing the insulator oxide thickness, which limits the thermal conductance,
the saturated currents can be significantly enhanced. We also calculate the
surface polar phonon contribution to the low-field mobility as a function of
carrier density, temperature, and distance from the substrate.Comment: 8 pages 9 figure
Graphene field-effect transistors based on boron nitride gate dielectrics
Graphene field-effect transistors are fabricated utilizing single-crystal
hexagonal boron nitride (h-BN), an insulating isomorph of graphene, as the gate
dielectric. The devices exhibit mobility values exceeding 10,000 cm2/V-sec and
current saturation down to 500 nm channel lengths with intrinsic
transconductance values above 400 mS/mm. The work demonstrates the favorable
properties of using h-BN as a gate dielectric for graphene FETs.Comment: 4 pages, 8 figure
Electronic compressibility of layer polarized bilayer graphene
We report on a capacitance study of dual gated bilayer graphene. The measured
capacitance allows us to probe the electronic compressibility as a function of
carrier density, temperature, and applied perpendicular electrical displacement
D. As a band gap is induced with increasing D, the compressibility minimum at
charge neutrality becomes deeper but remains finite, suggesting the presence of
localized states within the energy gap. Temperature dependent capacitance
measurements show that compressibility is sensitive to the intrinsic band gap.
For large displacements, an additional peak appears in the compressibility as a
function of density, corresponding to the presence of a 1-dimensional van Hove
singularity (vHs) at the band edge arising from the quartic bilayer graphene
band structure. For D > 0, the additional peak is observed only for electrons,
while D < 0 the peak appears only for holes. This asymmetry that can be
understood in terms of the finite interlayer separation and may be useful as a
direct probe of the layer polarization
234 EXPECTATIONS REGARDING KNEE OSTEOARTHRITIS MANAGEMENT: VIEWS FROM PATIENTS AND PRACTITIONERS. A QUALITATIVE INTERVIEW STUDY
Graphene microwave transistors on sapphire substrates
We have developed metal-oxide graphene field-effect transistors (MOGFETs) on
sapphire substrates working at microwave frequencies. For monolayers, we obtain
a transit frequency up to ~ 80 GHz for a gate length of 200 nm, and a power
gain maximum frequency of about ~ 3 GHz for this specific sample. Given the
strongly reduced charge noise for nanostructures on sapphire, the high
stability and high performance of this material at low temperature, our MOGFETs
on sapphire are well suited for a cryogenic broadband low-noise amplifier
Proton tracking in a high-granularity Digital Tracking Calorimeter for proton CT purposes
Radiation therapy with protons as of today utilizes information from x-ray CT
in order to estimate the proton stopping power of the traversed tissue in a
patient. The conversion from x-ray attenuation to proton stopping power in
tissue introduces range uncertainties of the order of 2-3% of the range,
uncertainties that are contributing to an increase of the necessary planning
margins added to the target volume in a patient. Imaging methods and
modalities, such as Dual Energy CT and proton CT, have come into consideration
in the pursuit of obtaining an as good as possible estimate of the proton
stopping power. In this study, a Digital Tracking Calorimeter is benchmarked
for proof-of-concept for proton CT purposes. The Digital Tracking Calorimeteris
applied for reconstruction of the tracks and energies of individual high energy
protons. The presented prototype forms the basis for a proton CT system using a
single technology for tracking and calorimetry. This advantage simplifies the
setup and reduces the cost of a proton CT system assembly, and it is a unique
feature of the Digital Tracking Calorimeter. Data from the AGORFIRM beamline at
KVI-CART in Groningen in the Netherlands and Monte Carlo simulation results are
used to in order to develop a tracking algorithm for the estimation of the
residual ranges of a high number of concurrent proton tracks. The range of the
individual protons can at present be estimated with a resolution of 4%. The
readout system for this prototype is able to handle an effective proton
frequency of 1 MHz by using 500 concurrent proton tracks in each readout frame,
which is at the high end range of present similar prototypes. A future further
optimized prototype will enable a high-speed and more accurate determination of
the ranges of individual protons in a therapeutic beam.Comment: 21 pages, 8 figure
Coordinated prophylactic surgical management for women with hereditary breast-ovarian cancer syndrome
<p>Abstract</p> <p>Background</p> <p>Women with <it>BRCA1 </it>or <it>BRCA2 </it>mutations have a substantially increased risk of breast and ovarian cancer compared with the general population. Therefore, prophylactic mastectomy (PM) and bilateral salpingo-oophorectomy (BSO) have been proposed as risk-reduction strategies for <it>BRCA1/2 </it>mutation carriers. We aimed to assess the feasibility of coordinated PM and BSO in hereditary breast-ovarian cancer syndrome.</p> <p>Methods</p> <p>High risk women for breast and ovarian cancer who underwent coordinated PM and BSO were included in this study. Clinical characteristics and surgical and oncologic outcomes were retrospectively reviewed.</p> <p>Results</p> <p>Twelve patients underwent coordinated PM and BSO. Ten had history of previous breast cancer. Autologous breast reconstruction was performed in ten patients. The mean age at surgery was 43 (range 34–65). Mean operating time was 9.3 hours (range 3–16) with a mean postoperative hospitalization of 5.4 days (range 4–8). Intraoperatively, there were no major surgical complications. Postoperatively, one patient developed an abdominal wound dehiscence, another reoperation for flap congestion; one had umbilical superficial epidermolysis, and one patient developed aspiration pneumonia. At a mean follow-up of 84 months, 10 of patients were cancer-free. Although no patients developed a new primary cancer, two developed a distant recurrence.</p> <p>Conclusion</p> <p>Coordinated PM and BSO is a feasible procedure with acceptable morbidity in selected high-risk patients that desire to undergo surgery at one operative setting.</p
Is the United States Claims Court Constitutional?
This article will deal with two major constitutional problems that have resulted from the creation of the Claims Court. The first issue is the constitutionality of the appointment of existing Court of Claims Commissioners to be judges on the Claims Court during a four-year transition period. By legislatively designating the persons who are to serve as judges on the new court, Congress has usurped the presidential appointment power. The second issue relates to the constitutional status of the Claims Court. The Court of Claims which it replaces was created under article III of the Constitution, and the judges on it were therefore entitled to life tenure and salaries that could not be reduced during their terms in office. The new Claims Court, on the other hand, is designated by Congress as an article I court; the judges are to be appointed for only fifteen year terms, and their salaries are subject to control by Congress. The new court exercises full judicial authority, however, and has jurisdiction over cases of national importance in which the government of the United States has a great financial stake. Although the analysis of this issue is far from simple, this author concludes that Congress has exceeded its constitutional authority by failing to comply with the requirements of article III of the Constitution in establishing the Claims Court
Photocurrent measurements of supercollision cooling in graphene
The cooling of hot electrons in graphene is the critical process underlying
the operation of exciting new graphene-based optoelectronic and plasmonic
devices, but the nature of this cooling is controversial. We extract the hot
electron cooling rate near the Fermi level by using graphene as novel
photothermal thermometer that measures the electron temperature () as it
cools dynamically. We find the photocurrent generated from graphene
junctions is well described by the energy dissipation rate , where the heat capacity is and is the
base lattice temperature. These results are in disagreement with predictions of
electron-phonon emission in a disorder-free graphene system, but in excellent
quantitative agreement with recent predictions of a disorder-enhanced
supercollision (SC) cooling mechanism. We find that the SC model provides a
complete and unified picture of energy loss near the Fermi level over the wide
range of electronic (15 to 3000 K) and lattice (10 to 295 K) temperatures
investigated.Comment: 7pages, 5 figure
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