21,702 research outputs found
SymFET: A Proposed Symmetric Graphene Tunneling Field Effect Transistor
In this work, an analytical model to calculate the channel potential and
current-voltage characteristics in a Symmetric tunneling
Field-Effect-Transistor (SymFET) is presented. The current in a SymFET flows by
tunneling from an n-type graphene layer to a p-type graphene layer. A large
current peak occurs when the Dirac points are aligned at a particular drain-to-
source bias VDS . Our model shows that the current of the SymFET is very weakly
dependent on temperature. The resonant current peak is controlled by chemical
doping and applied gate bias. The on/off ratio increases with graphene
coherence length and doping. The symmetric resonant peak is a good candidate
for high-speed analog applications, and can enable digital logic similar to the
BiSFET. Our analytical model also offers the benefit of permitting simple
analysis of features such as the full-width-at-half-maximum (FWHM) of the
resonant peak and higher order harmonics of the nonlinear current. The SymFET
takes advantage of the perfect symmetry of the bandstructure of 2D graphene, a
feature that is not present in conventional semiconductors
Two paths of cluster evolution: global expansion versus core collapse
All gravitationally bound clusters expand, due to both gas loss from their
most massive members and binary heating. All are eventually disrupted tidally,
either by passing molecular clouds or the gravitational potential of their host
galaxies. However, their interior evolution can follow two very different
paths. Only clusters of sufficiently large initial population and size undergo
the combined interior contraction and exterior expansion that leads eventually
to core collapse. In all other systems, core collapse is frustrated by binary
heating. These clusters globally expand for their entire lives, up to the point
of tidal disruption.
Using a suite of direct N-body calculations, we trace the "collapse line" in
r_v-N space that separates these two paths. Here, r_v and N are the cluster's
initial virial radius and population, respectively. For realistic starting
radii, the dividing N-value is from 10^4 to over 10^5. We also show that there
exists a minimum population, N_min, for core collapse. Clusters with N < N_min
tidally disrupt before core collapse occurs. At the Sun's Galactocentric
radius, R_G = 8.5 kpc, we find N_min >~ 300. The minimum population scales with
Galactocentric radius as R_G^{-9/8}.
The position of an observed cluster relative to the collapse line can be used
to predict its future evolution. Using a small sample of open clusters, we find
that most lie below the collapse line, and thus will never undergo core
collapse. Most globular clusters, on the other hand, lie well above the line.
In such a case, the cluster may or may not go through core collapse, depending
on its initial size. We show how an accurate age determination can help settle
this issue.Comment: Accepted for publication in MNRAS. 14 Pages, 9 Figures, 2 Table
Measurements of SCRF cavity dynamic heat load in horizontal test system
The Horizontal Test System (HTS) at Fermilab is currently testing fully
assembled, dressed superconducting radio frequency (SCRF) cavities. These
cavities are cooled in a bath of superfluid helium at 1.8K. Dissipated RF power
from the cavities is a dynamic heat load on the cryogenic system. The magnitude
of heat flux from these cavities into the helium is also an important variable
for understanding cavity performance. Methods and hardware used to measure this
dynamic heat load are presented. Results are presented from several cavity
tests and testing accuracy is discussed.Comment: 6 pp. Cryogenic Engineering Conference and International Cryogenic
Materials Conference 28 Jun - 2 Jul 2009. Tucson, Arizon
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