19,607 research outputs found
Piezoelectric rotator for studying quantum effects in semiconductor nanostructures at high magnetic fields and low temperatures
We report the design and development of a piezoelectric sample rotation
system, and its integration into an Oxford Instruments Kelvinox 100 dilution
refrigerator, for orientation-dependent studies of quantum transport in
semiconductor nanodevices at millikelvin temperatures in magnetic fields up to
10T. Our apparatus allows for continuous in situ rotation of a device through
>100deg in two possible configurations. The first enables rotation of the field
within the plane of the device, and the second allows the field to be rotated
from in-plane to perpendicular to the device plane. An integrated angle sensor
coupled with a closed-loop feedback system allows the device orientation to be
known to within +/-0.03deg whilst maintaining the sample temperature below
100mK.Comment: 8 pages, 5 figure
Rocking and rolling: a can that appears to rock might actually roll
A beer bottle or soda can on a table, when slightly tipped and released,
falls to an upright position and then rocks up to a somewhat opposite tilt.
Superficially this rocking motion involves a collision when the flat circular
base of the container slaps the table before rocking up to the opposite tilt. A
keen eye notices that the after-slap rising tilt is not generally just
diametrically opposite the initial tilt but is veered to one side or the other.
Cushman and Duistermaat (2006) recently noticed such veering when a flat disk
with rolling boundary conditions is dropped nearly flat. Here, we generalize
these rolling disk results to arbitrary axi-symmetric bodies and to
frictionless sliding. More specifically, we study motions that almost but do
not quite involve a face-down collision of the round container's bottom with
the table-top. These motions involve a sudden rapid motion of the contact point
around the circular base. Surprisingly, like for the rolling disk, the net
angle of motion of this contact point is nearly independent of initial
conditions. This angle of turn depends simply on the geometry and mass
distribution but not on the moment of inertia about the symmetry axis. We
derive simple asymptotic formulas for this "angle of turn" of the contact point
and check the result with numerics and with simple experiments. For tall
containers (height much bigger than radius) the angle of turn is just over
and the sudden rolling motion superficially appears as a nearly symmetric
collision leading to leaning on an almost diametrically opposite point on the
bottom rim.Comment: 10 pages, 5 figure
Does rising inequality delay marriage? Evidence from India
Top incomes in India have been rising sharply since the 1980s. In this chapter, we show that this phenomenon may have resulted in delays to female marriage. Our analysis takes advantage of cross-sectional variation in earnings distribution across narrowly defined marriage markets. We find that female marriage rates decline in response to increases in top male incomes. We also find marked effects on women’s educational attainment (in terms of years of schooling, as well as high school and college completion rates). We examine a number of hypotheses, and conclude that the pattern of results suggests a mechanism in which increases in top male incomes prolong the duration of marital search on the part of women
Steady State Analysis and Heavy Traffic Limits for Regulated Markov Chains.
Consider a continuous time finite state irreducible Markov chain whose jump transitions are partitioned into one group that is regulated and the other group that is not. The regulated transitions are only allowed to occur if there is a token available. We collect the tokens in a buer and allow a regulated transition to occur simultaneously with the removal of a token from the buffer. New tokens are added to the buer at a constant Poisson rate but the regulated transitions will be blocked if they occur too quickly. We will apply matrix analysis to the joint distribution for the state of the Markov chain and the number of tokens in the buffer. We will give a simple stability condition for the joint process and show that its steady state distribution will have a matrix geometric distribution. Moreover, we obtain from our analysis a heavy traffic limit for this joint steady state distribution which has a product form structure. This Markov chain model and steady state analysis generalizes the work of many earlier papers on specific queueing systems such as Konheim and Reiser or Latouche and Neuts, but most significantly the work of Kogan and Puhalskii.Markov Chains, Matrix-Geometric Solution, Heavy-Traffic Limits, Product Form Solution, Tensor and Kronecker Products.
Bivalirudin versus unfractionated heparin: a meta-analysis of patients receiving percutaneous coronary intervention for acute coronary syndromes
OBJECTIVE: Acute coronary syndrome (ACS) encompasses ST segment elevation myocardial infarction (STEMI), with generally high thrombus burden and non-ST segment elevation ACS (NSTE-ACS), with lower thrombus burden. In the setting of percutaneous coronary intervention (PCI) for ACS, bivalirudin appears superior to unfractionated heparin (UFH), driven by reduced major bleeding. Recent trials suggest that the benefit of bivalirudin may be reduced with use of transradial access and evolution in antiplatelet therapy. Moreover, a differential role of bivalirudin in ACS cohorts is unknown. METHODS: A meta-analysis of randomised trials comparing bivalirudin and UFH in patients with ACS receiving PCI, with separate analyses in STEMI and NSTE-ACS groups. Overall estimates of treatment effect were calculated with random-effects model. RESULTS: In 5 trials of STEMI (10 358 patients), bivalirudin increased the risk of acute stent thrombosis (ST) (OR 3.62; CI 1.95 to 6.74; p<0.0001) compared with UFH. Bivalirudin reduced the risk of major bleeding only when compared with UFH plus planned glycoprotein IIb/IIIa inhibitors (GPI) (OR 0.49; CI 0.36 to 0.67; p<0.00001). In 14 NSTE-ACS trials (25 238 patients), there was no difference between bivalirudin and UFH in death, myocardial infarction or ST. However, bivalirudin reduced the risk of major bleeding compared with UFH plus planned GPI (OR 0.52; CI 0.43 to 0.62; p<0.00001), or UFH plus provisional GPI (OR 0.68; CI 0.46 to 1.01; p=0.05). The reduction in major bleeding with bivalirudin was not related to vascular access site. CONCLUSIONS: Bivalirudin increases the risk of acute ST in STEMI, but may confer an advantage over UFH in NSTE-ACS while undergoing PCI, reducing major bleeding without an increase in ST
Advanced turboprop vibratory characteristics
The assembly of SR5 advanced turboprop blades to develop a structural dynamic data base for swept props is reported. Steady state blade deformation under centrifugal loading and vibratory characteristics of the rotor assembly were measured. Vibration was induced through a system of piezoelectric crystals attached to the blades. Data reduction procedures are used to provide deformation, mode shape, and frequencies of the assembly at predetermined speeds
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