2,416 research outputs found
Large magnetoresistance at room-temperature in semiconducting polymer sandwich devices
We report on the discovery of a large, room temperature magnetoresistance
(MR) effect in polyfluorene sandwich devices in weak magnetic fields. We
characterize this effect and discuss its dependence on voltage, temperature,
film thickness, electrode materials, and (unintentional) impurity
concentration. We usually observed negative MR, but positive MR can also be
achieved under high applied electric fields. The MR effect reaches up to 10% at
fields of 10mT at room temperature. The effect shows only a weak temperature
dependence and is independent of the sign and direction of the magnetic field.
We find that the effect is related to the hole current in the devices.Comment: 3 pages, 4 figure
Cosmological Perturbations from Cosmic Strings
Some aspects of the theory of cosmological perturbations from cosmic strings
and other topological defects are outlined, with particular reference to a
simple example: a spatially flat CDM-dominated universe. The conserved
energy-momentum pseudo-tensor is introduced, and the equation for the density
perturbation derived from it. It is shown how the scaling hypothesis for defect
evolution results in a Harrison-Zel'dovich spectrum for wavelengths well inside
the horizon.Comment: LaTeX, 6pp. From Proceedings of `Trends in Astroparticle Physics',
Stockholm, Sweden 22-25 September 1994, edited by L. Bergstr\"om, P. Carlson,
P.O. Hulth and H. Snellman (to be published in Nucl.~Phys~B, Proceedings
Supplements Section
Beyond the Small-Angle Approximation For MBR Anisotropy from Seeds
In this paper we give a general expression for the energy shift of massless
particles travelling through the gravitational field of an arbitrary matter
distribution as calculated in the weak field limit in an asymptotically flat
space-time. It is {\it not} assumed that matter is non-relativistic. We
demonstrate the surprising result that if the matter is illuminated by a
uniform brightness background that the brightness pattern observed at a given
point in space-time (modulo a term dependent on the oberver's velocity) depends
only on the matter distribution on the observer's past light-cone. These
results apply directly to the cosmological MBR anisotropy pattern generated in
the immediate vicinity of of an object like a cosmic string or global texture.
We apply these results to cosmic strings, finding a correction to previously
published results for in the small-angle approximation. We also derive the
full-sky anisotropy pattern of a collapsing texture knot.Comment: 23 pages, FERMILAB-Pub-94/047-
Pre-operative fixed flexion deformity a curse to the knee arthroplasty surgeon?
Background: The aim of the study was to study the effect of preoperative flexion deformity on the postoperative knee range of motion following total knee arthroplasty.Methods: A total of 28 knees were studied out of which twenty were osteoarthritis and eight were rheumatoid arthritis. Cruciate substituting and cruciate retaining implants were used. Patients above the age of 45 years with a minimum follow up period of two years from April 2010 were studied.Results: 28 knees had an average preoperative range of motion of 87.678. Patients with preoperative flexion deformity had postoperative range of motion of 97.5 degrees. The average postoperative flexion in 8 rheumatoid patients was 96.25 and 20 osteoarthritis patients was 107.462.Conclusions: The average postoperative range of motion was 105.538. Patients with a higher pre-operative range of motion had higher postoperative range of motion. Rheumatoid patients had a significantly low range of motion when compared to osteoarthritis patients. Patients with increased fixed flexion deformity had a significantly low postoperative range of motion
Hyperfine interaction and magnetoresistance in organic semiconductors
We explore the possibility that hyperfine interaction causes the recently
discovered organic magnetoresistance (OMAR) effect. Our study employs both
experiment and theoretical modelling. An excitonic pair mechanism model based
on hyperfine interaction, previously suggested by others to explain magnetic
field effects in organics, is examined. Whereas this model can explain a few
key aspects of the experimental data, we, however, uncover several fundamental
contradictions as well. By varying the injection efficiency for minority
carriers in the devices, we show experimentally that OMAR is only weakly
dependent on the ratio between excitons formed and carriers injected, likely
excluding any excitonic effect as the origin of OMAR.Comment: 10 pages, 7 figures, 1 tabl
Large magnetoresistance at room-temperature in small molecular weight organic semiconductor sandwich devices
We present an extensive study of a large, room temperature negative
magnetoresistance (MR) effect in tris-(8-hydroxyquinoline) aluminum sandwich
devices in weak magnetic fields. The effect is similar to that previously
discovered in polymer devices. We characterize this effect and discuss its
dependence on field direction, voltage, temperature, film thickness, and
electrode materials. The MR effect reaches almost 10% at fields of
approximately 10 mT at room temperature. The effect shows only a weak
temperature dependence and is independent of the sign and direction of the
magnetic field. Measuring the devices' current-voltage characteristics, we find
that the current depends on the voltage through a power-law. We find that the
magnetic field changes the prefactor of the power-law, whereas the exponent
remains unaffected. We also studied the effect of the magnetic field on the
electroluminescence (MEL) of the devices and analyze the relationship between
MR and MEL. We find that the largest part of MEL is simply a consequence of a
change in device current caused by the MR effect.Comment: 8 figure
Location Management Cost Reduction Using Adaptive Velocity-movement Based Scheme
Wireless personal communication networks (PCNs) consist of a fixed wireless
network and a large number of mobile terminals. These terminals are free to
travel within the PC coverage area without service interruption. Each terminal
periodically reports its location to the network by a process called location
update (or registration). When a call arrives for a particular mobile terminal,
the network will determine the exact location of the destination terminal by a
process called terminal paging. One major problem that arises in this scenario
is the cost associated with paging and registration. Several papers in the
literature attempt to reduce the cost by devising new schemes for paging and
registration. One of the many interesting schemes was presented by Wan and Lin (1998) that considers a dynamic paging scheme based on the semi-real
time velocity information of an individual mobile user, which allows a more
accurate prediction of the user location when a call arrives. In this paper, we
modified the scheme presented by Wan and Lin by creating an adaptive
velocity timer that changes according to the speed of the mobile and applies
the same analysis to the movement-based scheme. The investigation shows that
the proposed approach of Wan and Lin has better results than what was
reported therein and our new approach helps reduce the total cost drastically
compared to the original scheme. Results also show that the movement threshold
and the adaptive velocity time unit, when they are adaptive, provide significant
savings of cost under different cell sizes and velocities in high and low mobility
systems
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