353 research outputs found
A review of congenital heart block
Congenital heart block is a rare disorder. It has an incidence of about 1 in 22,000 live
births. It may be associated with high mortality and morbidity. This should generate a
high index of suspicion for early diagnosis and aggressive therapy when appropriate.
The congenital heart block associated with neonatal lupus is considered a form of
passively acquired autoimmune disease in which maternal autoantibodies to the
intracellular ribonucleoproteins Ro (SS-A) and La (SS-B), cross the placenta and
injure the previously normal fetal heart. Women with serum titers of anti-Ro antibody
carry a 3% risk of having a child with neonatal lupus syndrome. Recurrence rates are
about 18%. We believe that serial echocardiograms should be acquired so that early
diagnosis is made and aggressive therapy administered, if signs of conduction
system disease such as PR interval prolongation by Doppler are found, so as to
optimize the outcome. Establishment of guidelines for therapy have been set
empirically, should signs of congenital heart block develop. Those patients whose
congenital heart block is associated with structural heart disease have a higher
morbidity and mortality, which is determined more by the underlying structural
congenital heart disease than it is by the need for a pacemaker per se.peer-reviewe
Quantum tunnelling of magnetization in Mn12-ac studied by 55Mn NMR
We present an ultra-low temperature study (down to T = 20 mK) of the nuclear
spin-lattice relaxation (SLR) in the 55Mn nuclei of the molecular magnet
Mn12-ac. The nuclear spins act as local probes for the electronic spin
fluctuations, due to thermal excitations and to tunnelling events. In the
quantum regime (below T = 0.75 K), the nuclear SLR becomes
temperature-independent and is driven by fluctuations of the cluster's
electronic spin due to the quantum tunnelling of magnetization in the ground
doublet. The quantitative analysis of the nuclear SLR shows that the presence
of fast-tunnelling molecules, combined with nuclear intercluster spin
diffusion, plays an important role in the relaxation process.Comment: RevTex, 5 pages, 3 eps figures; presented at the Internation
Conference on Molecular Magnets (Valencia, 5 - 10 Oct. 2002); to be published
in Polyhedro
Processor Allocation for Optimistic Parallelization of Irregular Programs
Optimistic parallelization is a promising approach for the parallelization of
irregular algorithms: potentially interfering tasks are launched dynamically,
and the runtime system detects conflicts between concurrent activities,
aborting and rolling back conflicting tasks. However, parallelism in irregular
algorithms is very complex. In a regular algorithm like dense matrix
multiplication, the amount of parallelism can usually be expressed as a
function of the problem size, so it is reasonably straightforward to determine
how many processors should be allocated to execute a regular algorithm of a
certain size (this is called the processor allocation problem). In contrast,
parallelism in irregular algorithms can be a function of input parameters, and
the amount of parallelism can vary dramatically during the execution of the
irregular algorithm. Therefore, the processor allocation problem for irregular
algorithms is very difficult.
In this paper, we describe the first systematic strategy for addressing this
problem. Our approach is based on a construct called the conflict graph, which
(i) provides insight into the amount of parallelism that can be extracted from
an irregular algorithm, and (ii) can be used to address the processor
allocation problem for irregular algorithms. We show that this problem is
related to a generalization of the unfriendly seating problem and, by extending
Tur\'an's theorem, we obtain a worst-case class of problems for optimistic
parallelization, which we use to derive a lower bound on the exploitable
parallelism. Finally, using some theoretically derived properties and some
experimental facts, we design a quick and stable control strategy for solving
the processor allocation problem heuristically.Comment: 12 pages, 3 figures, extended version of SPAA 2011 brief announcemen
OL-085 A candidate genes study for the association of host single nucleotide polymorphisms with liver cirrhosis risk in Chinese hepatitis B patients
Single and double qubit gates by manipulating degeneracy
A novel mechanism is proposed for single and double qubit state manipulations
in quantum computation with four-fold degenerate energy levels. The principle
is based on starting with a four fold degeneracy, lifting it stepwise
adiabatically by a set of control parameters and performing the quantum gate
operations on non-degenerate states. A particular realization of the proposed
mechanism is suggested by using inductively coupled rf-squid loops in the
macroscopic quantum tunnelling regime where the energy eigen levels are
directly connected with the measurable flux states. The one qubit and two qubit
controlled operations are demonstrated explicitly. The appearance of the flux
states also allows precise read-in and read-out operations by the measurement
of flux.Comment: 6 pages + 5 figures (separately included
Measurement of coherent charge transfer in an adiabatic Cooper pair pump
We study adiabatic charge transfer in a superconducting Cooper pair pump,
focusing on the influence of current measurement on coherence. We investigate
the limit where the Josephson coupling energy between the various parts
of the system is small compared to the Coulomb charging energy . In this
case the charge transferred in a pumping cycle , the charge of one
Cooper pair: the main contribution is due to incoherent Cooper pair tunneling.
We are particularly interested in the quantum correction to , which is due
to coherent tunneling of pairs across the pump and which depends on the
superconducting phase difference between the electrodes: . A measurement of tends to destroy the phase
coherence. We first study an arbitrary measuring circuit and then specific
examples and show that coherent Cooper pair transfer can in principle be
detected using an inductively shunted ammeter
Nonorientable spacetime tunneling
Misner space is generalized to have the nonorientable topology of a Klein
bottle, and it is shown that in a classical spacetime with multiply connected
space slices having such a topology, closed timelike curves are formed.
Different regions on the Klein bottle surface can be distinguished which are
separated by apparent horizons fixed at particular values of the two angular
variables that eneter the metric. Around the throat of this tunnel (which we
denote a Klein bottlehole), the position of these horizons dictates an ordinary
and exotic matter distribution such that, in addition to the known diverging
lensing action of wormholes, a converging lensing action is also present at the
mouths. Associated with this matter distribution, the accelerating version of
this Klein bottlehole shows four distinct chronology horizons, each with its
own nonchronal region. A calculation of the quantum vacuum fluctuations
performed by using the regularized two-point Hadamard function shows that each
chronology horizon nests a set of polarized hypersurfaces where the
renormalized momentum-energy tensor diverges. This quantum instability can be
prevented if we take the accelerating Klein bottlehole to be a generalization
of a modified Misner space in which the period of the closed spatial direction
is time-dependent. In this case, the nonchronal regions and closed timelike
curves cannot exceed a minimum size of the order the Planck scale.Comment: 11 pages, RevTex, Accepted in Phys. Rev.
Black Holes and Instabilities of Negative Tension Branes
We consider the collision in 2+1 dimensions of a black hole and a negative
tension brane on an orbifold. Because there is no gravitational radiation in
2+1 dimensions, the horizon area shrinks when part of the brane falls through.
This provides a potential violation of the generalized second law of
thermodynamics. However, tracing the details of the dynamical evolution one
finds that it does not proceed from equilibrium configuration to equilibrium
configuration. Instead, a catastrophic space-time singularity develops similar
to the `big crunch' of FRW space-times. In the context of classical
general relativity, our result demonstrates a new instability of constructions
with negative tension branes.Comment: 18 pages, 3 figures, uses RevTeX. Minor typos fixed. References and
one footnote adde
Magnetization steps in a diluted Heisenberg antiferromagnetic chain: Theory and experiments on TMMC:Cd
A theory for the equilibrium low-temperature magnetization M of a diluted
Heisenberg antiferromagnetic chain is presented. The magnetization curve, M
versus B, is calculated using the exact contributions of finite chains with 1
to 5 spins, and the "rise and ramp approximation" for longer chains. Some
non-equilibrium effects that occur in a rapidly changing B, are also
considered. Specific non-equilibrium models based on earlier treatments of the
phonon bottleneck, and of spin flips associated with cross relaxation and with
level crossings, are discussed. Magnetization data on powders of TMMC diluted
with cadmium [i.e., (CH_3)_4NMn_xCd_(1-x)Cl_3, with 0.16<=x<=0.50 were measured
at 0.55 K in 18 T superconducting magnets. The field B_1 at the first MST from
pairs is used to determine the NN exchange constant, J, which changes from -5.9
K to -6.5 K as x increases from 0.16 to 0.50. The magnetization curves obtained
in the superconducting magnets are compared with simulations based on the
equilibrium theory. Data for the differential susceptibility, dM/dB, were taken
in pulsed magnetic fields (7.4 ms duration) up to 50 T, with the powder samples
in a 1.5 K liquid-helium bath. Non-equilibrium effects, which became more
severe as x decreased, were observed. The non-equilibrium effects are
tentatively interpreted using the "Inadequate Heat Flow Scenario," or to
cross-relaxation, and crossings of energy levels, including those of excited
states.Comment: 16 pages, 14 figure
L-VRAP-a lunar volatile resources analysis package for lunar exploration
The Lunar Volatile Resources Analysis Package (L-VRAP) has been conceived to deliver some of the objectives of the proposed Lunar Lander mission currently being studied by the European Space Agency. The purpose of the mission is to demonstrate and develop capability; the impetus is very much driven by a desire to lay the foundations for future human exploration of the Moon. Thus, LVRAP has design goals that consider lunar volatiles from the perspective of both their innate scientific interest and also their potential for in situ utilisation as a resource. The device is a dual mass spectrometer system and is capable of meeting the requirements of the mission with respect to detection, quantification and characterisation of volatiles. Through the use of appropriate sampling techniques, volatiles from either the regolith or atmosphere (exosphere) can be analysed. Furthermore, since L-VRAP has the capacity to determine isotopic compositions, it should be possible for the instrument to determine the sources of the volatiles that are found on the Moon (be they lunar per se, extra-lunar, or contaminants imparted by the mission itself
- …