7,386 research outputs found
Ultrasonic Doppler measurement of renal artery blood flow
An extensive evaluation of the practical and theoretical limitations encountered in the use of totally implantable CW Doppler flowmeters is provided. Theoretical analyses, computer models, in-vitro and in-vivo calibration studies describe the sources and magnitudes of potential errors in the measurement of blood flow through the renal artery, as well as larger vessels in the circulatory system. The evaluation of new flowmeter/transducer systems and their use in physiological investigations is reported
Ultrasonic Doppler measurement of renal artery blood flow
Studies were made of (1) blood flow redistribution during lower body negative pressure (LBNP), (2) the profile of blood flow across the mitral annulus of the heart (both perpendicular and parallel to the commissures), (3) testing and evaluation of a number of pulsed Doppler systems, (4) acute calibration of perivascular Doppler transducers, (5) redesign of the mitral flow transducers to improve reliability and ease of construction, and (6) a frequency offset generator designed for use in distinguishing forward and reverse components of blood flow by producing frequencies above and below the offset frequency. Finally methodology was developed and initial results were obtained from a computer analysis of time-varying Doppler spectra
Variational cluster approach to the Hubbard model: Phase-separation tendency and finite-size effects
Using the variational cluster approach (VCA), we study the transition from
the antiferromagnetic to the superconducting phase of the two-dimensional
Hubbard model at zero temperature. Our calculations are based on a new method
to evaluate the VCA grand potential which employs a modified Lanczos algorithm
and avoids integrations over the real or imaginary frequency axis. Thereby,
very accurate results are possible for cluster sizes not accessible to full
diagonalization. This is important for an improved treatment of short-range
correlations, including correlations between Cooper pairs in particular. We
investigate the cluster-size dependence of the phase-separation tendency that
has been proposed recently on the basis of calculations for smaller clusters.
It is shown that the energy barrier driving the phase separation decreases with
increasing cluster size. This supports the conjecture that the ground state
exhibits microscopic inhomogeneities rather than macroscopic phase separation.
The evolution of the single-particle spectum as a function of doping is studied
in addtion and the relevance of our results for experimental findings is
pointed out.Comment: 7 pages, 6 figures, published versio
Quantum radiation by electrons in lasers and the Unruh effect
In addition to the Larmor radiation known from classical electrodynamics,
electrons in a laser field may emit pairs of entangled photons -- which is a
pure quantum effect. We investigate this quantum effect and discuss why it is
suppressed in comparison with the classical Larmor radiation (which is just
Thomson backscattering of the laser photons). Further, we provide an intuitive
explanation of this process (in a simplified setting) in terms of the Unruh
effect.Comment: 4 pages, 3 figure
Computing with cells: membrane systems - some complexity issues.
Membrane computing is a branch of natural computing which abstracts computing models from the structure and the functioning of the living cell. The main ingredients of membrane systems, called P systems, are (i) the membrane structure, which consists of a hierarchical arrangements of membranes which delimit compartments where (ii) multisets of symbols, called objects, evolve according to (iii) sets of rules which are localised and associated with compartments. By using the rules in a nondeterministic/deterministic maximally parallel manner, transitions between the system configurations can be obtained. A sequence of transitions is a computation of how the system is evolving. Various ways of controlling the transfer of objects from one membrane to another and applying the rules, as well as possibilities to dissolve, divide or create membranes have been studied. Membrane systems have a great potential for implementing massively concurrent systems in an efficient way that would allow us to solve currently intractable problems once future biotechnology gives way to a practical bio-realization. In this paper we survey some interesting and fundamental complexity issues such as universality vs. nonuniversality, determinism vs. nondeterminism, membrane and alphabet size hierarchies, characterizations of context-sensitive languages and other language classes and various notions of parallelism
A low stray light, high current, low energy electron source
A design of an electron gun system is presented whose stray light emission is reduced by about three orders of magnitude compared to a regular low-energy electron diffraction gun. This is achieved by a combination of a BaO cathode run at rather low temperature and a 30° tandem parallel-plate analyzer used as an optical baffle. The system provides a high beam current of several microampers at 50 eV beam energy. The system can be used down to ∼10 eV
Magnetism and d-wave superconductivity on the half-filled square lattice with frustration
The role of frustration and interaction strength on the half-filled Hubbard
model is studied on the square lattice with nearest and next-nearest neighbour
hoppings t and t' using the Variational Cluster Approximation (VCA). At
half-filling, we find two phases with long-range antiferromagnetic (AF) order:
the usual Neel phase, stable at small frustration t'/t, and the so-called
collinear (or super-antiferromagnet) phase with ordering wave-vector
or , stable for large frustration. These are separated by a phase with
no detectable long-range magnetic order. We also find the d-wave
superconducting (SC) phase (), which is favoured by frustration if
it is not too large. Intriguingly, there is a broad region of coexistence where
both AF and SC order parameters have non-zero values. In addition, the physics
of the metal-insulator transition in the normal state is analyzed. The results
obtained with the help of the VCA method are compared with the large-U
expansion of the Hubbard model and known results for the frustrated J1-J2
Heisenberg model. These results are relevant for pressure studies of undoped
parents of the high-temperature superconductors: we predict that an insulator
to d-wave SC transition may appear under pressure.Comment: 12 pages, 10 figure
Experimental analysis of lateral impact on planar brittle material
The fragmentation of alumina and glass plates due to lateral impact is
studied. A few hundred plates have been fragmented at different impact
velocities and the produced fragments are analyzed. The method employed in this
work allows one to investigate some geometrical properties of the fragments,
besides the traditional size distribution usually studied in former
experiments. We found that, although both materials exhibit qualitative similar
fragment size distribution function, their geometrical properties appear to be
quite different. A schematic model for two-dimensional fragmentation is also
presented and its predictions are compared to our experimental results. The
comparison suggests that the analysis of the fragments' geometrical properties
constitutes a more stringent test of the theoretical models' assumptions than
the size distribution
Fractional Fokker-Planck dynamics: Numerical algorithm and simulations
Anomalous transport in a tilted periodic potential is investigated
numerically within the framework of the fractional Fokker-Planck dynamics via
the underlying CTRW. An efficient numerical algorithm is developed which is
applicable for an arbitrary potential. This algorithm is then applied to
investigate the fractional current and the corresponding nonlinear mobility in
different washboard potentials. Normal and fractional diffusion are compared
through their time evolution of the probability density in state space.
Moreover, we discuss the stationary probability density of the fractional
current values.Comment: 10 pages, 9 figure
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