4,545 research outputs found
Estimation of elastic and viscous properties of the left ventricle based on annulus plane harmonic behavior
Assessment of left ventricular (LV) function
with an emphasis on contractility has been a challenge
in cardiac mechanics during the recent decades. The LV
function is usually described by the LV pressurevolume
(P-V) diagram. The standard P-V diagrams are
easy to interpret but difficult to obtain and require
invasive instrumentation for measuring the
corresponding volume and pressure data. In the present
study, we introduce a technique that can estimate the
viscoelastic properties of the LV based on harmonic
behavior of the ventricular chamber and it can be
applied non-invasively as well. The estimation technique
is based on modeling the actual long axis displacement
of the mitral annulus plane toward the cardiac base as a
linear damped oscillator with time-varying coefficients.
The time-varying parameters of the model were
estimated by a standard Recursive Linear Least
Squares (RLLS) technique. LV stiffness at end-systole
and end diastole was in the range of 61.86-136.00
dyne/g.cm and 1.25-21.02 dyne/g.cm, respectively. The
only input used in this model was the long axis
displacement of the annulus plane, which can also be
obtained non-invasively using tissue Doppler or MR
imaging
Apparent suppression of turbulent magnetic dynamo action by a dc magnetic field
Numerical studies of the effect of a dc magnetic field on dynamo action
(development of magnetic fields with large spatial scales), due to
helically-driven magnetohydrodynamic turbulence, are reported. The apparent
effect of the dc magnetic field is to suppress the dynamo action, above a
relatively low threshold. However, the possibility that the suppression results
from an improper combination of rectangular triply spatially-periodic boundary
conditions and a uniform dc magnetic field is addressed: heretofore a common
and convenient computational convention in turbulence investigations. Physical
reasons for the observed suppression are suggested. Other geometries and
boundary conditions are offered for which the dynamo action is expected not to
be suppressed by the presence of a dc magnetic field component.Comment: To appear in Physics of Plasma
Correlation between magnetic interactions and domain structure in A1 FePt ferromagnetic thin films
We have investigated the relationship between the domain structure and the
magnetic interactions in a series of FePt ferromagnetic thin films of varying
thickness. As-made films grow in the magnetically soft and chemically
disordered A1 phase that may have two distinct domain structures. Above a
critical thickness nm the presence of an out of plane
anisotropy induces the formation of stripes, while for planar
domains occur.
Magnetic interactions have been characterized using the well known DCD-IRM
remanence protocols, plots, and magnetic viscosity measurements. We
have observed a strong correlation between the domain configuration and the
sign of the magnetic interactions. Planar domains are associated with positive
exchange-like interactions, while stripe domains have a strong negative
dipolar-like contribution. In this last case we have found a close correlation
between the interaction parameter and the surface dipolar energy of the stripe
domain structure. Using time dependent magnetic viscosity measurements, we have
also estimated an average activation volume for magnetic reversal, nm which is approximately
independent of the film thickness or the stripe period.Comment: 25 pages, 11 figure
Speech recognition through physical reservoir computing with neuromorphic nanowire networks
The hardware implementation of the reservoir computing paradigm represents a key aspect for taking into advantage of neuromorphic data processing. In this context, self-organised nanonetworks represent a versatile and scalable computational substrate for multiple tasks by exploiting the emerging collective behaviour of the system arising from complexity. The emerging behaviour allows spatio-temporal processing of multiple input signals and relies on the nonlinear interaction in between a multitude of nanoscale memristive elements. By means of a physics-based grid-graph modeling, we report on the implementation of reservoir computing for a speech recognition task in a memristive nanonetwork based on nanowires (NWs) acting as a physical reservoir. Besides analysing the pre-processing step for the transduction of the audio samples in electrical stimuli to be applied to the physical reservoir, we analyse the effect of the network size and the adoption of virtual nodes on computing performances. Results show that memristive nanonetworks allow in materia implementation of reservoir computing for the realisation of brain-inspired neuromorphic systems with reduced training cost
Memristive devices based on single ZnO nanowires-from material synthesis to neuromorphic functionalities
Memristive and resistive switching devices are considered promising building blocks for the realization of artificial neural networks and neuromorphic systems. Besides conventional top-down memristive devices based on thin films, resistive switching devices based on nanowires (NWs) have attracted great attention, not only for the possibility of going beyond current scaling limitations of the top-down approach, but also as model systems for the localization and investigation of the physical mechanism of switching. This work reports on the fabrication of memristive devices based on ZnO NWs, from NW synthesis to single NW-based memristive cell fabrication and characterization. The bottom-up synthesis of ZnO NWs was performed by low-pressure chemical vapor deposition according to a self-seeding vapor-solid (VS) mechanism on a Pt substrate over large scale (∼cm2), without the requirement of previous seed deposition. The grown ZnO NWs are single crystalline with wurtzite crystal structure and are vertically aligned respect to the growth substrate. Single NWs were then contacted by means of asymmetric contacts, with an electrochemically active and an electrochemically inert electrode, to form NW-based electrochemical metallization memory cells that show reproducible resistive switching behaviour and neuromorphic functionalities including short-term synaptic plasticity and paired pulse facilitation. Besides representing building blocks for NW-based memristive and neuromorphic systems, these single crystalline devices can be exploited as model systems to study physicochemical processing underlaying memristive functionalities thanks to the high localization of switching events on the ZnO crystalline surface
Magnetic field dependence of antiferromagnetic resonance in NiO
We report on measurements of magnetic field and temperature dependence of antiferromagnetic resonances in the prototypical antiferromagnet NiO. The frequencies of the magnetic resonances in the vicinity of 1 THz have been determined in the time-domain via time-resolved Faraday measurements after selective excitation by narrow-band superradiant terahertz (THz) pulses at temperatures down to 3 K and in magnetic fields up to 10 T. The measurements reveal two antiferromagnetic resonance modes, which can be distinguished by their characteristic magnetic field dependencies. The nature of the two modes is discussed by comparison to an eight-sublattice antiferromagnetic model, which includes superexchange between the next-nearest-neighbor Ni spins, magnetic dipolar interactions, cubic magneto-crystalline anisotropy, and Zeeman interaction with the external magnetic field. Our study indicates that a two-sublattice model is insufficient for the description of spin dynamics in NiO, while the magnetic-dipolar interactions and magneto-crystalline anisotropy play important roles
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