6,510 research outputs found
Fracton pairing mechanism for "strange" superconductors: Self-assembling organic polymers and copper-oxide compounds
Self-assembling organic polymers and copper-oxide compounds are two classes
of "strange" superconductors, whose challenging behavior does not comply with
the traditional picture of Bardeen, Cooper, and Schrieffer (BCS)
superconductivity in regular crystals. In this paper, we propose a theoretical
model that accounts for the strange superconducting properties of either class
of the materials. These properties are considered as interconnected
manifestations of the same phenomenon: We argue that superconductivity occurs
in the both cases because the charge carriers (i.e., electrons or holes)
exchange {\it fracton excitations}, quantum oscillations of fractal lattices
that mimic the complex microscopic organization of the strange superconductors.
For the copper oxides, the superconducting transition temperature as
predicted by the fracton mechanism is of the order of K. We suggest
that the marginal ingredient of the high-temperature superconducting phase is
provided by fracton coupled holes that condensate in the conducting
copper-oxygen planes owing to the intrinsic field-effect-transistor
configuration of the cuprate compounds. For the gate-induced superconducting
phase in the electron-doped polymers, we simultaneously find a rather modest
transition temperature of K owing to the limitations imposed by
the electron tunneling processes on a fractal geometry. We speculate that
hole-type superconductivity observes larger onset temperatures when compared to
its electron-type counterpart. This promises an intriguing possibility of the
high-temperature superconducting states in hole-doped complex materials. A
specific prediction of the present study is universality of ac conduction for
.Comment: 12 pages (including separate abstract page), no figure
Current-Mode Techniques for the Implementation of Continuous- and Discrete-Time Cellular Neural Networks
This paper presents a unified, comprehensive approach
to the design of continuous-time (CT) and discrete-time
(DT) cellular neural networks (CNN) using CMOS current-mode
analog techniques. The net input signals are currents instead
of voltages as presented in previous approaches, thus avoiding
the need for current-to-voltage dedicated interfaces in image
processing tasks with photosensor devices. Outputs may be either
currents or voltages. Cell design relies on exploitation of current
mirror properties for the efficient implementation of both linear
and nonlinear analog operators. These cells are simpler and
easier to design than those found in previously reported CT
and DT-CNN devices. Basic design issues are covered, together
with discussions on the influence of nonidealities and advanced
circuit design issues as well as design for manufacturability
considerations associated with statistical analysis. Three prototypes
have been designed for l.6-pm n-well CMOS technologies.
One is discrete-time and can be reconfigured via local logic for
noise removal, feature extraction (borders and edges), shadow
detection, hole filling, and connected component detection (CCD)
on a rectangular grid with unity neighborhood radius. The other
two prototypes are continuous-time and fixed template: one for
CCD and other for noise removal. Experimental results are given
illustrating performance of these prototypes
Describing the Flow Curve of Shear-Banding Fluids Through a Structural Minimal Model
Main characteristics of colloidal systems that develop fluid phases with
different mechanical properties, namely shear-banding fluids, are briefly
reviewed both from experimental and theoretical (modelling) point of view. A
non-monotonic shear stress vs. shear rate constitutive relation is presented.
This relation derives from a phenomenological model of a shear ratedependent
viscosity describing structural changes and involves the possibility of
multivalued shear rates under a given shear stress. In the case of a
stress-dependent viscosity, the same model allows one to predict vorticity
banding. Predictions of this model under controlled stress are discussed,
namely occurrence of a kind of top- and bottom-jumping of the shear rate in
response to stress increasing-decreasing. Applying this model to evaluation of
the flow curve of such colloidal systems is performed. Particular emphasis is
placed on the adequate computation of the shear rate function in cylindrical
Couette cells in order to handle the corresponding flow curve which exhibits
the well-known shear stress plateau. Indeed, as different fluid phases coexist
in the flow domain, measured (torque vs. angular velocity) data cannot be
directly converted into rheometric (shear stress vs. shear rate) functions. As
the lacking non-local terms in the model prevents the direct determination of
the stress-plateau, this value is included as an adjustable parameter. Thus
model predictions satisfactorily match up experimental data of wormlike
micellar solutions from the literature.Comment: 22 pages, 9 fi
A study of transonic aerodynamic analysis methods for use with a hypersonic aircraft synthesis code
A means of performing routine transonic lift, drag, and moment analyses on hypersonic all-body and wing-body configurations were studied. The analysis method is to be used in conjunction with the Hypersonic Vehicle Optimization Code (HAVOC). A review of existing techniques is presented, after which three methods, chosen to represent a spectrum of capabilities, are tested and the results are compared with experimental data. The three methods consist of a wave drag code, a full potential code, and a Navier-Stokes code. The wave drag code, representing the empirical approach, has very fast CPU times, but very limited and sporadic results. The full potential code provides results which compare favorably to the wind tunnel data, but with a dramatic increase in computational time. Even more extreme is the Navier-Stokes code, which provides the most favorable and complete results, but with a very large turnaround time. The full potential code, TRANAIR, is used for additional analyses, because of the superior results it can provide over empirical and semi-empirical methods, and because of its automated grid generation. TRANAIR analyses include an all body hypersonic cruise configuration and an oblique flying wing supersonic transport
Domain Agnostic Fourier Neural Operators
Fourier neural operators (FNOs) can learn highly nonlinear mappings between
function spaces, and have recently become a popular tool for learning responses
of complex physical systems. However, to achieve good accuracy and efficiency,
FNOs rely on the Fast Fourier transform (FFT), which is restricted to modeling
problems on rectangular domains. To lift such a restriction and permit FFT on
irregular geometries as well as topology changes, we introduce domain agnostic
Fourier neural operator (DAFNO), a novel neural operator architecture for
learning surrogates with irregular geometries and evolving domains. The key
idea is to incorporate a smoothed characteristic function in the integral layer
architecture of FNOs, and leverage FFT to achieve rapid computations, in such a
way that the geometric information is explicitly encoded in the architecture.
In our empirical evaluation, DAFNO has achieved state-of-the-art accuracy as
compared to baseline neural operator models on two benchmark datasets of
material modeling and airfoil simulation. To further demonstrate the capability
and generalizability of DAFNO in handling complex domains with topology
changes, we consider a brittle material fracture evolution problem. With only
one training crack simulation sample, DAFNO has achieved generalizability to
unseen loading scenarios and substantially different crack patterns from the
trained scenario. Our code and data accompanying this paper are available at
https://github.com/ningliu-iga/DAFNO
Shear-induced transitions and instabilities in surfactant wormlike micelles
In this review, we report recent developments on the shear-induced
transitions and instabilities found in surfactant wormlike micelles. The survey
focuses on the non-linear shear rheology and covers a broad range of surfactant
concentrations, from the dilute to the liquid-crystalline states and including
the semi-dilute and concentrated regimes. Based on a systematic analysis of
many surfactant systems, the present approach aims to identify the essential
features of the transitions. It is suggested that these features define classes
of behaviors. The review describes three types of transitions and/or
instabilities : the shear-thickening found in the dilute regime, the
shear-banding which is linked in some systems to the isotropic-to-nematic
transition, and the flow-aligning and tumbling instabilities characteristic of
nematic structures. In these three classes of behaviors, the shear-induced
transitions are the result of a coupling between the internal structure of the
fluid and the flow, resulting in a new mesoscopic organization under shear.
This survey finally highlights the potential use of wormlike micelles as model
systems for complex fluids and for applications.Comment: 64 pages, 31 figures, 2 table
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