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## ‣ Mechanics of Individual, Isolated Vortices in a Cuprate Superconductor

Fonte: Nature Publishing Group
Publicador: Nature Publishing Group

Português

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Superconductors often contain quantized microscopic whirlpools of electrons, called vortices, that can be modelled as one-dimensional elastic objects1. Vortices are a diverse area of study for condensed matter because of the interplay between thermal fluctuations, vortex–vortex interactions and the interaction of the vortex core with the three-dimensional disorder landscape. Although vortex matter has been studied extensively, the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa

_{2}Cu_{3}O_{6.991}single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.; PhysicsLink permanente para citações:

## ‣ Why is Understanding Glassy Polymer Mechanics So Difficult?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Materials Science

In this Perspective, I describe recent work on systems in which the
traditional distinctions between (i) unentangled vs. well-entangled systems and
(ii) melts vs. glasses seem least useful, and argue for the broader use in
glassy polymer mechanics of two more dichotomies: systems which possess (iii)
unary vs. binary and (iv) cooperative vs. nonccoperative relaxation dynamics. I
discuss the applicability of (iii-iv) to understanding the functional form of
glassy strain hardening. Results from molecular dynamics simulations show that
the "dramatic" strain hardening observed in densely entangled systems is
associated with a crossover from unary, noncooperative to binary, cooperative
relaxation as strain increases; chains stretch between entanglement points,
altering the character of local plasticity. Promising approaches for future
research along these lines are discussed.; Comment: Results and conclusions same but manuscript extensively edited for
clarity. Accepted for publication in J. Polym. Sci. - Polym. Phys

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## ‣ Rough contact mechanics for graded bulk rheology: The role of small-scale wavelengths on rubber friction

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 13/10/2015
Português

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We present a numerical model for the prediction of the rough contact
mechanics of a viscoelastic block, with graded rheology, in steady sliding
contact with a randomly rough rigid surface. In particular, we derive the
effective surface response of a stepwise or continuously-graded block in the
Fourier domain, which is then embedded in a Fourier-based residuals molecular
dynamic formulation of the contact mechanics. Finally we discuss on the role of
small-scale wavelengths on rubber friction and contact area, and we demonstrate
that the rough contact mechanics exhibits effective interface properties which
converge to asymptotes upon increase of the small-scale roughness content, when
a realistic rheology of the confinement is taken into account.

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## ‣ Twisted equivariant matter

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#High Energy Physics - Theory#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Strongly Correlated Electrons#Mathematical Physics#Mathematics - Algebraic Topology

We show how general principles of symmetry in quantum mechanics lead to
twisted notions of a group representation. This framework generalizes both the
classical 3-fold way of real/complex/quaternionic representations as well as a
corresponding 10-fold way which has appeared in condensed matter and nuclear
physics. We establish a foundation for discussing continuous families of
quantum systems. Having done so, topological phases of quantum systems can be
defined as deformation classes of continuous families of gapped Hamiltonians.
For free particles there is an additional algebraic structure on the
deformation classes leading naturally to notions of twisted equivariant
K-theory. In systems with a lattice of translational symmetries we show that
there is a canonical twisting of the equivariant K-theory of the Brillouin
torus. We give precise mathematical definitions of two invariants of the
topological phases which have played an important role in the study of
topological insulators. Twisted equivariant K-theory provides a finer
classification of topological insulators than has been previously available.; Comment: 93 pages, 1 figure; v2 has minor corrections and clarifications for
publication in AHP

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## ‣ Jamming of Soft Particles: Geometry, Mechanics, Scaling and Isostaticity

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 06/11/2009
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Amorphous materials as diverse as foams, emulsions, colloidal suspensions and
granular media can jam into a rigid, disordered state where they withstand
finite shear stresses before yielding. Here we review the current understanding
of the transition to jamming and the nature of the jammed state for disordered
packings of particles that act through repulsive contact interactions and are
at zero temperature and zero shear stress. We first discuss the breakdown of
affine assumptions that underlies the rich mechanics near jamming. We then
extensively discuss jamming of frictionless soft spheres. At the jamming point,
these systems are marginally stable (isostatic) in the sense of constraint
counting, and many geometric and mechanical properties scale with distance to
this jamming point. Finally we discuss current explorations of jamming of
frictional and non-spherical (ellipsoidal) particles. Both friction and
asphericity tune the contact number at jamming away from the isostatic limit,
but in opposite directions. This allows one to disentangle distance to jamming
and distance to isostaticity. The picture that emerges is that most quantities
are governed by the contact number and scale with distance to isostaticity,
while the contact number itself scales with distance to jamming.; Comment: Submitted to J Phys Cond Matt 25 pages...

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## ‣ Effective approach to non-relativistic quantum mechanics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Non-trivial self-adjoint extensions in quantum mechanics have been used in
the past to model contact interactions when those interactions are expected a
priori. However, real physical systems can only ever be understood to a finite
resolution, so there exists a generic ignorance of possible short-range
interactions. To address this issue, a real-space approach is described here
wherein an artificial boundary is inserted at an intermediate scale to
explicitly hide possible short-distance effects. Boundary conditions may then
encode physical effects that are hidden behind the boundary, thereby
effectively capturing the set of possible UV completions. Using this approach,
a non-relativistic analysis is performed of the free particle, harmonic
oscillator, and Coulomb potential in three dimensions. Requiring measurable
quantities, such as spectra and cross sections, to be independent of the
position of the boundary, renormalization group-type equations are derived that
determine how the boundary conditions run. Generically, observables differ from
their canonical values and symmetries are anomalously broken. Connections are
made to well-studied physical systems, such as the deuteron and condensed
matter systems that employ Feshbach resonances.; Comment: 6 pages...

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## ‣ Quantum entanglement based on surface phonon polaritons in condensed matter systems

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/10/2012
Português

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Condensed matter systems are potential candidates to realize the integration
of quantum information circuits. Surface phonon polariton (SPhP) is a special
propagation mode in condensed matter systems. We present an investigation on
the entanglement of SPhP modes. The entangled pairs are generated from
entangled photons injected to the system. Quantum performances of entangled
SPhPs are investigated by using the interaction Hamiltonian and the
perturbation theory. The wave mechanics approach is taken to describe the
coupling process as a comparison. Finally, the correlation of system is
examined. A whole set of descriptions of SPhP entanglement thus are presented.; Comment: 16 pages, 4 figures

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## ‣ Quantum coherence in momentum space of light-matter condensates

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Quantum Gases#Quantum Physics

We show that the use of momentum-space optical interferometry, which avoids
any spatial overlap between two parts of a macroscopic quantum state, presents
a unique way to study coherence phenomena in polariton condensates. In this
way, we address the longstanding question in quantum mechanics: "\emph{Do two
components of a condensate, which have never seen each other, possess a
definitive phase?}" [P. W. Anderson, \emph{Basic Notions of Condensed Matter
Physics} (Benjamin, 1984)]. A positive answer to this question is
experimentally obtained here for light-matter condensates, created under
precise symmetry conditions, in semiconductor microcavities taking advantage of
the direct relation between the angle of emission and the in-plane momentum of
polaritons.; Comment: 6 pages, 3 figures

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## ‣ Systematic analysis of Persson's contact mechanics theory of randomly rough elastic surfaces

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/06/2014
Português

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We systematically check explicit and implicit assumptions of Persson's
contact mechanics theory. It casts the evolution of the pressure distribution
${\rm Pr}(p)$ with increasing resolution of surface roughness as a diffusive
process, in which resolution plays the role of time. The tested key assumptions
of the theory are: (a) the diffusion coefficient is independent of pressure
$p$, (b) the diffusion process is drift-free at any value of $p$, (c) the point
$p=0$ acts as an absorbing barrier, i.e., once a point falls out of contact, it
never reenters again, (d) the Fourier component of the elastic energy is only
populated if the appropriate wave vector is resolved, and (e) it no longer
changes when even smaller wavelengths are resolved. Using high-resolution
numerical simulations, we quantify deviations from these approximations and
find quite significant discrepancies in some cases. For example, the drift
becomes substantial for small values of $p$, which typically represent points
in real space close to a contact line. On the other hand, there is a
significant flux of points reentering contact. These and other identified
deviations cancel each other to a large degree, resulting in an overall
excellent description for contact area...

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## ‣ Mechanics and force transmission in soft composites of rods in elastic gels

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/04/2011
Português

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Physics - Biological Physics

We report detailed theoretical investigations of the micro-mechanics and bulk
elastic properties of composites consisting of randomly distributed stiff
fibers embedded in an elastic matrix in two and three dimensions. Recent
experiments published in Physical Review Letters [102, 188303 (2009)] have
suggested that the inclusion of stiff microtubules in a softer, nearly
incompressible biopolymer matrix can lead to emergent compressibility. This can
be understood in terms of the enhancement of the compressibility of the
composite relative to its shear compliance as a result of the addition of stiff
rod-like inclusions. We show that the Poisson's ratio $\nu$ of such a composite
evolves with increasing rod density towards a particular value, or {\em fixed
point}, independent of the material properties of the matrix, so long as it has
a finite initial compressibility. This fixed point is $\nu=1/4$ in three
dimensions and $\nu=1/3$ in two dimensions. Our results suggest an important
role for stiff filaments such as microtubules and stress fibers in cell
mechanics. At the same time, our work has a wider elasticity context, with
potential applications to composite elastic media with a wide separation of
scales in stiffness of its constituents such as carbon nanotube-polymer
composites...

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## ‣ Linear elastic fracture mechanics predicts the propagation distance of frictional slip

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/08/2014
Português

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When a frictional interface is subject to a localized shear load, it is often
(experimentally) observed that local slip events initiate at the stress
concentration and propagate over parts of the interface by arresting naturally
before reaching the edge. We develop a theoretical model based on linear
elastic fracture mechanics to describe the propagation of such precursory slip.
The model's prediction of precursor lengths as a function of external load is
in good quantitative agreement with laboratory experiments as well as with
dynamic simulations, and provides thereby evidence to recognize frictional slip
as a fracture phenomenon. We show that predicted precursor lengths depend,
within given uncertainty ranges, mainly on the kinetic friction coefficient,
and only weakly on other interface and material parameters. By simplifying the
fracture mechanics model we also reveal sources for the observed non-linearity
in the growth of precursor lengths as a function of the applied force. The
discrete nature of precursors as well as the shear tractions caused by
frustrated Poisson's expansion are found to be the dominant factors. Finally,
we apply our model to a different, symmetric set-up and provide a prediction of
the propagation distance of frictional slip for future experiments.

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## ‣ Mesoscopic Mechanics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 29/07/2003
Português

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#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Strongly Correlated Electrons

This article is concerned with the existence, status and description of the
so-called emergent phenomena believed to occur in certain principally planar
electronic systems. In fact, two distinctly different if inseparable tasks are
accomplished. First, a rigorous mathematical model is proposed of emergent
character, which is conceptually bonded with Quantum Mechanics while apparently
non-derivable from the many-body Schr\"{o}dinger equation. I call the resulting
conceptual framework the Mesoscopic Mechanics (MeM). Its formulation is
space-independent and comprises a nonlinear and holistic extension of the free
electron model. Secondly, the question of relevancy of the proposed ``emergent
mechanics" to the actually observed phenomena is discussed. In particular, I
postulate a probabilistic interpretation, and indicate how the theory could be
applied and verified by experiment.
The Mesoscopic Mechanics proposed here has been deduced from the Nonlinear
Maxwell Theory (NMT)--a classical in character nonlinear field theory. This
latter theory has already been shown to provide a consistent phenomenological
model of such phenomena as superconductivity, charge stripes, magnetic vortex
lattice, and magnetic oscillations. The NMT, which arose from geometric
considerations...

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## ‣ Possible Connection between Probability, Spacetime Geometry and Quantum Mechanics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/07/2007
Português

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Following our discussion [Physica A, 375 (2007) 123] to associate an
analogous probabilistic description with spacetime geometry in the
Schwarzschild metric from the macro- to the micro-domain, we argue that there
is a possible connection among normalized probabilities, spacetime geometry (in
the form of Schwarzschild radii) and quantum mechanics (in the form of complex
wave functions). We show how this association along different (n)-nested
surfaces --representing curve space due to an inhomogeneous density of matter--
preserves the postulates of quantum mechanics at different geometrical scales.; Comment: in press

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## ‣ Strong Coupling Perturbation Theory in Quantum Mechanics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 31/07/2003
Português

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We present a full introduction to the recent devised perturbation theory for
strong coupling in quantum mechanics. In order to put the theory in a proper
historical perspective, the approach devised in quantum field theory is rapidly
presented, showing how it implies a kind of duality in perturbation theory,
from the start. The approach of renormalization group in perturbation theory is
then presented. This method permits to resum secularities in perturbation
theory and makes fully algorithmical the resummation, transforming the
perturbation calculations in a step by step computational procedure. The
general theorem on which is founded a proper application of the strong coupling
expansion, based on a result in the quantum adiabatic theory, is then exposed.
This theorem gives the leading order of a strong coupling expansion. Then,
after the introduction of the principle of duality in perturbation theory that
puts in a proper context the quantum field theory method, the resulting theory
of the strong coupling expansion and the free picture are presented. An
algorithm for the computation of the perturbation series is finally given. This
approach has a lot of applications in fields as quantum optics, condensed
matter and so on, extending the original expectations of the quantum field
theory method. So...

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## ‣ A condensed matter analogy of impact crater formation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/03/2014
Português

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Impact craters exist on various solid objects in the planetary system. A
simplified analogy of the process of their formation is here analyzed by
standard solid state physics and the so called dynamic quantized fracture
mechanics. An expression which links the crater volume to the parameters of the
impactor and the target is obtained within the two approaches. For low impactor
energy, this expression is of the same mathematical form as the one resulting
from recent experiments.It is shown that the formation of an impact crater is
possible even without heating of the target, if the critical stress in the
target satisfies certain conditions. The critical value of the stress needed
for the occurence of a fracture is calculated for three craters: two
terrestrial and one lunar crater. The approach presented here uses only
measurable material parameters, and is therefore more realistic than the
treatement of the same problem using the cohesive energy of materials.; Comment: plain LaTeX and 1 EPS file packed together

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## ‣ A multiscale Molecular Dynamics approach to Contact Mechanics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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The friction and adhesion between elastic bodies are strongly influenced by
the roughness of the surfaces in contact. Here we develop a multiscale
molecular dynamics approach to contact mechanics, which can be used also when
the surfaces have roughness on many different length-scales, e.g., for self
affine fractal surfaces. As an illustration we consider the contact between
randomly rough surfaces, and show that the contact area varies linearly with
the load for small load. We also analyze the contact morphology and the
pressure distribution at different magnification, both with and without
adhesion. The calculations are compared with analytical contact mechanics
models based on continuum mechanics.; Comment: Format Revtex4, two columns, 13 pages, 19 pictures. Submitted for
publication in the European Physical Journal E. Third revision with minimal
changes: Corrected a few mistyping

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## ‣ Yield Stress Materials in Soft Condensed Matter

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/02/2015
Português

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We present a comprehensive review of the physical behavior of yield stress
materials in soft condensed matter, which encompasses a broad range of soft
materials from colloidal assemblies and gels to emulsions and non-Brownian
suspensions. All these disordered materials display a nonlinear response to an
external mechanical forcing, which results from the existence of a finite force
threshold for flow to occur, the yield stress. We discuss both the physical
origin and the rheological consequences associated with this nonlinear
behavior. We give an overview of the different experimental techniques
developed to measure the yield stress. We discuss extensively the recent
progress concerning a microscopic description of the flow dynamics of yield
stress materials, emphasizing in particular the role played by relaxation
timescales, the interplay between shear flow and aging behavior, the existence
of inhomogeneous shear flows and shear bands, wall slip, and non-local effects
in confined geometries. We finally review the status of modeling of the shear
rheology of yield stress materials in the framework of continuum mechanics.; Comment: Review article: 58 pages, 38 figs, 487 refs

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## ‣ Mechanics of Individual, Isolated Vortices in a Cuprate Superconductor

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/09/2008
Português

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Superconductors often contain quantized microscopic whirlpools of electrons,
called vortices, that can be modeled as one-dimensional elastic objects.
Vortices are a diverse playground for condensed matter because of the interplay
between thermal fluctuations, vortex-vortex interactions, and the interaction
of the vortex core with the three-dimensional disorder landscape. While vortex
matter has been studied extensively, the static and dynamic properties of an
individual vortex have not. Here we employ magnetic force microscopy (MFM) to
image and manipulate individual vortices in detwinned, single crystal
YBa2Cu3O6.991 (YBCO), directly measuring the interaction of a moving vortex
with the local disorder potential. We find an unexpected and dramatic
enhancement of the response of a vortex to pulling when we wiggle it
transversely. In addition, we find enhanced vortex pinning anisotropy that
suggests clustering of oxygen vacancies in our sample and demonstrates the
power of MFM to probe vortex structure and microscopic defects that cause
pinning.

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## ‣ Non-commutative mechanics, in mathematical & in condensed matter physics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#Condensed Matter - Mesoscale and Nanoscale Physics#High Energy Physics - Theory#Physics - Classical Physics

Non-commutative structures were introduced, independently and around the same
time, in mathematical and in condensed matter physics (see Table~1). Souriau's
construction applied to the two-parameter central extension of the planar
Galilei group leads to the ``exotic'' particle, which has non-commuting
position coordinates. A Berry-phase argument applied to the Bloch electron
yields in turn a semiclassical model that has been used to explain the
anomalous/spin/optical Hall effects. The non-commutative parameter is
momentum-dependent in this case, and can take the form of a monopole in
momentum space.; Comment: This is a contribution to the Proc. of the O'Raifeartaigh Symposium
on Non-Perturbative and Symmetry Methods in Field Theory (June 2006,
Budapest, Hungary), published in SIGMA (Symmetry, Integrability and Geometry:
Methods and Applications) at http://www.emis.de/journals/SIGMA/

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## ‣ The jamming surface of granular matter determined from soil mechanics results

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 17/06/2005
Português

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Classical soil mechanics results are used to propose the equation of the
jamming transition surface in the (stress, specific volume) space. Taking
axis-ymmetric conditions, labelling q the deviatoric stress and p' the mean
pressure applied on the granular skeleton, and considering normal range of
pressure (10 kPa-10MPa) the equation of the surface of jamming transition is v
= vo-m ln(p'/p'o)+ md ln(1+q q/(M' M' p' p')); M' is related to the friction
angle, m and md are two constants which depend on soil characteristics; p'o is
a "unit" pressure.; Comment: 2 pages + 1 page, 0 figure

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