10,152 research outputs found
Relevance of Abelian Symmetry and Stochasticity in Directed Sandpiles
We provide a comprehensive view on the role of Abelian symmetry and
stochasticity in the universality class of directed sandpile models, in context
of the underlying spatial correlations of metastable patterns and scars. It is
argued that the relevance of Abelian symmetry may depend on whether the dynamic
rule is stochastic or deterministic, by means of the interaction of metastable
patterns and avalanche flow. Based on the new scaling relations, we conjecture
critical exponents for avalanche, which is confirmed reasonably well in
large-scale numerical simulations.Comment: 4 pages, 3 figures; published versio
4D Lorentz Electron Microscopy Imaging: Magnetic Domain Wall Nucleation, Reversal, and Wave Velocity
Magnetization reversal is an important topic of research in the fields of both basic and applied ferromagnetism. For the study of magnetization reversal dynamics and magnetic domain wall (DW) motion in ferromagnetic thin films, imaging techniques are indispensable. Here, we report 4D imaging of DWs by the out-of-focus Fresnel method in Lorentz ultrafast electron microscopy (UEM), with in situ spatial and temporal resolutions. The temporal change in magnetization, as revealed by changes in image contrast, is clocked using an impulsive optical field to produce structural deformation of the specimen, thus modulating magnetic field components in the specimen plane. Directly visualized are DW nucleation and subsequent annihilation and oscillatory reappearance (periods of 32 and 45 ns) in nickel films on two different substrates. For the case of Ni films on a Ti/Si_(3)N_4 substrate, under conditions of minimum residual external magnetic field, the oscillation is associated with a unique traveling wave train of periodic magnetization reversal. The velocity of DW propagation in this wave train is measured to be 172 m/s with a wavelength of 7.8 μm. The success of this study demonstrates the promise of Lorentz UEM for real-space imaging of spin switching, ferromagnetic resonance, and laser-induced demagnetization in ferromagnetic nanostructures
Universality classes and crossover behaviors in non-Abelian directed sandpiles
We study universality classes and crossover behaviors in non-Abelian directed
sandpile models, in terms of the metastable pattern analysis. The non-Abelian
property induces spatially correlated metastable patterns, characterized by the
algebraic decay of the grain density along the propagation direction of an
avalanche. Crossover scaling behaviors are observed in the grain density due to
the interplay between the toppling randomness and the parity of the threshold
value. In the presence of such crossovers, we show that the broadness of the
grain distribution plays a crucial role in resolving the ambiguity of the
universality class. Finally, we claim that the metastable pattern analysis is
important as much as the conventional analysis of avalanche dynamics.Comment: 10 pages, 7 figures, 1 table; published in PRE as the full paper of
PRL v101, 218001 (2008
Nonchaotic Nonlinear Motion Visualized in Complex Nanostructures by Stereographic 4D Electron Microscopy
Direct electron imaging with sufficient time resolution is a powerful tool for visualizing the three-dimensional (3D) mechanical motion and resolving the four-dimensional (4D) trajectories of many different components of a nanomachine, e.g., a NEMS device. Here, we report a nanoscale nonchaotic motion of a nano- and microstructured NiTi shape memory alloy in 4D electron microscopy. A huge amplitude oscillatory mechanical motion following laser heating is observed repetitively, likened to a 3D motion of a conductor’s baton. By time-resolved 4D stereographic reconstruction of the motion, prominent vibrational frequencies (3.0, 3.8, 6.8, and 14.5 MHz) are fully characterized, showing evidence of nonlinear behavior. Moreover, it is found that a stress (fluence)−strain (displacement) profile shows nonlinear elasticity. The observed resonances of the nanostructure are reminiscent of classical molecular quasi-periodic behavior, but here both the amplitude and frequency of the motion are visualized using ultrafast electron microscopy
Consequences of the partial restoration of chiral symmetry in AdS/QCD
Chiral symmetry is an essential concept in understanding QCD at low energy.
We treat the chiral condensate, which measures the spontaneous breaking of
chiral symmetry, as a free parameter to investigate the effect of partially
restored chiral symmetry on the physical quantities in the frame work of an
AdS/QCD model. We observe an interesting scaling behavior among the nucleon
mass, pion decay constant and chiral condensate. We propose a phenomenological
way to introduce the temperature dependence of a physical quantity in the
AdS/QCD model with the thermal AdS metric.Comment: 11 pages, 6 figure
Einstein Manifolds As Yang-Mills Instantons
It is well-known that Einstein gravity can be formulated as a gauge theory of
Lorentz group where spin connections play a role of gauge fields and Riemann
curvature tensors correspond to their field strengths. One can then pose an
interesting question: What is the Einstein equations from the gauge theory
point of view? Or equivalently, what is the gauge theory object corresponding
to Einstein manifolds? We show that the Einstein equations in four dimensions
are precisely self-duality equations in Yang-Mills gauge theory and so Einstein
manifolds correspond to Yang-Mills instantons in SO(4) = SU(2)_L x SU(2)_R
gauge theory. Specifically, we prove that any Einstein manifold with or without
a cosmological constant always arises as the sum of SU(2)_L instantons and
SU(2)_R anti-instantons. This result explains why an Einstein manifold must be
stable because two kinds of instantons belong to different gauge groups,
instantons in SU(2)_L and anti-instantons in SU(2)_R, and so they cannot decay
into a vacuum. We further illuminate the stability of Einstein manifolds by
showing that they carry nontrivial topological invariants.Comment: v4; 17 pages, published version in Mod. Phys. Lett.
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