2,224 research outputs found
Unidirectional Amplification and Shaping of Optical Pulses by Three-Wave Mixing with Negative Phonons
A possibility to greatly enhance frequency-conversion efficiency of
stimulated Raman scattering is shown by making use of extraordinary properties
of three-wave mixing of ordinary and backward waves. Such processes are
commonly attributed to negative-index plasmonic metamaterials. This work
demonstrates the possibility to replace such metamaterials that are very
challenging to engineer by readily available crystals which support elastic
waves with contra-directed phase and group velocities. The main goal of this
work is to investigate specific properties of indicated nonlinear optical
process in short pulse regime and to show that it enables elimination of
fundamental detrimental effect of fast damping of optical phonons on the
process concerned. Among the applications is the possibility of creation of a
family of unique photonic devices such as unidirectional Raman amplifiers and
femtosecond pulse shapers with greatly improved operational properties.Comment: 6 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1304.681
Key role of elastic vortices in the initiation of intersonic shear cracks
Using the particle-based method of movable cellular automata, we analyze the initiation and propagation of intersonic mode II cracks along a weak interface. We show that the stress concentration in front of the crack tip, which is believed to be the mechanism of acceleration of the crack beyond the speed of shear waves, is due to the formation of an elastic vortex. The vortex develops in front of the crack during the short initial period of crack propagation. It expands and moves away from the crack tip and finally detaches from it. Maximum stress concentration in the vortex is achieved at the moment of detachment of the vortex. The crack can accelerate towards the longitudinal wave speed if the magnitude of shear stresses in the elastic vortex reaches the material shear strength before vortex detachment. We have found that for given material parameters, the condition for the unstable accelerated crack propagation depends only on the ratio of the initial crack length to its width (e.g., due to surface roughness)
On the role of scales in contact mechanics and friction between elastomers and randomly rough self-affine surfaces
The paper is devoted to a qualitative analysis of friction of elastomers from the point of view of scales contributing to the force of friction. We argue that – contrary to widespread opinion – friction between a randomly rough self-affine fractal surface and an elastomer is not a multiscale phenomenon, but is governed mostly by the interplay of only two scales – as a rule the largest and the smallest scales of roughness of the contacting bodies. The hypothesis of two-scale character of elastomer friction is illustrated by computer simulations in the framework of the paradigm of Greenwood, Tabor and Grosch using a simplified one-dimensional model
Structure, Properties, and Phase Transformations of Water Nanoconfined between Brucite-like Layers: The Role of Wall Surface Polarity
The interaction of water with confining surfaces is primarily governed by the wetting properties of the wall material—in particular, whether it is hydrophobic or hydrophilic. The hydrophobicity or hydrophilicity itself is determined primarily by the atomic structure and polarity of the surface groups. In the present work, we used molecular dynamics to study the structure and properties of nanoscale water layers confined between layered metal hydroxide surfaces with a brucite-like structure. The influence of the surface polarity of the confining material on the properties of nanoconfined water was studied in the pressure range of 0.1–10 GPa. This pressure range is relevant for many geodynamic phenomena, hydrocarbon recovery, contact spots of tribological systems, and heterogeneous materials under extreme mechanical loading. Two phase transitions were identified in water confined within 2 nm wide slit-shaped nanopores: (1) at p1 = 3.3–3.4 GPa, the liquid transforms to a solid phase with a hexagonal close-packed (HCP) crystal structure, and (2) at p2 = 6.7–7.1 GPa, a further transformation to face-centered cubic (FCC) crystals occurs. It was found that the behavior of the confined water radically changes when the partial charges (and, therefore, the surface polarity) are reduced. In this case, water transforms directly from the liquid phase to an FCC-like phase at 3.2–3.3 GPa. Numerical simulations enabled determination of the amount of hydrogen bonding and diffusivity of nanoconfined water, as well as the relationship between pressure and volumetric strain
Piecewise Parabolic Method on a Local Stencil for Magnetized Supersonic Turbulence Simulation
Stable, accurate, divergence-free simulation of magnetized supersonic
turbulence is a severe test of numerical MHD schemes and has been surprisingly
difficult to achieve due to the range of flow conditions present. Here we
present a new, higher order-accurate, low dissipation numerical method which
requires no additional dissipation or local "fixes" for stable execution. We
describe PPML, a local stencil variant of the popular PPM algorithm for solving
the equations of compressible ideal magnetohydrodynamics. The principal
difference between PPML and PPM is that cell interface states are evolved
rather that reconstructed at every timestep, resulting in a compact stencil.
Interface states are evolved using Riemann invariants containing all transverse
derivative information. The conservation laws are updated in an unsplit
fashion, making the scheme fully multidimensional. Divergence-free evolution of
the magnetic field is maintained using the higher order-accurate constrained
transport technique of Gardiner and Stone. The accuracy and stability of the
scheme is documented against a bank of standard test problems drawn from the
literature. The method is applied to numerical simulation of supersonic MHD
turbulence, which is important for many problems in astrophysics, including
star formation in dark molecular clouds. PPML accurately reproduces in
three-dimensions a transition to turbulence in highly compressible isothermal
gas in a molecular cloud model. The low dissipation and wide spectral bandwidth
of this method make it an ideal candidate for direct turbulence simulations.Comment: 28 pages, 18 figure
Vortex Dynamics and the Hall-Anomaly: a Microscopic Analysis
We present a microscopic derivation of the equation of motion for a vortex in
a superconductor. A coherent view on vortex dynamics is obtained, in which {\it
both} hydrodynamics {\it and} the vortex core contribute to the forces acting
on a vortex. The competition between these two provides an interpretation of
the observed sign change in the Hall angle in superconductors with mean free
path of the order of the coherence length in terms of broken
particle-hole symmetry, which is related to details of the microscopic
mechanism of superconductivity.Comment: 12 pages, late
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