29 research outputs found
Phonon nanocapacitor for storage and lasing of terahertz lattice waves
We introduce a novel ultra-compact nanocapacitor of coherent phonons formed
by high-finesse interference mirrors based on atomic-scale semiconductor
metamaterials. Our molecular dynamics simulations show that the nanocapacitor
stores THz monochromatic lattice waves, which can be used for phonon lasing -
the emission of coherent phonons. Either one- or two-color phonon lasing can be
realized depending on the geometry of the nanodevice. The two color regimes of
the capacitor originates from the distinct transmittance dependance on the
phonon wave packet incident angle for the two phonon polarizations at their
respective resonances. Phonon nanocapacitor can be charged by cooling the
sample equilibrated at room temperature or by the pump-probe technique. The
nanocapacitor can be discharged by applying tunable reversible strain,
resulting in the emission of coherent THz acoustic beams.Comment: 12 pages, 5 figure
Nonlinear waves in a model for silicate layers
Some layered silicates are composed of positive ions, surrounded by layers of ions with opposite sign. Mica muscovite is a particularly interesting material, because there exist fossil and experimental evidence for nonlinear excitations transporting localized energy and charge along the cation rows within the potassium layers. This evidence suggest that there are different kinds of excitations with different energies and properties. Some of the authors proposed recently a one-dimensional model based in physical principles and the silicate structure. The main characteristic of the model is that it has a hard substrate potential and two different repulsion terms, between ions and nuclei. In a previous work with this model, it was found the propagation of crowdions, i.e., lattice kinks in a lattice with substrate potential that transport mass and charge. They have a single specific velocity and energy coherent with the experimental data. In the present work we perform a much more thorough search for nonlinear excitations in the same model using the pseudospectral method to obtain exact nanopteron solutions, which are single kinks with tails, crowdions and bi-crowdions. We analyze their velocities, energies and stability or instability and the possible reasons for the latter. We relate the different excitations with their possible origin from recoils from different beta decays and with the fossil tracks. We explore the consequences of some variation of the physical parameters because their values are not perfectly known. Through a different method, we also have found stationary and moving breathers, that is, localized nonlinear excitations with an internal vibration. Moving breathers have small amplitude and energy, which is also coherent with the fossil evidence.MINECO (Spain) FIS2015-65998-C2-2-PJunta de Andalucía 2017/FQM-280Universidad de Sevilla (España) grants VI PPIT-US-201
Kinks in Coulomb’s chains
Chains of identical ions, for which the dominant interaction is the electrostatic repulsion, appear in layered
silicates. The ions can move almost from site to site. The chains do not explode because the surrounding
media has a net negative charge which screens the Coulomb’s repulsion and become attractive when the ions
separate two much. Moreover there is a border effect which keeps the ions within the crystal.
We have been able to obtain moving supersonic kinks that keep their shape and cross nicely one with each
other and can travel over the surrounding sea of phonons. Their energies can be very different, from the order
of eVs to hundreds of them. Therefore they can influence many different processes within silicates.Ministerio de Ciencia e Innovación FIS2008-0484
On modulational instability and energy localization in anharmonic lattices at finite energy density
The localization of vibrational energy, induced by the modulational
instability of the Brillouin-zone-boundary mode in a chain of classical
anharmonic oscillators with finite initial energy density, is studied within a
continuum theory. We describe the initial localization stage as a gas of
envelope solitons and explain their merging, eventually leading to a single
localized object containing a macroscopic fraction of the total energy of the
lattice. The initial-energy-density dependences of all characteristic time
scales of the soliton formation and merging are described analytically. Spatial
power spectra are computed and used for the quantitative explanation of the
numerical results.Comment: 12 pages, 7 figure
Ultradiscrete kinks with supersonic speed in a layered crystal with realistic potentials
We develop a dynamical model of the propagating nonlinear localized
excitations, supersonic kinks, in the cation layer in a silicate mica crystal.
We start from purely electrostatic Coulomb interaction and add the
Ziegler-Biersack-Littmark short-range repulsive potential and the periodic
potential produced by other atoms of the lattice. This approach allows the
construction of supersonic kinks which can propagate in the lattice within a
large range of energies and velocities. The interparticle distances in the
lattice kinks with high energy are physically reasonable values. The
introduction of the periodic lattice potential results in the important feature
that the kinks propagate with a single velocity and a single energy which are
independent on the excitation conditions. The found kinks are ultra-discrete
and can be described with the "magic wave number" , which was
previously revealed in the nonlinear sinusoidal waves and supersonic kinks in
the Fermi-Pasta-Ulam lattice. The extreme discreteness of the supersonic kinks,
with basically two particles moving at the same time, allows the interpretation
of their double-kink structure. The energy of the supersonic kinks is between
the possible source of K recoil in beta decay and the energy necessary
for the ejection of an atom at the border as has been found experimentally.Comment: 14 pages, 15 figure
Confined photon modes with triangular symmetry in hexagonal microcavities in 2D photonic Crystals
We present theoretical and experimental studies of the size and thickness
dependencies of the optical emission spectra from microcavities with hexagonal
shape in films of two-dimensional photonic crystal. A semiclassical plane-wave
model, which takes into account the electrodynamic properties of quasi-2D
planar photonic microcavity, is developed to predict the eigenfrequencies of
the confined photon modes as a function of both the hexagon-cavity size and the
film thickness. Modes with two different symmetries, triangular and hexagonal,
are critically analyzed. It is shown that the model of confined photon modes
with triangular symmetry gives a better agreement between the predicted
eigenmodes and the observed resonances.Comment: 14 pages, 6 figure