17,444 research outputs found
Dynamical phase transition in vibrational surface modes
We consider the dynamical properties of a simple model of vibrational surface
modes. We obtain the exact spectrum of surface excitations and discuss their
dynamical features. In addition to the usually discussed localized and
oscillatory regimes we also find a second phase transition where surface mode
frequency becomes purely imaginary and describes an overdamped regime.
Noticeably, this transition has an exact correspondence to the oscillatory -
overdamped transition of the standard oscillator with a frictional force
proportional to velocity.Comment: 4 pages, 3 figures. To appear in Braz. J. Phy
Pair Partitioning in time reversal acoustics
Time reversal of acoustic waves can be achieved efficiently by the persistent
control of excitations in a finite region of the system. The procedure, called
Time Reversal Mirror, is stable against the inhomogeneities of the medium and
it has numerous applications in medical physics, oceanography and
communications. As a first step in the study of this robustness, we apply the
Perfect Inverse Filter procedure that accounts for the memory effects of the
system. In the numerical evaluation of such procedures we developed the Pair
Partitioning method for a system of coupled oscillators. The algorithm,
inspired in the Trotter strategy for quantum dynamics, obtains the dynamic for
a chain of coupled harmonic oscillators by the separation of the system in
pairs and applying a stroboscopic sequence that alternates the evolution of
each pair. We analyze here the formal basis of the method and discuss his
extension for including energy dissipation inside the medium.Comment: 6 pages, 4 figure
Exponentially fitted fifth-order two-step peer explicit methods
The so called peer methods for the numerical solution of Initial Value Problems (IVP) in ordinary differential systems were introduced by R. Weiner et al [6, 7, 11, 12, 13] for solving different types of problems either in sequential or parallel computers. In this work, we study exponentially fitted three-stage peer schemes that are able to fit functional spaces with dimension six. Finally, some numerical experiments are presented to show the behaviour of the new peer schemes for some periodic problems
Exact time-reversal focusing of acoustic and quantum excitations in open cavities: The perfect inverse filter
The time-reversal mirror (TRM) prescribes the reverse playback of a signal to
focalize an acoustic excitation as a Loschmidt echo. In the quantum domain, the
perfect inverse filter (PIF) processes this signal to ensure an exact reversion
provided that the excitation originated outside the cavity delimited by the
transducers. We show that PIF takes a simple form when the initial excitation
is created inside this cavity. This also applies to the acoustical case, where
it corrects the TRM and improves the design of an acoustic bazooka. We solve an
open chaotic cavity modeling a quantum bazooka and a simple model for a
Helmholtz resonator, showing that the PIF becomes decisive to compensate the
group velocities involved in a highly localized excitation and to achieve
subwavelength resolution.Comment: 6 pages, 2 figure
Semiclassical Theory of Time-Reversal Focusing
Time reversal mirrors have been successfully implemented for various kinds of
waves propagating in complex media. In particular, acoustic waves in chaotic
cavities exhibit a refocalization that is extremely robust against external
perturbations or the partial use of the available information. We develop a
semiclassical approach in order to quantitatively describe the refocusing
signal resulting from an initially localized wave-packet. The time-dependent
reconstructed signal grows linearly with the temporal window of injection, in
agreement with the acoustic experiments, and reaches the same spatial extension
of the original wave-packet. We explain the crucial role played by the chaotic
dynamics for the reconstruction of the signal and its stability against
external perturbations.Comment: 4 pages, 1 figur
Time Reversal Mirror and Perfect Inverse Filter in a Microscopic Model for Sound Propagation
Time reversal of quantum dynamics can be achieved by a global change of the
Hamiltonian sign (a hasty Loschmidt daemon), as in the Loschmidt Echo
experiments in NMR, or by a local but persistent procedure (a stubborn daemon)
as in the Time Reversal Mirror (TRM) used in ultrasound acoustics. While the
first is limited by chaos and disorder, the last procedure seems to benefit
from it. As a first step to quantify such stability we develop a procedure, the
Perfect Inverse Filter (PIF), that accounts for memory effects, and we apply it
to a system of coupled oscillators. In order to ensure a many-body dynamics
numerically intrinsically reversible, we develop an algorithm, the pair
partitioning, based on the Trotter strategy used for quantum dynamics. We
analyze situations where the PIF gives substantial improvements over the TRM.Comment: Submitted to Physica
Tuning laser-induced bandgaps in graphene
Could a laser field lead to the much sought-after tunable bandgaps in
graphene? By using Floquet theory combined with Green's functions techniques,
we predict that a laser field in the mid-infrared range can produce observable
bandgaps in the electronic structure of graphene. Furthermore, we show how they
can be tuned by using the laser polarization. Our results could serve as a
guidance to design opto-electronic nano-devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
Floquet interface states in illuminated three-dimensional topological insulators
Recent experiments showed that the surface of a three dimensional topological
insulator develops gaps in the Floquet-Bloch band spectrum when illuminated
with a circularly polarized laser. These Floquet-Bloch bands are characterized
by non-trivial Chern numbers which only depend on the helicity of the
polarization of the radiation field. Here we propose a setup consisting of a
pair of counter-rotating lasers, and show that one-dimensional chiral states
emerge at the interface between the two lasers. These interface states turn out
to be spin-polarized and may trigger interesting applications in the field of
optoelectronics and spintronics.Comment: 5 pages with 3 figures + supplemental materia
Non-perturbative laser effects on the electrical properties of graphene nanoribbons
The use of Floquet theory combined with a realistic description of the
electronic structure of illuminated graphene and graphene nanoribbons is
developed to assess the emergence of non-adiabatic and non-perturbative effects
on the electronic properties. Here, we introduce an efficient computational
scheme and illustrate its use by applying it to graphene nanoribbons in the
presence of both linear and circular polarization. The interplay between
confinement due to the finite sample size and laser-induced transitions is
shown to lead to sharp features on the average conductance and density of
states. Particular emphasis is given to the emergence of the bulk limit
response.Comment: 14 pages, 8 figures, to appear in J. Phys.: Condens. Matter, special
issue on "Ultrafast and nonlinear optics in carbon nanomaterials
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