65 research outputs found
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
Decoherent time-dependent transport beyond the Landauer-B\"uttiker formulation: a quantum-drift alternative to quantum jumps
We present a model for decoherence in time-dependent transport. It boils down
into a form of wave function that undergoes a smooth stochastic drift of the
phase in a local basis, the Quantum Drift (QD) model. This drift is nothing
else but a local energy fluctuation. Unlike Quantum Jumps (QJ) models, no jumps
are present in the density as the evolution is unitary. As a first application,
we address the transport through a resonant state
that undergoes decoherence. We show the equivalence with the decoherent steady
state transport in presence of a B\"{u}ttiker's voltage probe. In order to test
the dynamics, we consider two many-spin systems whith a local energy
fluctuation. A two-spin system is reduced to a two level system (TLS) that
oscillates among and . We show that QD model recovers not only
the exponential damping of the oscillations in the low perturbation regime, but
also the non-trivial bifurcation of the damping rates at a critical point, i.e.
the quantum dynamical phase transition. We also address the spin-wave like
dynamics of local polarization in a spin chain. The QD average solution has
about half the dispersion respect to the mean dynamics than QJ. By evaluating
the Loschmidt Echo (LE), we find that the pure states and are quite robust against the
local decoherence. In contrast, the LE, and hence coherence, decays faster when
the system is in a superposition state. Because its simple implementation, the
method is well suited to assess decoherent transport problems as well as to
include decoherence in both one-body and many-body dynamics.Comment: 10 pages, 5 figure
Gaussian to Exponential Crossover in the Attenuation of Polarization Echoes in NMR
An ingenious pulse sequence devised by S. Zhang, B. H. Meier, and R. R. Ernst
(Phys. Rev. Lett. {\bf 69}, 2149 (1992)) reverses the time evolution (``spin
diffusion'') of the local polarization in a dipolar coupled H spin
system. This refocusing originates a Polarization Echo whose amplitude
attenuates by increasing the time elapsed until the dynamics is reversed.
Different functional attenuations are found for a set of dipolar coupled
systems: ferrocene, (CH)Fe, cymantrene, (CH)Mn(CO), and
cobaltocene, (CH)Co. To control a relevant variable involved in
this attenuation a pulse sequence has been devised to progressively reduce the
dipolar dynamics. Since it reduces the evolution of the polarization echo it is
referred as REPE sequence. Two extreme behaviors were found while
characterizing the materials: In systems with a strong source of relaxation and
slow dynamics, the attenuation follows an exponential law (cymantrene). In
systems with a strong dipolar dynamics the attenuation is mainly Gaussian. By
the application of the REPE sequence the characteristic time of the Gaussian
decay is increased until the presence of an underlying dissipative mechanism is
revealed (cobaltocene). For ferrocene, however, the attenuation remains
Gaussian within the experimental time scale. These two behaviors suggest that
the many body quantum dynamics presents an extreme intrinsic instability which,
in the presence of small perturbations, leads to the onset of irreversibility.
This experimental conclusion is consistent with the tendencies displayed by the
numerical solutions of model systems.Comment: 7 pages + 7 Postscript figure
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
Spin projection chromatography
We formulate the many-body spin dynamics at high temperature within the
non-equilibrium Keldysh formalism. For the simplest XY interaction, analytical
expressions in terms of the one particle solutions are obtained for linear and
ring configurations. For small rings of even spin number, the group velocities
of excitations depend on the parity of the total spin projection. This should
enable a dynamical filtering of spin projections with a given parity i.e. a
Spin projection chromatography.Comment: 13 pages, 3 figure
Quantum Interference Phenomena in the Local Polarization Dynamics of Mesoscopic Systems: An NMR Observation
It was predicted that local spin polarization in a ring of five dipolar
coupled spins should present a particular fingerprint of quantum interferences
reflecting both the discrete and finite nature of the system [Phys. Rev. Lett.
75 (1995) 4310]. We report its observation for the proton system of a
(CH)Fe molecule using a rare C as {\it local probe}. Novel
high frequency (Hz) polarization oscillations appear because
incomplete C-H cross-polarization transfer {\it splits} the
polarization state, in a portion that wanders in the proton system and one that
remains in the C. They interfere with each other after rejoining.Comment: 12 pages, RevTex, 4 Figures available upon request, to appear in
Chemical Physics Letter
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