7,444 research outputs found
Transport of quantum excitations coupled to spatially extended nonlinear many-body systems
The role of noise in the transport properties of quantum excitations is a
topic of great importance in many fields, from organic semiconductors for
technological applications to light-harvesting complexes in photosynthesis. In
this paper we study a semi-classical model where a tight-binding Hamiltonian is
fully coupled to an underlying spatially extended nonlinear chain of atoms. We
show that the transport properties of a quantum excitation are subtly modulated
by (i) the specific type (local vs non-local) of exciton-phonon coupling and by
(ii) nonlinear effects of the underlying lattice. We report a non-monotonic
dependence of the exciton diffusion coefficient on temperature, in agreement
with earlier predictions, as a direct consequence of the lattice-induced
fluctuations in the hopping rates due to long-wavelength vibrational modes. A
standard measure of transport efficiency confirms that both nonlinearity in the
underlying lattice and off-diagonal exciton-phonon coupling promote transport
efficiency at high temperatures, preventing the Zeno-like quench observed in
other models lacking an explicit noise-providing dynamical system
Coherent quantum transport in disordered systems I: The influence of dephasing on the transport properties and absorption spectra on one-dimensional systems
Excitonic transport in static disordered one dimensional systems is studied
in the presence of thermal fluctuations that are described by the
Haken-Strobl-Reineker model. For short times, non-diffusive behavior is
observed that can be characterized as the free-particle dynamics in the
Anderson localized system. Over longer time scales, the environment-induced
dephasing is sufficient to overcome the Anderson localization caused by the
disorder and allow for transport to occur which is always seen to be diffusive.
In the limiting regimes of weak and strong dephasing quantum master equations
are developed, and their respective scaling relations imply the existence of a
maximum in the diffusion constant as a function of the dephasing rate that is
confirmed numerically. In the weak dephasing regime, it is demonstrated that
the diffusion constant is proportional to the square of the localization length
which leads to a significant enhancement of the transport rate over the
classical prediction. Finally, the influence of noise and disorder on the
absorption spectrum is presented and its relationship to the transport
properties is discussed.Comment: 23 pages, 7 figure
The semiclassical tool in mesoscopic physics
Semiclassical methods are extremely valuable in the study of transport and
thermodynamical properties of ballistic microstructures. By expressing the
conductance in terms of classical trajectories, we demonstrate that quantum
interference phenomena depend on the underlying classical dynamics of
non-interacting electrons. In particular, we are able to calculate the
characteristic length of the ballistic conductance fluctuations and the weak
localization peak in the case of chaotic dynamics. Integrable cavities are not
governed by single scales, but their non-generic behavior can also be obtained
from semiclassical expansions (over isolated trajectories or families of
trajectories, depending on the system). The magnetic response of a
microstructure is enhanced with respect to the bulk (Landau) susceptibility,
and the semiclassical approach shows that this enhancement is the largest for
integrable geometries, due to the existence of families of periodic orbits. We
show how the semiclassical tool can be adapted to describe weak residual
disorder, as well as the effects of electron-electron interactions. The
interaction contribution to the magnetic susceptibility also depends on the
nature of the classical dynamics of non-interacting electrons, and is
parametrically larger in the case of integrable systems.Comment: Latex, Cimento-varenna style, 82 pages, 21 postscript figures;
lectures given in the CXLIII Course "New Directions in Quantum Chaos" on the
International School of Physics "Enrico Fermi"; Varenna, Italy, July 1999; to
be published in Proceeding
Observation of spin Coulomb drag in a two-dimensional electron gas
An electron propagating through a solid carries spin angular momentum in
addition to its mass and charge. Of late there has been considerable interest
in developing electronic devices based on the transport of spin, which offer
potential advantages in dissipation, size, and speed over charge-based devices.
However, these advantages bring with them additional complexity. Because each
electron carries a single, fixed value (-e) of charge, the electrical current
carried by a gas of electrons is simply proportional to its total momentum. A
fundamental consequence is that the charge current is not affected by
interactions that conserve total momentum, notably collisions among the
electrons themselves. In contrast, the electron's spin along a given spatial
direction can take on two values, "up" and "down", so that the spin current and
momentum need not be proportional. Although the transport of spin polarization
is not protected by momentum conservation, it has been widely assumed that,
like the charge current, spin current is unaffected by electron-electron (e-e)
interactions. Here we demonstrate experimentally not only that this assumption
is invalid, but that over a broad range of temperature and electron density,
the flow of spin polarization in a two-dimensional gas of electrons is
controlled by the rate of e-e collisions
Nonequilibrium mesoscopic transport: a genealogy
Models of nonequilibrium quantum transport underpin all modern electronic
devices, from the largest scales to the smallest. Past simplifications such as
coarse graining and bulk self-averaging served well to understand electronic
materials. Such particular notions become inapplicable at mesoscopic
dimensions, edging towards the truly quantum regime. Nevertheless a unifying
thread continues to run through transport physics, animating the design of
small-scale electronic technology: microscopic conservation and nonequilibrium
dissipation. These fundamentals are inherent in quantum transport and gain even
greater and more explicit experimental meaning in the passage to atomic-sized
devices. We review their genesis, their theoretical context, and their
governing role in the electronic response of meso- and nanoscopic systems.Comment: 21p
Persistent memory for a Brownian walker in a random array of obstacles
We show that for particles performing Brownian motion in a frozen array of
scatterers long-time correlations emerge in the mean-square displacement.
Defining the velocity autocorrelation function (VACF) via the second
time-derivative of the mean-square displacement, power-law tails govern the
long-time dynamics similar to the case of ballistic motion. The physical origin
of the persistent memory is due to repeated encounters with the same obstacle
which occurs naturally in Brownian dynamics without involving other scattering
centers. This observation suggests that in this case the VACF exhibits these
anomalies already at first order in the scattering density. Here we provide an
analytic solution for the dynamics of a tracer for a dilute planar Lorentz gas
and compare our results to computer simulations. Our result support the idea
that quenched disorder provides a generic mechanism for persistent correlations
irrespective of the microdynamics of the tracer particle.Comment: 11 pages, 4 figures, accepted in Chemical Physic
SS433's jet trace from ALMA imaging and Global Jet Watch spectroscopy: evidence for post-launch particle acceleration
We present a comparison of Doppler-shifted H-alpha line emission observed by
the Global Jet Watch from freshly-launched jet ejecta at the nucleus of the
Galactic microquasar SS433 with subsequent ALMA imaging at mm-wavelengths of
the same jet ejecta. There is a remarkable similarity between the
transversely-resolved synchrotron emission and the prediction of the jet trace
from optical spectroscopy: this is an a priori prediction not an a posteriori
fit, confirming the ballistic nature of the jet propagation. The mm-wavelength
of the ALMA polarimetry is sufficiently short that the Faraday rotation is
negligible and therefore that the observed E-vector directions are accurately
orthogonal to the projected local magnetic field. Close to the nucleus the
B-field vectors are perpendicular to the direction of propagation. Further out
from the nucleus, the B-field vectors that are coincident with the jet instead
become parallel to the ridge line; this occurs at a distance where the jet
bolides are expected to expand into one another. X-ray variability has also
been observed at this location; this has a natural explanation if shocks from
the expanding and colliding bolides cause particle acceleration. In regions
distinctly separate from the jet ridge line, the fractional polarisation
approaches the theoretical maximum for synchrotron emission.Comment: To appear in ApJ Letter
Superdiffusive heat conduction in semiconductor alloys -- II. Truncated L\'evy formalism for experimental analysis
Nearly all experimental observations of quasi-ballistic heat flow are
interpreted using Fourier theory with modified thermal conductivity. Detailed
Boltzmann transport equation (BTE) analysis, however, reveals that the
quasi-ballistic motion of thermal energy in semiconductor alloys is no longer
Brownian but instead exhibits L\'evy dynamics with fractal dimension . Here, we present a framework that enables full 3D experimental analysis by
retaining all essential physics of the quasi-ballistic BTE dynamics
phenomenologically. A stochastic process with just two fitting parameters
describes the transition from pure L\'evy superdiffusion as short length and
time scales to regular Fourier diffusion. The model provides accurate fits to
time domain thermoreflectance raw experimental data over the full modulation
frequency range without requiring any `effective' thermal parameters and
without any a priori knowledge of microscopic phonon scattering mechanisms.
Identified values for InGaAs and SiGe match ab initio BTE predictions
within a few percent. Our results provide experimental evidence of fractal
L\'evy heat conduction in semiconductor alloys. The formalism additionally
indicates that the transient temperature inside the material differs
significantly from Fourier theory and can lead to improved thermal
characterization of nanoscale devices and material interfaces
- …