9,498 research outputs found
Disorder and interference: localization phenomena
The specific problem we address in these lectures is the problem of transport
and localization in disordered systems, when interference is present, as
characteristic for waves, with a focus on realizations with ultracold atoms.Comment: Notes of a lecture delivered at the Les Houches School of Physics on
"Ultracold gases and quantum information" 2009 in Singapore. v3: corrected
mistakes, improved script for numerics, Chapter 9 in "Les Houches 2009 -
Session XCI: Ultracold Gases and Quantum Information" edited by C. Miniatura
et al. (Oxford University Press, 2011
Interaction instability of localization in quasiperiodic systems
Integrable models form pillars of theoretical physics because they allow for
full analytical understanding. Despite being rare, many realistic systems can
be described by models that are close to integrable. Therefore, an important
question is how small perturbations influence the behavior of solvable models.
This is particularly true for many-body interacting quantum systems where no
general theorems about their stability are known. Here, we show that no such
theorem can exist by providing an explicit example of a one-dimensional
many-body system in a quasiperiodic potential whose transport properties
discontinuously change from localization to diffusion upon switching on
interaction. This demonstrates an inherent instability of a possible many-body
localization in a quasiperiodic potential at small interactions. We also show
how the transport properties can be strongly modified by engineering potential
at only a few lattice sites.Comment: 10 pages; (v2: additional explanations, data, and references
Spatially heterogeneous dynamics in a thermosensitive soft suspension before and after the glass transition
The microscopic dynamics and aging of a soft thermosensitive suspension was
investigated by looking at the thermal fluctuations of tracers in the
suspension. Below and above the glass transition, the dense microgel particles
suspension was found to develop an heterogeneous dynamics, featured by a non
Gaussian Probability Distribution Function (PDF) of the probes' displacements,
with an exponential tail. We show that non Gaussian shapes are a characteristic
of the ensemble-averaged PDF, while local PDF remain Gaussian. This shows that
the scenario behind the non Gaussian van Hove functions is a spatially
heterogeneous dynamics, characterized by a spatial distribution of locally
homogeneous dynamical environments through the sample, on the considered time
scales. We characterize these statistical distributions of dynamical
environments, in the liquid, supercooled, and glass states, and show that it
can explain the observed exponential tail of the van Hove functions observed in
the concentrated states. The intensity of spatial heterogeneities was found to
amplify with increasing volume fraction. In the aging regime, it tends to
increase as the glass gets more arrested.Comment: 19 pages, 10 figures, Soft Matter accepte
Theoretical study of scattering in graphene ribbons in the presence of structural and atomistic edge roughness
We investigate the diffusive electron-transport properties of charge-doped
graphene ribbons and nanoribbons with imperfect edges. We consider different
regimes of edge scattering, ranging from wide graphene ribbons with (partially)
diffusive edge scattering to ribbons with large width variations and
nanoribbons with atomistic edge roughness. For the latter, we introduce an
approach based on pseudopotentials, allowing for an atomistic treatment of the
band structure and the scattering potential, on the self-consistent solution of
the Boltzmann transport equation within the relaxation-time approximation and
taking into account the edge-roughness properties and statistics. The resulting
resistivity depends strongly on the ribbon orientation, with zigzag (armchair)
ribbons showing the smallest (largest) resistivity and intermediate ribbon
orientations exhibiting intermediate resistivity values. The results also show
clear resistivity peaks, corresponding to peaks in the density of states due to
the confinement-induced subband quantization, except for armchair-edge ribbons
that show a very strong width dependence because of their claromatic behavior.
Furthermore, we identify a strong interplay between the relative position of
the two valleys of graphene along the transport direction, the correlation
profile of the atomistic edge roughness, and the chiral valley modes, leading
to a peculiar strongly suppressed resistivity regime, most pronounced for the
zigzag orientation.Comment: 13 pages, 7 figure
Microscopic Aspects of Stretched Exponential Relaxation (SER) in Homogeneous Molecular and Network Glasses and Polymers
Because the theory of SER is still a work in progress, the phenomenon itself
can be said to be the oldest unsolved problem in science, as it started with
Kohlrausch in 1847. Many electrical and optical phenomena exhibit SER with
probe relaxation I(t) ~ exp[-(t/{\tau}){\beta}], with 0 < {\beta} < 1. Here
{\tau} is a material-sensitive parameter, useful for discussing chemical
trends. The "shape" parameter {\beta} is dimensionless and plays the role of a
non-equilibrium scaling exponent; its value, especially in glasses, is both
practically useful and theoretically significant. The mathematical complexity
of SER is such that rigorous derivations of this peculiar function were not
achieved until the 1970's. The focus of much of the 1970's pioneering work was
spatial relaxation of electronic charge, but SER is a universal phenomenon, and
today atomic and molecular relaxation of glasses and deeply supercooled liquids
provide the most reliable data. As the data base grew, the need for a
quantitative theory increased; this need was finally met by the
diffusion-to-traps topological model, which yields a remarkably simple
expression for the shape parameter {\beta}, given by d*/(d* + 2). At first
sight this expression appears to be identical to d/(d + 2), where d is the
actual spatial dimensionality, as originally derived. The original model,
however, failed to explain much of the data base. Here the theme of earlier
reviews, based on the observation that in the presence of short-range forces
only d* = d = 3 is the actual spatial dimensionality, while for mixed short-
and long-range forces, d* = fd = d/2, is applied to four new spectacular
examples, where it turns out that SER is useful not only for purposes of
quality control, but also for defining what is meant by a glass in novel
contexts. (Please see full abstract in main text
Obtaining localization properties efficiently using the Kubo-Greenwood formalism
We establish, through numerical calculations and comparisons with a recursive
Green's function based implementation of the Landauer-B\"uttiker formalism, an
efficient method for studying Anderson localization in quasi-one-dimensional
and two-dimensional systems using the Kubo-Greenwood formalism. Although the
recursive Green's function method can be used to obtain the localization length
of a mesoscopic conductor, it is numerically very expensive for systems that
contain a large number of atoms transverse to the transport direction. On the
other hand, linear-scaling has been achieved with the Kubo-Greenwood method,
enabling the study of effectively two-dimensional systems. While the
propagating length of the charge carriers will eventually saturate to a finite
value in the localized regime, the conductances given by the Kubo-Greenwood
method and the recursive Green's function method agree before the saturation.
The converged value of the propagating length is found to be directly
proportional to the localization length obtained from the exponential decay of
the conductance.Comment: 7 pages, 6 figure
Non-Markovian data-driven modeling of single-cell motility
Trajectories of human breast cancer cells moving on one-dimensional circular tracks are modeled by thenon-Markovian version of the Langevin equation that includes an arbitrary memory function. When averagedover cells, the velocity distribution exhibits spurious non-Gaussian behavior, while single cells are characterizedby Gaussian velocity distributions. Accordingly, the data are described by a linear memory model whichincludes different random walk models that were previously used to account for various aspects of cell motilitysuch as migratory persistence, non-Markovian effects, colored noise, and anomalous diffusion. The memoryfunction is extracted from the trajectory data without restrictions or assumptions, thus making our approachtruly data driven, and is used for unbiased single-cell comparison. The cell memory displays time-delayedsingle-exponential negative friction, which clearly distinguishes cell motion from the simple persistent randomwalk model and suggests a regulatory feedback mechanism that controls cell migration. Based on the extractedmemory function we formulate a generalized exactly solvable cell migration model which indicates thatnegative friction generates cell persistence over long timescales. The nonequilibrium character of cell motionis investigated by mapping the non-Markovian Langevin equation with memory onto a Markovian model thatinvolves a hidden degree of freedom and is equivalent to the underdamped active Ornstein-Uhlenbeck process
- …