1,487 research outputs found
Inferring diffusion in single live cells at the single molecule level
The movement of molecules inside living cells is a fundamental feature of
biological processes. The ability to both observe and analyse the details of
molecular diffusion in vivo at the single molecule and single cell level can
add significant insight into understanding molecular architectures of diffusing
molecules and the nanoscale environment in which the molecules diffuse. The
tool of choice for monitoring dynamic molecular localization in live cells is
fluorescence microscopy, especially so combining total internal reflection
fluorescence (TIRF) with the use of fluorescent protein (FP) reporters in
offering exceptional imaging contrast for dynamic processes in the cell
membrane under relatively physiological conditions compared to competing single
molecule techniques. There exist several different complex modes of diffusion,
and discriminating these from each other is challenging at the molecular level
due to underlying stochastic behaviour. Analysis is traditionally performed
using mean square displacements of tracked particles, however, this generally
requires more data points than is typical for single FP tracks due to
photophysical instability. Presented here is a novel approach allowing robust
Bayesian ranking of diffusion processes (BARD) to discriminate multiple complex
modes probabilistically. It is a computational approach which biologists can
use to understand single molecule features in live cells.Comment: combined ms (1-37 pages, 8 figures) and SI (38-55, 3 figures
Dynamical response of a pinned two-dimensional Wigner crystal
We re-examine a long-standing problem of a finite-frequency conductivity of a
weakly pinned two-dimensional classical Wigner crystal. In this system an
inhomogeneously broadened absorption line (pinning mode) centered at disorder
and magnetic field dependent frequency is known to appear. We show
that the relative linewidth of the pinning mode is
of the order of one in weak magnetic fields, exhibits a power-law decrease in
intermediate fields, and eventually saturates at a small value in strong
magnetic fields. The linewidth narrowing is due to a peculiar mechanism of
mixing between the stiffer longitudinal and the softer transverse components of
the collective excitations. The width of the high-field resonance proves to be
related to the density of states in the low-frequency tail of the zero-field
phonon spectrum. We find a qualitative agreement with recent experiments and
point out differences from the previous theoretical work on the subject.Comment: 19 pages, 11 figures. Supersedes cond-mat/990424
On the Large -charge Expansion in Superconformal Field Theories
In this note we study two point functions of Coulomb branch chiral ring
elements with large -charge, in quantum field theories with superconformal symmetry in four spacetime dimensions. Focusing on the case
of one-dimensional Coulomb branch, we use the effective-field-theoretic methods
of arXiv:1706.05743, to estimate the two-point function in the limit where the operator insertion On has large total
-charge . We show that
has a nontrivial but universal asymptotic expansion at large , of
the form where approaches a constant as
, and is an -independent constant
describing on the normalization of the operator relative to the effective
Abelian gauge coupling. The exponent is a positive number proportional
to the difference between the -anomaly coefficient of the underlying CFT and
that of the effective theory of the Coulomb branch. For Lagrangian SCFT, we
check our predictions against exact results from supersymmetric localization of
Baggio et. al. and Gerchkovitz et. al., and find precise agreement for the
logarithm , up to and including order
. We also give predictions for the growth of two-point
functions in all rank-one SCFT in the classification of Argyres et. al. In this
way, we show the large--charge expansion serves as a bridge from the world
of unbroken superconformal symmetry, OPE data, and bootstraps, to the world of
the low-energy dynamics of the moduli space of vacua.Comment: minor change
Onset of many-body chaos in the model
The growth of commutators of initially commuting local operators diagnoses
the onset of chaos in quantum many-body systems. We compute such commutators of
local field operators with components in the -dimensional
nonlinear sigma model to leading order in . The system is taken to be in
thermal equilibrium at a temperature above the zero temperature quantum
critical point separating the symmetry broken and unbroken phases. The
commutator grows exponentially in time with a rate denoted . At
large the growth of chaos as measured by is slow because the
model is weakly interacting, and we find . The
scaling with temperature is dictated by conformal invariance of the underlying
quantum critical point. We also show that operators grow ballistically in space
with a "butterfly velocity" given by where is the
Lorentz-invariant speed of particle excitations in the system. We briefly
comment on the behavior of and in the neighboring symmetry
broken and unbroken phases.Comment: (1+55) pages, 13 figures; (v2) Final published versio
Anomalous transport in the crowded world of biological cells
A ubiquitous observation in cell biology is that diffusion of macromolecules
and organelles is anomalous, and a description simply based on the conventional
diffusion equation with diffusion constants measured in dilute solution fails.
This is commonly attributed to macromolecular crowding in the interior of cells
and in cellular membranes, summarising their densely packed and heterogeneous
structures. The most familiar phenomenon is a power-law increase of the MSD,
but there are other manifestations like strongly reduced and time-dependent
diffusion coefficients, persistent correlations, non-gaussian distributions of
the displacements, heterogeneous diffusion, and immobile particles. After a
general introduction to the statistical description of slow, anomalous
transport, we summarise some widely used theoretical models: gaussian models
like FBM and Langevin equations for visco-elastic media, the CTRW model, and
the Lorentz model describing obstructed transport in a heterogeneous
environment. Emphasis is put on the spatio-temporal properties of the transport
in terms of 2-point correlation functions, dynamic scaling behaviour, and how
the models are distinguished by their propagators even for identical MSDs.
Then, we review the theory underlying common experimental techniques in the
presence of anomalous transport: single-particle tracking, FCS, and FRAP. We
report on the large body of recent experimental evidence for anomalous
transport in crowded biological media: in cyto- and nucleoplasm as well as in
cellular membranes, complemented by in vitro experiments where model systems
mimic physiological crowding conditions. Finally, computer simulations play an
important role in testing the theoretical models and corroborating the
experimental findings. The review is completed by a synthesis of the
theoretical and experimental progress identifying open questions for future
investigation.Comment: review article, to appear in Rep. Prog. Phy
Localization and chiral symmetry in 2+1 flavor domain wall QCD
We present results for the dependence of the residual mass of domain wall
fermions (DWF) on the size of the fifth dimension and its relation to the
density and localization properties of low-lying eigenvectors of the
corresponding hermitian Wilson Dirac operator relevant to simulations of 2+1
flavor domain wall QCD. Using the DBW2 and Iwasaki gauge actions, we generate
ensembles of configurations with a space-time volume and an
extent of 8 in the fifth dimension for the sea quarks. We demonstrate the
existence of a regime where the degree of locality, the size of chiral symmetry
breaking and the rate of topology change can be acceptable for inverse lattice
spacings GeV.Comment: 59 Pages, 23 figures, 1 MPG linke
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