762 research outputs found
Correlated percolation models of structured habitat in ecology
Percolation offers acknowledged models of random media when the relevant
medium characteristics can be described as a binary feature. However, when
considering habitat modeling in ecology, a natural constraint comes from
nearest-neighbor correlations between the suitable/unsuitable states of the
spatial units forming the habitat. Such constraints are also relevant in the
physics of aggregation where underlying processes may lead to a form of
correlated percolation. However, in ecology, the processes leading to habitat
correlations are in general not known or very complex. As proposed by Hiebeler
[Ecology {\bf 81}, 1629 (2000)], these correlations can be captured in a
lattice model by an observable aggregation parameter , supplementing the
density of suitable sites. We investigate this model as an instance of
correlated percolation. We analyze the phase diagram of the percolation
transition and compute the cluster size distribution, the pair-connectedness
function and the correlation function . We find that while
displays a power-law decrease associated with long-range correlations in a wide
domain of parameter values, critical properties are compatible with the
universality class of uncorrelated percolation. We contrast the correlation
structures obtained respectively for the correlated percolation model and for
the Ising model, and show that the diversity of habitat configurations
generated by the Hiebeler model is richer than the archetypal Ising model. We
also find that emergent structural properties are peculiar to the implemented
algorithm, leading to questioning the notion of a well-defined model of
aggregated habitat. We conclude that the choice of model and algorithm have
strong consequences on what insights ecological studies can get using such
models of species habitat
Critical Opalescence in Baryonic QCD Matter
We show that critical opalescence, a clear signature of second-order phase
transition in conventional matter, manifests itself as critical intermittency
in QCD matter produced in experiments with nuclei. This behaviour is revealed
in transverse momentum spectra as a pattern of power laws in factorial moments,
to all orders, associated with baryon production. This phenomenon together with
a similar effect in the isoscalar sector of pions (sigma mode) provide us with
a set of observables associated with the search for the QCD critical point in
experiments with nuclei at high energies.Comment: 7 pages, 1 figur
Quantifying the tangling of trajectories using the topological entropy
We present a simple method to efficiently compute a lower limit of the
topological entropy and its spatial distribution for two-dimensional mappings.
These mappings could represent either two-dimensional time-periodic fluid flows
or three-dimensional magnetic fields, which are periodic in one direction. This
method is based on measuring the length of a material line in the flow.
Depending on the nature of the flow, the fluid can be mixed very efficiently
which causes the line to stretch. Here we study a method that adaptively
increases the resolution at locations along the line where folds lead to high
curvature. This reduces the computational cost greatly which allows us to study
unprecedented parameter regimes. We demonstrate how this efficient
implementation allows the computation of the variation of the finite-time
topological entropy in the mapping. This measure quantifies spatial variations
of the braiding efficiency, important in many practical applications.Comment: 11 pages, 9 figure
Quantized conductance in a one-dimensional ballistic oxide nanodevice
Electric-field effect control of two-dimensional electron gases (2-DEG) has
enabled the exploration of nanoscale electron quantum transport in
semiconductors. Beyond these classical materials, transition metal-oxide-based
structures have d-electronic states favoring the emergence of novel quantum
orders absent in conventional semiconductors. In this context, the
LaAlO3/SrTiO3 interface that combines gate-tunable superconductivity and
sizeable spin-orbit coupling is emerging as a promising platform to realize
topological superconductivity. However, the fabrication of nanodevices in which
the electronic properties of this oxide interface can be controlled at the
nanoscale by field-effect remains a scientific and technological challenge.
Here, we demonstrate the quantization of conductance in a ballistic quantum
point contact (QPC), formed by electrostatic confinement of the LaAlO3/SrTiO3
2-DEG with a split-gate. Through finite source-drain voltage, we perform a
comprehensive spectroscopic investigation of the 3d energy levels inside the
QPC, which can be regarded as a spectrometer able to probe Majorana states in
an oxide 2-DEG
Competition between electron pairing and phase coherence in superconducting interfaces
In LaAlO3/SrTiO3 heterostructures, a gate tunable superconducting electron gas is confined in a quantum well at the interface between two insulating oxides. Remarkably, the gas coexists with both magnetism and strong Rashba spin–orbit coupling. However, both the origin of superconductivity and the nature of the transition to the normal state over the whole doping range remain elusive. Here we use resonant microwave transport to extract the superfluid stiffness and the superconducting gap energy of the LaAlO3/SrTiO3 interface as a function of carrier density. We show that the superconducting phase diagram of this system is controlled by the competition between electron pairing and phase coherence. The analysis of the superfluid density reveals that only a very small fraction of the electrons condenses into the superconducting state. We propose that this corresponds to the weak filling of high- energy dxz/dyz bands in the quantum well, more apt to host superconductivity
Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices
The recent development in the fabrication of artificial oxide
heterostructures opens new avenues in the field of quantum materials by
enabling the manipulation of the charge, spin and orbital degrees of freedom.
In this context, the discovery of two-dimensional electron gases (2-DEGs) at
LAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba
spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on
the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical
properties, including superconductivity and SOC, can be tuned over a wide range
by a top-gate voltage. We derive a phase diagram, which emphasises a
field-effect-induced superconductor-to-insulator quantum phase transition.
Magneto-transport measurements indicate that the Rashba coupling constant
increases linearly with electrostatic doping. Our results pave the way for the
realisation of mesoscopic devices, where these two properties can be
manipulated on a local scale by means of top-gates
On the Inability of Markov Models to Capture Criticality in Human Mobility
We examine the non-Markovian nature of human mobility by exposing the
inability of Markov models to capture criticality in human mobility. In
particular, the assumed Markovian nature of mobility was used to establish a
theoretical upper bound on the predictability of human mobility (expressed as a
minimum error probability limit), based on temporally correlated entropy. Since
its inception, this bound has been widely used and empirically validated using
Markov chains. We show that recurrent-neural architectures can achieve
significantly higher predictability, surpassing this widely used upper bound.
In order to explain this anomaly, we shed light on several underlying
assumptions in previous research works that has resulted in this bias. By
evaluating the mobility predictability on real-world datasets, we show that
human mobility exhibits scale-invariant long-range correlations, bearing
similarity to a power-law decay. This is in contrast to the initial assumption
that human mobility follows an exponential decay. This assumption of
exponential decay coupled with Lempel-Ziv compression in computing Fano's
inequality has led to an inaccurate estimation of the predictability upper
bound. We show that this approach inflates the entropy, consequently lowering
the upper bound on human mobility predictability. We finally highlight that
this approach tends to overlook long-range correlations in human mobility. This
explains why recurrent-neural architectures that are designed to handle
long-range structural correlations surpass the previously computed upper bound
on mobility predictability
- …