608 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
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
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
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
Anomalous thickness-dependent electrical conductivity in van der Waals layered transition metal halide, Nb<sub>3</sub>Cl<sub>8</sub>
Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb3Cl8, over a wide range of thicknesses both with and without hexagonal boron nitride (hBN) encapsulation. The conductivity is found to increase by more than three orders of magnitude when the thickness is decreased from 280 µm to 5 nm, at 300 K. At low temperatures and below ~50 nm, the conductance becomes thickness independent, implying surface conduction is dominating. Temperature dependent conductivity measurements indicate Nb3Cl8 is an insulator, however, the effective activation energy decreases from a bulk value of 310 meV to 140 meV by 5 nm. X-ray photoelectron spectroscopy (XPS) shows mild surface oxidation in devices without hBN capping, however, no significant difference in transport is observed when compared to the capped devices, implying the thickness dependent transport behavior is intrinsic to the material. A conduction mechanism comprised of a higher conductivity surface channel in parallel with a lower conductivity interlayer channel is discussed
Pace: An Advanced Structure for Handling Multitechnique NDT Data
The analysis of NDT data is a problem of growing importance and complexity, encountered in many industrial fields, such as electricity production, oil industry, aeronautics, infrastructure inspection or metal production. This is why dedicated systems have begun to appear in the previous years to handle this particular part of a nondestructive inspection, located after the data acquisition and leading to the diagnosis
Nanopatterning of weak links in superconducting oxide interfaces
The interface between two wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO), hosts a quasi-two-dimensional electron gas (q2DEG), two-dimensional superconductivity, ferromagnetism, and giant Rashba spin-orbit coupling. The co-existence of two-dimensional superconductivity with gate-tunable spin-orbit coupling and multiband occupation is of particular interest for the realization of unconventional superconducting pairing. To investigate the symmetry of the superconducting order parameter, phase sensitive measurements of the Josephson effect are required. We describe an approach for the fabrication of artificial superconducting weak links at the LAO/STO interface using direct high-resolution electron beam lithography and low-energy argon ion beam irradiation. The method does not require lift-off steps or sacrificial layers. Therefore, resolution is only limited by the electron beam lithography and pattern transfer. We have realized superconducting weak links with a barrier thickness of 30–100 nm. The barrier transparency of the weak links can be controlled by the irradiation dose and further tuned by a gate voltage. Our results open up new possibilities for the realization of quantum devices in oxide interfaces
Phase transitions and configuration space topology
Equilibrium phase transitions may be defined as nonanalytic points of
thermodynamic functions, e.g., of the canonical free energy. Given a certain
physical system, it is of interest to understand which properties of the system
account for the presence of a phase transition, and an understanding of these
properties may lead to a deeper understanding of the physical phenomenon. One
possible approach of this issue, reviewed and discussed in the present paper,
is the study of topology changes in configuration space which, remarkably, are
found to be related to equilibrium phase transitions in classical statistical
mechanical systems. For the study of configuration space topology, one
considers the subsets M_v, consisting of all points from configuration space
with a potential energy per particle equal to or less than a given v. For
finite systems, topology changes of M_v are intimately related to nonanalytic
points of the microcanonical entropy (which, as a surprise to many, do exist).
In the thermodynamic limit, a more complex relation between nonanalytic points
of thermodynamic functions (i.e., phase transitions) and topology changes is
observed. For some class of short-range systems, a topology change of the M_v
at v=v_t was proved to be necessary for a phase transition to take place at a
potential energy v_t. In contrast, phase transitions in systems with long-range
interactions or in systems with non-confining potentials need not be
accompanied by such a topology change. Instead, for such systems the
nonanalytic point in a thermodynamic function is found to have some
maximization procedure at its origin. These results may foster insight into the
mechanisms which lead to the occurrence of a phase transition, and thus may
help to explore the origin of this physical phenomenon.Comment: 22 pages, 6 figure
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