4,430 research outputs found
A morphospace of functional configuration to assess configural breadth based on brain functional networks
The best approach to quantify human brain functional reconfigurations in
response to varying cognitive demands remains an unresolved topic in network
neuroscience. We propose that such functional reconfigurations may be
categorized into three different types: i) Network Configural Breadth, ii)
Task-to-Task transitional reconfiguration, and iii) Within-Task
reconfiguration. In order to quantify these reconfigurations, we propose a
mesoscopic framework focused on functional networks (FNs) or communities. To do
so, we introduce a 2D network morphospace that relies on two novel mesoscopic
metrics, Trapping Efficiency (TE) and Exit Entropy (EE), which capture topology
and integration of information within and between a reference set of FNs. In
this study, we use this framework to quantify the Network Configural Breadth
across different tasks. We show that the metrics defining this morphospace can
differentiate FNs, cognitive tasks and subjects. We also show that network
configural breadth significantly predicts behavioral measures, such as episodic
memory, verbal episodic memory, fluid intelligence and general intelligence. In
essence, we put forth a framework to explore the cognitive space in a
comprehensive manner, for each individual separately, and at different levels
of granularity. This tool that can also quantify the FN reconfigurations that
result from the brain switching between mental states.Comment: main article: 24 pages, 8 figures, 2 tables. supporting information:
11 pages, 5 figure
Probing defects and correlations in the hydrogen-bond network of ab initio water
The hydrogen-bond network of water is characterized by the presence of
coordination defects relative to the ideal tetrahedral network of ice, whose
fluctuations determine the static and time-dependent properties of the liquid.
Because of topological constraints, such defects do not come alone, but are
highly correlated coming in a plethora of different pairs. Here we discuss in
detail such correlations in the case of ab initio water models and show that
they have interesting similarities to regular and defective solid phases of
water. Although defect correlations involve deviations from idealized
tetrahedrality, they can still be regarded as weaker hydrogen bonds that retain
a high degree of directionality. We also investigate how the structure and
population of coordination defects is affected by approximations to the
inter-atomic potential, finding that in most cases, the qualitative features of
the hydrogen bond network are remarkably robust
Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity
In recent years, a class of solid state materials, called three-dimensional
topological insulators, has emerged. In the bulk, a topological insulator
behaves like an ordinary insulator with a band gap. At the surface, conducting
gapless states exist showing remarkable properties such as helical Dirac
dispersion and suppression of backscattering of spin-polarized charge carriers.
The characterization and control of the surface states via transport
experiments is often hindered by residual bulk contributions yet at cryogenic
temperatures. Here, we show that surface currents in Bi2Se3 can be controlled
by circularly polarized light on a picosecond time scale with a fidelity near
unity even at room temperature. We re-veal the temporal separation of such
ultrafast helicity-dependent surface currents from photo-induced thermoelectric
and drift currents in the bulk. Our results uncover the functionality of
ultrafast optoelectronic devices based on surface currents in topological
insulators.Comment: 19 pages, 4 figures, supplementary informatio
Ephemeral point-events: is there a last remnant of physical objectivity?
For the past two decades, Einstein's Hole Argument (which deals with the
apparent indeterminateness of general relativity due to the general covariance
of the field equations) and its resolution in terms of Leibniz equivalence (the
statement that Riemannian geometries related by active diffeomorphisms
represent the same physical solution) have been the starting point for a lively
philosophical debate on the objectivity of the point-events of space-time. It
seems that Leibniz equivalence makes it impossible to consider the points of
the space-time manifold as physically individuated without recourse to
dynamical individuating fields. Various authors have posited that the metric
field itself can be used in this way, but nobody so far has considered the
problem of explicitly distilling the metrical fingerprint of point-events from
the gauge-dependent components of the metric field. Working in the Hamiltonian
formulation of general relativity, and building on the results of Lusanna and
Pauri (2002), we show how Bergmann and Komar's intrinsic pseudo-coordinates
(based on the value of curvature invariants) can be used to provide a physical
individuation of point-events in terms of the true degrees of freedom (the
Dirac observables) of the gravitational field, and we suggest how this
conceptual individuation could in principle be implemented with a well-defined
empirical procedure. We argue from these results that point-events retain a
significant kind of physical objectivity.Comment: LaTeX, natbib, 34 pages. Final journal versio
End states and subgap structure in proximity-coupled chains of magnetic adatoms
A recent experiment [Nadj-Perge et al., Science 346, 602 (2014)] provides
evidence for Majorana zero modes in iron (Fe) chains on the superconducting
Pb(110) surface. Here, we study this system by scanning tunneling microscopy
using superconducting tips. This high-resolution technique resolves a rich
subgap structure, including zero-energy excitations in some chains. We compare
the symmetry properties of the data under voltage reversal against theoretical
expectations and provide evidence that the putative Majorana signature overlaps
with a previously unresolved low-energy resonance. Interpreting the data within
a Majorana framework suggests that the topological gap is significantly smaller
than previously believed. Aided by model calculations, we also analyze
higher-energy features of the subgap spectrum and their relation to high-bias
peaks which we associate with the Fe d-bands.Comment: 5+5 pages, 5+6 figure
Pressure-induced electronic topological transitions in low dimensional superconductors
In the high-Tc cuprates, the unusual dependence of Tc on external pressure
results from the combination of the nonmonotonic dependence of Tc on hole
doping or hole-doping distribution among inequivalent layers, and from an
``intrinsic'' contribution. After reviewing our work on the interplay among Tc,
hole content, and pressure in the bilayered and multilayered cuprate
superconductors, we will discuss how the proximity to an electronic topological
transition (ETT) may give a microscopic justification of the ``intrinsic''
pressure dependence of Tc in the cuprates. As a function of the proximity to an
ETT, we recover a nonmonotonic behaviour of the superconducting gap at T=0,
regardless of the pairing symmetry of the order parameter. This is in agreement
with the trend observed for Tc as a function of pressure and other material
specific quantities in several high-Tc cuprates. In the case of epitaxially
strained cuprate thin films, we argue that an ETT can be driven by a
strain-induced modification of the in-plane band structure, at constant hole
content, at variance with a doping-induced ETT, as is usually assumed. We also
find that an increase of the in-plane anisotropy enhances the effect of
fluctuations above Tc on the normal-state transport properties, which is a
fingerprint of quantum criticality at T=0.Comment: EHPRG Award Lecture, http://www.ehprg.org. To be published in J.
Phys.: Cond. Matte
Spin liquids in graphene
We reveal that local interactions in graphene allow novel spin liquids
between the semi-metal and antiferromagnetic Mott insulating phases, identified
with algebraic spin liquid and Z spin liquid, respectively. We argue that
the algebraic spin liquid can be regarded as the two dimensional realization of
one dimensional spin dynamics, where antiferromagnetic correlations show
exactly the same power-law dependence as valence bond correlations. Nature of
the Z spin liquid turns out to be singlet pairing, but time
reversal symmetry is preserved, taking in one valley and
in the other valley. We propose the quantized thermal valley Hall effect as an
essential feature of this gapped spin liquid state. Quantum phase transitions
among the semi-metal, algebraic spin liquid, and Z spin liquid are shown
to be continuous while the transition from the Z spin liquid to the
antiferromagnetic Mott insulator turns out to be the first order. We emphasize
that both algebraic spin liquid and Z spin liquid can be
verified by the quantum Monte Carlo simulation, showing the enhanced symmetry
in the algebraic spin liquid and the quantized thermal valley Hall effect in
the Z spin liquid
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