95 research outputs found
Hidden geometric correlations in real multiplex networks
Real networks often form interacting parts of larger and more complex
systems. Examples can be found in different domains, ranging from the Internet
to structural and functional brain networks. Here, we show that these multiplex
systems are not random combinations of single network layers. Instead, they are
organized in specific ways dictated by hidden geometric correlations between
the individual layers. We find that these correlations are strong in different
real multiplexes, and form a key framework for answering many important
questions. Specifically, we show that these geometric correlations facilitate:
(i) the definition and detection of multidimensional communities, which are
sets of nodes that are simultaneously similar in multiple layers; (ii) accurate
trans-layer link prediction, where connections in one layer can be predicted by
observing the hidden geometric space of another layer; and (iii) efficient
targeted navigation in the multilayer system using only local knowledge, which
outperforms navigation in the single layers only if the geometric correlations
are sufficiently strong. Our findings uncover fundamental organizing principles
behind real multiplexes and can have important applications in diverse domains.Comment: Supplementary Materials available at
http://www.nature.com/nphys/journal/v12/n11/extref/nphys3812-s1.pd
Inferring short-term volatility indicators from Bitcoin blockchain
In this paper, we study the possibility of inferring early warning indicators
(EWIs) for periods of extreme bitcoin price volatility using features obtained
from Bitcoin daily transaction graphs. We infer the low-dimensional
representations of transaction graphs in the time period from 2012 to 2017
using Bitcoin blockchain, and demonstrate how these representations can be used
to predict extreme price volatility events. Our EWI, which is obtained with a
non-negative decomposition, contains more predictive information than those
obtained with singular value decomposition or scalar value of the total Bitcoin
transaction volume
Investigation of the thermal stability of Mg/Co periodic multilayers for EUV applications
We present the results of the characterization of Mg/Co periodic multilayers
and their thermal stability for the EUV range. The annealing study is performed
up to a temperature of 400\degree C. Images obtained by scanning transmission
electron microscopy and electron energy loss spectroscopy clearly show the good
quality of the multilayer structure. The measurements of the EUV reflectivity
around 25 nm (~49 eV) indicate that the reflectivity decreases when the
annealing temperature increases above 300\degreeC. X-ray emission spectroscopy
is performed to determine the chemical state of the Mg atoms within the Mg/Co
multilayer. Nuclear magnetic resonance used to determine the chemical state of
the Co atoms and scanning electron microscopy images of cross sections of the
Mg/Co multilayers reveal changes in the morphology of the stack from an
annealing temperature of 305\degreee;C. This explains the observed reflectivity
loss.Comment: Published in Applied Physics A: Materials Science \& Processing
Published at
http://www.springerlink.com.chimie.gate.inist.fr/content/6v396j6m56771r61/ 21
page
Electron Thermalization and Relaxation in Laser-Heated Nickel by Few-Femtosecond Core-Level Transient Absorption Spectroscopy
Direct measurements of photoexcited carrier dynamics in nickel are made using
few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at
the nickel M edge. It is observed that the core-level absorption
lineshape of photoexcited nickel can be described by a Gaussian broadening
() and a red shift () of the ground state absorption
spectrum. Theory predicts, and the experimental results verify that after
initial rapid carrier thermalization, the electron temperature increase
() is linearly proportional to the Gaussian broadening factor
, providing quantitative real-time tracking of the relaxation of the
electron temperature. Measurements reveal an electron cooling time for 50 nm
thick polycrystalline nickel films of 64080 fs. With hot thermalized
carriers, the spectral red shift exhibits a power-law relationship with the
change in electron temperature of . Rapid
electron thermalization via carrier-carrier scattering accompanies and follows
the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal
equilibrium. Entwined with a <6 fs instrument response function, carrier
thermalization times ranging from 34 fs to 13 fs are estimated from
experimental data acquired at different pump fluences and it is observed that
the electron thermalization time decreases with increasing pump fluence. The
study provides an initial example of measuring electron temperature and
thermalization in metals in real time with XUV light, and it lays a foundation
for further investigation of photoinduced phase transitions and carrier
transport in metals with core-level absorption spectroscopy.Comment: 20 pages, 8 figure
Subfemtosecond steering of hydrocarbon deprotonation through superposition of vibrational modes
Subfemtosecond control of the breaking and making of chemical bonds in polyatomic molecules is poised to open new pathways for the laser-driven synthesis of chemical products. The break-up of the C-H bond in hydrocarbons is an ubiquitous process during laser-induced dissociation. While the yield of the deprotonation of hydrocarbons has been successfully manipulated in recent studies, full control of the reaction would also require a directional control (that is, which C-H bond is broken). Here, we demonstrate steering of deprotonation from symmetric acetylene molecules on subfemtosecond timescales before the break-up of the molecular dication. On the basis of quantum mechanical calculations, the experimental results are interpreted in terms of a novel subfemtosecond control mechanism involving non-resonant excitation and superposition of vibrational degrees of freedom. This mechanism permits control over the directionality of chemical reactions via vibrational excitation on timescales defined by the subcycle evolution of the laser waveform
Spin-polarized LEED from Xe-Pt(111)
Hilgers G, Kleineberg U, Nolting K, Wirth S, Müller N, Heinzmann U. Spin-polarized LEED from Xe-Pt(111). Vacuum. 1990;41(1-3):325-327
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