515 research outputs found
Accelerating random walks by disorder
We investigate the dynamic impact of heterogeneous environments on
superdiffusive random walks known as L\'evy flights. We devote particular
attention to the relative weight of source and target locations on the rates
for spatial displacements of the random walk. Unlike ordinary random walks
which are slowed down for all values of the relative weight of source and
target, non-local superdiffusive processes show distinct regimes of attenuation
and acceleration for increased source and target weight, respectively.
Consequently, spatial inhomogeneities can facilitate the spread of
superdiffusive processes, in contrast to common belief that external disorder
generally slows down stochastic processes. Our results are based on a novel
type of fractional Fokker-Planck equation which we investigate numerically and
by perturbation theory for weak disorder.Comment: 8 pages, 5 figure
L\'evy-like behavior in deterministic models of intelligent agents exploring heterogeneous environments
Many studies on animal and human movement patterns report the existence of
scaling laws and power-law distributions. Whereas a number of random walk
models have been proposed to explain observations, in many situations
individuals actually rely on mental maps to explore strongly heterogeneous
environments. In this work we study a model of a deterministic walker, visiting
sites randomly distributed on the plane and with varying weight or
attractiveness. At each step, the walker minimizes a function that depends on
the distance to the next unvisited target (cost) and on the weight of that
target (gain). If the target weight distribution is a power-law, , in some range of the exponent , the foraging medium induces
movements that are similar to L\'evy flights and are characterized by
non-trivial exponents. We explore variations of the choice rule in order to
test the robustness of the model and argue that the addition of noise has a
limited impact on the dynamics in strongly disordered media.Comment: 15 pages, 7 figures. One section adde
A plethora of structural transitions, distortions and modulations in Cu-doped BiMn7O12 quadruple perovskites
The presence of strongly competing electronic instabilities in a crystalline material can produce fascinating structural phenomena. For example, the A-site-ordered quadruple perovskite BiMn7O12 hosts both active polar instabilities of the Bi3+ lone pair electrons and JahnâTeller instabilities of Mn3+ cations that drive the following sequence of phase transformations on cooling, Im-3 > I2/m > Im > P1, corresponding to orbital ordering and polar distortions. Carrier doping by Cu2+ tunes the two instabilities in BiCuxMn7âxO12 solid solutions and significantly complicates the system behavior. The x = 0.05 and 0.1 members show the following sequence of phase transformations on cooling, Im-3 > I2/m > R-1(αÎČÎł)0 > R3(00Îł)t, and are examples of materials with the electric dipole helicoidal texture in the ground state and a dipole density wave structure in the intermediate R-1(αÎČÎł)0 phase (Science 2020, 369, 680â684). Here, the detailed behavior of the BiCuxMn7âxO12 solid solutions with x = 0.2â0.8 was investigated by laboratory X-ray, synchrotron X-ray, and neutron powder diffraction between 5 K and 620 K, and differential scanning calorimetry measurements. Nearly every composition (with a step Îx = 0.1) has a unique behavior when considering both the sequence of phase transitions and the presence of incommensurate superstructure reflections. The sequence Im-3 > HT-Immm(t)* > Immm* > LT-Immm(t)* is realized for x = 0.2 and 0.3 (where t denotes pseudo-tetragonal), Im-3 > I2/m* > Immm(t)* â for x = 0.4, Im-3 > I2/m* > I2/m* â for x = 0.5, Im-3 > I2/m* > Im-3 â for x = 0.6 and 0.7, and Im-3 > R-3 > I2/m > Im-3 â for x = 0.8, where asterisks denote the presence of additional incommensurate reflections. Re-entrance of the high-temperature cubic phase was observed at low temperatures for x = 0.6â0.8 suggesting strong competition between the different electronic instabilities. The re-entrant cubic phases have nearly zero thermal expansion
Unravelling daily human mobility motifs
Human mobility is differentiated by time scales. While the mechanism for long time scales has been studied, the underlying mechanism on the daily scale is still unrevealed. Here, we uncover the mechanism responsible for the daily mobility patterns by analysing the temporal and spatial trajectories of thousands of persons as individual networks. Using the concept of motifs from network theory, we find only 17 unique networks are present in daily mobility and they follow simple rules. These networks, called here motifs, are sufficient to capture up to 90 per cent of the population in surveys and mobile phone datasets for different countries. Each individual exhibits a characteristic motif, which seems to be stable over several months. Consequently, daily human mobility can be reproduced by an analytically tractable framework for Markov chains by modelling periods of high-frequency trips followed by periods of lower activity as the key ingredient.Volkswagen FoundationNEC Corporation (Fund)Massachusetts Institute of Technology (Solomon Buchsbaum Research Fund)New England University Transportation Center (Year 23 grant
Antiferrodistortive phase transition in EuTiO3
X-ray diffraction, dynamical mechanical analysis and infrared reflectivity
studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics.
Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal
I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in
Glazer notation). The phase transition is analogous to SrTiO3. However, some
ceramics as well as single crystals of EuTiO3 show different infrared
reflectivity spectra bringing evidence of a different crystal structure. In
such samples electron diffraction revealed an incommensurate tetragonal
structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with
modulated structure are activated in the IR spectra due to folding of the
Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly
influence the temperature of the phase transition to antiferrodistortive phase
as well as the tendency to incommensurate modulation in EuTiO3.Comment: PRB, in pres
Trabalho assalariado na agricultura paulista: evolução e importĂąncia no perĂodo 2004?2014.
Em 2014, do total de 702 mil pessoas com mais de 10 anos de idade e ocupadas em atividades agrĂcolas no estado de SĂŁo Paulo, 460 mil eram empregados (permanentes e temporĂĄrios). TambĂ©m vale ressaltar que, no perĂodo 2004?2014, a população economicamente ativa (PEA) agrĂcola paulista ocupada sofreu fortĂssima redução de 328 mil pessoas (taxa de -3,7% ao ano). E deste total, 187 mil (ou 57,1%) eram empregados. Em virtude disso, o objetivo do artigo Ă© analisar a evolução e a importĂąncia desta categoria de trabalhadores na agricultura paulista no perĂodo 2004?2014, tendo como base de informaçÔes os dados primĂĄrios da Pesquisa Nacional por Amostra de DomicĂlios (PNAD), realizada pelo Instituto Brasileiro de Geografia e EstatĂstica (IBGE). Especial ĂȘnfase serĂĄ dada para os seguintes aspectos do mercado de trabalho agrĂcola no perĂodo em questĂŁo: a queda contĂnua das ocupaçÔes na agropecuĂĄria; a masculinização da população economicamente ativa (PEA) ocupada nas atividades agrĂcolas; e o predomĂnio das relaçÔes de trabalho assalariado.TĂtulo em inglĂȘs: Wage labor force in the State of SĂŁo Paulo?s agriculture: evolution and importance in the 2004-2014 period
Continuous metal-insulator transition of the antiferromagnetic perovskite NaOsO3
Newly synthesized perovskite NaOsO3 shows Curie-Weiss metallic nature at high
temperature and suddenly goes into an antiferromagnetically insulating state at
410 K on cooling. Electronic specific heat at the low temperature limit is
absent, indicating that the band gap fully opens. In situ observation in
electron microscopy undetected any lattice anomalies in the vicinity of the
transition temperature. It is most likely that the antiferromagnetic
correlation plays an essential role of the gap opening.Comment: 13 pages, 5 figures, PHYS. REV. B 80, 161104(R),(2009
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