906 research outputs found
Cross-waves induced by the vertical oscillation of a fully immersed vertical plate
Capillary waves excited by the vertical oscillations of a thin elongated
plate below an air-water interface are analyzed using time-resolved
measurements of the surface topography. A parametric instability is observed
above a well defined acceleration threshold, resulting in a so-called
cross-wave, a staggered wave pattern localized near the wavemaker and
oscillating at half the forcing frequency. This cross-wave, which is stationary
along the wavemaker but propagative away from it, is described as the
superposition of two almost anti-parallel propagating parametric waves making a
small angle of the order of with the wavemaker edge. This
contrasts with the classical Faraday parametric waves, which are exactly
stationnary because of the homogeneity of the forcing. Our observations suggest
that the selection of the cross-wave angle results from a resonant mechanism
between the two parametric waves and a characteristic length of the surface
deformation above the wavemaker.Comment: to appear in Physics of Fluid
Sharing Positive Affective States Amongst Rodents
Group living is thought to benefit from the ability to empathize with others. Much attention has been paid to empathy for the pain of others as an inhibitor of aggression. Empathizing with the positive affect of others has received less attention although it could promote helping by making it vicariously rewarding. Here, we review this latter, nascent literature to show that three components of the ability to empathize with positive emotions are already present in rodents, namely, the ability to perceive, share, and prefer actions that promote positive emotional states of conspecifics. While it has often been argued that empathy evolved as a motivation to care for others, we argue that these tendencies may have selfish benefits that could have stabilized their evolution: approaching others in a positive state can provide information about the source of valuable resources; becoming calmer and optimistic around animals in a calm or positive mood can help adapt to the socially sensed safety level in the environment; and preferring actions also benefiting others can optimize foraging, reduce aggression, and trigger reciprocity. Together, these findings illustrate an emerging field shedding light on the emotional world of rodents and on the biology and evolution of our ability to cooperate in groups.</p
Quantum Hall resistance standards from graphene grown by chemical vapor deposition on silicon carbide
Replacing GaAs by graphene to realize more practical quantum Hall resistance
standards (QHRS), accurate to within in relative value, but operating
at lower magnetic fields than 10 T, is an ongoing goal in metrology. To date,
the required accuracy has been reported, only few times, in graphene grown on
SiC by sublimation of Si, under higher magnetic fields. Here, we report on a
device made of graphene grown by chemical vapour deposition on SiC which
demonstrates such accuracies of the Hall resistance from 10 T up to 19 T at 1.4
K. This is explained by a quantum Hall effect with low dissipation, resulting
from strongly localized bulk states at the magnetic length scale, over a wide
magnetic field range. Our results show that graphene-based QHRS can replace
their GaAs counterparts by operating in as-convenient cryomagnetic conditions,
but over an extended magnetic field range. They rely on a promising hybrid and
scalable growth method and a fabrication process achieving low-electron density
devices.Comment: 12 pages, 8 figure
Fermi-surface transformation across the pseudogap critical point of the cuprate superconductor LaNdSrCuO
The electrical resistivity and Hall coefficient R of the
tetragonal single-layer cuprate Nd-LSCO were measured in magnetic fields up to
T, large enough to access the normal state at , for closely
spaced dopings across the pseudogap critical point at .
Below , both coefficients exhibit an upturn at low temperature, which
gets more pronounced with decreasing . Taken together, these upturns show
that the normal-state carrier density at drops upon entering the
pseudogap phase. Quantitatively, it goes from at to at . By contrast, the mobility does not change appreciably, as
revealed by the magneto-resistance. The transition has a width in doping and
some internal structure, whereby R responds more slowly than to the
opening of the pseudogap. We attribute this difference to a Fermi surface that
supports both hole-like and electron-like carriers in the interval , with compensating contributions to R. Our data are in excellent
agreement with recent high-field data on YBCO and LSCO. The quantitative
consistency across three different cuprates shows that a drop in carrier
density from to is a universal signature of the pseudogap
transition at . We discuss the implication of these findings for the
nature of the pseudogap phase.Comment: 11 pages, 12 figure
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