19,036 research outputs found
Quantum magnetism with ultracold molecules
This article gives an introduction to the realization of effective quantum
magnetism with ultracold molecules in an optical lattice, reviews experimental
and theoretical progress, and highlights future opportunities opened up by
ongoing experiments. Ultracold molecules offer capabilities that are otherwise
difficult or impossible to achieve in other effective spin systems, such as
long-ranged spin-spin interactions with controllable degrees of spatial and
spin anisotropy and favorable energy scales. Realizing quantum magnetism with
ultracold molecules provides access to rich many-body behaviors, including many
exotic phases of matter and interesting excitations and dynamics.
Far-from-equilibrium dynamics plays a key role in our exposition, just as it
did in recent ultracold molecule experiments realizing effective quantum
magnetism. In particular, we show that dynamical probes allow the observation
of correlated many-body spin physics, even in polar molecule gases that are not
quantum degenerate. After describing how quantum magnetism arises in ultracold
molecules and discussing recent observations of quantum magnetism with polar
molecules, we survey prospects for the future, ranging from immediate goals to
long-term visions.Comment: 21 pages, 6 figures, 1 table. Review articl
Anisotropy Control in Photoelectron Spectra: A Coherent Two-Pulse Interference Strategy
Coherence among rotational ion channels during photoionization is exploited
to control the anisotropy of the resulting photoelectron angular distributions
at specific photoelectron energies. The strategy refers to a robust and single
parameter control using two ultra-short light pulses delayed in time. The first
pulse prepares a superposition of a few ion rotational states, whereas the
second pulse serves as a probe that gives access to a control of the molecular
asymmetry parameter for individual rotational channels. This is
achieved by tuning the time delay between the pulses leading to channel
interferences that can be turned from constructive to destructive. The
illustrative example is the ionization of the state of
Li. Quantum wave packet evolutions are conducted including both
electronic and nuclear degrees of freedom to reach angle-resolved photoelectron
spectra. A simple interference model based on coherent phase accumulation
during the field-free dynamics between the two pulses is precisely exploited to
control the photoelectron angular distributions from almost isotropic, to
marked anisotropic
Rapid inversion: running animals and robots swing like a pendulum under ledges.
Escaping from predators often demands that animals rapidly negotiate complex environments. The smallest animals attain relatively fast speeds with high frequency leg cycling, wing flapping or body undulations, but absolute speeds are slow compared to larger animals. Instead, small animals benefit from the advantages of enhanced maneuverability in part due to scaling. Here, we report a novel behavior in small, legged runners that may facilitate their escape by disappearance from predators. We video recorded cockroaches and geckos rapidly running up an incline toward a ledge, digitized their motion and created a simple model to generalize the behavior. Both species ran rapidly at 12-15 body lengths-per-second toward the ledge without braking, dove off the ledge, attached their feet by claws like a grappling hook, and used a pendulum-like motion that can exceed one meter-per-second to swing around to an inverted position under the ledge, out of sight. We discovered geckos in Southeast Asia can execute this escape behavior in the field. Quantification of these acrobatic behaviors provides biological inspiration toward the design of small, highly mobile search-and-rescue robots that can assist us during natural and human-made disasters. We report the first steps toward this new capability in a small, hexapedal robot
Phototactic Robot Tunable by Sensorial Delays
The presence of a delay between sensing and reacting to a signal can
determine the long-term behavior of autonomous agents whose motion is
intrinsically noisy. In a previous work [M. Mijalkov, A. McDaniel, J. Wehr, and
G. Volpe, Phys. Rev. X 6, 011008 (2016)], we have shown that sensorial delay
can alter the drift and the position probability distribution of an autonomous
agent whose speed depends on the illumination intensity it measures. Here,
using theory, simulations, and experiments with a phototactic robot, we
generalize this effect to an agent for which both speed and rotational
diffusion depend on the illumination intensity and are subject to two
independent sensorial delays. We show that both the drift and the probability
distribution are influenced by the presence of these sensorial delays. In
particular, the radial drift may have positive as well as negative sign, and
the position probability distribution peaks in different regions depending on
the delay. Furthermore, the presence of multiple sensorial delays permits us to
explore the role of the interaction between them.Comment: 13 pages, 6 figure
Core-Core Dynamics in Spin Vortex Pairs
We investigate magnetic nano-pillars, in which two thin ferromagnetic
nanoparticles are separated by a nanometer thin nonmagnetic spacer and can be
set into stable spin vortex-pair configurations. The 16 ground states of the
vortex-pair system are characterized by parallel or antiparallel chirality and
parallel or antiparallel core-core alignment. We detect and differentiate these
individual vortex-pair states experimentally and analyze their dynamics
analytically and numerically. Of particular interest is the limit of strong
core-core coupling, which we find can dominate the spin dynamics in the system.
We observe that the 0.2 GHz gyrational resonance modes of the individual
vortices are replaced with 2-6 GHz range collective rotational and vibrational
core-core resonances in the configurations where the cores form a bound pair.
These results demonstrate new opportunities in producing and manipulating spin
states on the nanoscale and may prove useful for new types of ultra-dense
storage devices where the information is stored as multiple vortex-core
configurations
Performance trade-offs in the flight initiation of Drosophila
The fruit fly Drosophila melanogaster performs at least two distinct types of flight initiation. One kind is a stereotyped escape response to a visual stimulus that is mediated by the hard-wired giant fiber neural pathway, and the other is a more variable `voluntary' response that can be performed without giant fiber activation. Because the simpler escape take-offs are apparently successful, it is unclear why the fly has multiple pathways to coordinate flight initiation. In this study we use high-speed videography to observe flight initiation in unrestrained wild-type flies and assess the flight performance of each of the two types of take-off. Three-dimensional kinematic analysis of take-off sequences indicates that wing use during the jumping phase of flight initiation is essential for stabilizing flight. During voluntary take-offs, early wing elevation leads to a slower and more stable take-off. In contrast, during visually elicited escapes, the wings are pulled down close to the body during take-off, resulting in tumbling flights in which the fly translates faster but also rotates rapidly about all three of its body axes. Additionally, we find evidence that the power delivered by the legs is substantially greater during visually elicited escapes than during voluntary take-offs. Thus, we find that the two types of Drosophila flight initiation result in different flight performances once the fly is airborne, and that these performances are distinguished by a trade-off between speed and stability
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