LP11880E

Sudden changes of direction of Marching Locust groups

One may find in nature many examples of animals which possess an
undeniable collective behavior, such as ants or bees. In some cases,
this collective behavior manifests itself in patterns of movement in
large groups of animals like fishes, insects or birds. Strikingly,
many of these groups show sudden changes of direction in their
displacements in the absence of any external stimuli. Movement in
groups is nowadays understood as an advantageous strategy in the
search of food or as protection against predation. But the mechanistic
underpinning of these sudden changes of direction has thus far
remained largely unresolved. In this work we analyze experimental data
on the collective movement of a group of marching locusts in an
experimental setup. Our results point to the Poissonian character of
the change of direction stochastic process. This implies that this
process is purely random and as such it may well be the consequence of
the accumulation of errors made by the individuals within the group as
they try to imitate the velocity of their neighbors. These small
errors cancel each other for short times; however there is a small
probability that they may add up to produce a change of direction, and
this small probability becomes indeed significant for long times. We
have also noted the similarity of this effect with the sudden
magnetization changes that appear in magnetic materials, which
apparently share a comparable mechanism.

***

CR10222


Alpha-gas state in heavier nuclear systems

A dilute gas-like state of alpha particles can exist in heavier nuclear systems.
In the last decade, alpha clustering in nuclei has become a topic of great interest,
involving conglomerates of alpha-particles forming inside the atomic nucleus.
Recently, the extension to systems termed nuclear molecules has broadened this interest.
One atomic analogy is Bose-Einstein condensation. Recent advances in nuclear
theory indicate that there are a class of alpha particles (4He nuclei) condense out into
a dilute gas-like state. These states have a considerably extended nuclear radius and
radically different properties to other nuclear states close-by. One of the most well known
examples in light nuclei is the second 0+ state in Carbon-12, the so called Hoyle state,
responsible for our existence, due to its role in nucleosynthesis in the stars.
We have gone a step further by proving from a theoretical point of view that such
a dilute state of alpha particles can exist in heavier systems. We have shown that
a three-alpha cluster around a Calcium-40 nucleus can have a dilute gas-like structure,
which is consistence with experimental results by Kokalova et al.

Picometer‑sized magnets in free‑electron‑laser light

Atomic clusters represent an ideal model system to study size‑driven
effects in solids. Indeed, such clusters provide a self‑contained
`laboratory' where, as a function of the number of constituent atoms,
condensed matter effects gradually appear. Particularly intriguing is
the evolution of the geometric and electronic structure, and their role
in the appearance of magnetic ordering in increasing the size of the
system from a single atom to a macroscopic unit.

Knowing the structure of a cluster is thus the first important step in
understanding more complex properties. This is a challenging task since
none of the usual spectroscopy techniques work on low‑density gas‑phase
clusters. However, we prove it possible using intense tunable infrared
light from a free electron laser. The laser radiation was used to
resonantly shake a small messenger molecule off the cluster surface.
Thus obtained vibrational spectrum was proven to be a unique fingerprint
of not only the geometric, but also the magnetic configuration of the
clusters. For example, a tiny object with only four Fe atoms was shown
to be a ferrimagnet with a large exchange energy.


***

LT12529

How to make water ‘bounce’ at the nanoscale

In our cells, water is stuck between molecules with only a few nanometers to spare. Such ‘nanoconfined’ water has long been suspected of having unique properties. Now a team of physicists at Wayne State University has measured the mechanical properties of water squeezed down to just a few molecules, and found that water can be switched from being a liquid to a bouncy solid by small changes in external conditions. Using a new Atomic Force Microscope technique developed at Wayne State, the team probed the mechanical properties of confined water layers without disturbing them. Oscillating a tiny probe, immersed in the liquid, with amplitudes the size of a hydrogen atom (0.1 nm), they recorded the response as the probe squeezed the water at extremely low speeds. Once squeezed to a layer four molecules thick or less, the water behaved like honey: more viscous than in bulk, but still liquid. However, at squeeze speeds of 0.8 nm/s and above, water became elastic. This speed is so slow, it would take 12 years to move one foot, yet it is enough to change the behavior of water drastically.

LS12549ER

Artificial Runners and Tumblers

A new class of artificial microswimmers with combined translational and
rotational self‑propulsion is fabricated and studied experimentally. The
chemically fueled microswimmers are made of doublets of Janus colloidal
beads with catalytic patches that are positioned at a fixed angle
relative to one another. Our work suggests strategies for designing
microswimmers that could follow prescribed cycloidal trajectories.

***

LR12495

Light‑Sharpened Ultra‑Precise Atomic Compasses

By a subtle manipulation of a beam of light, researchers have been able to improve, for the first time, the measurement of a magnetic field beyond a fundamental quantum limit. As early compasses did for navigation, today’s ultra‑precise magnetometers are opening new, ground‑breaking applications in medical and biological fields. Currently, the best magnetometers use atoms as compass needles and detect their magnetic direction with a laser beam. The sensitivity of these quantum compasses is limited by Heisenberg’s uncertainty principle, which affects the sharpness of the needle, limiting it to a “sphere of uncertainty”. Heisenberg’s principle cannot be violated, but nothing prevents us from squeezing the “sphere of uncertainty” to sharpen the needle in one direction (it will be blunt in the other, but that’s unimportant). The ICFO experiment used rubidium atoms as compasses, and a beam of so‑called squeezed light to read the atoms’ orientation and effectively "sharpen" the magnetic needles. The experiment beat the standard quantum limit, improving the magnetometer’s sensitivity by a factor of two. These results demonstrate the solid promise of quantum‑enhanced measurements, with important applications such as diagnosis of heart and neurological diseases.