Taking snapshots of a magnetic multilayer within 30 femtoseconds at the
X-ray laser FLASH
X-ray free electron lasers (XFEL) are unique machines providing
ultra-short flashes of x-ray photons of unmatched intensity. The number
of photons delivered in a single 20-30 femtosecond long XFEL pulse
equals the number of photons delivered within one second at a
synchrotron source. Spectacular new possibilities open up when using the
photon energy of today's FELs at X-ray energies where inner shell
electrons carrying magnetic moments can be excited. The very short X-ray
pulses will then allow to record magnetic diffraction pattern within
femtoseconds - the timescale of spin-flip processes in magnetic
materials. Our paper demonstrates this possibility of performing
resonant magnetic scattering experiments with a single pulse of the
(linearly polarized) UV/soft x-ray free-electron laser FLASH at DESY,
Hamburg. The magnetic domains This means that a new probe of magnetic
properties is now available, that allows to perform magnetic diffraction
experiments within a sub- 100 fs time-scale.
***
LY11228B
SILICON HARDWARE MAY OUTLIVE MOORE'S LAW
The end of the road for Moore's law, predicted to occur within a decade,
is associated to the fall of the computer-chip silicon supremacy. But this
may not be the case with the advent of new proposals for future silicon
computers, based on quantum behavior at a microscopic level. This paper
proposes a new strategy for quantum computation, which takes advantage of
the available silicon-based microelectronics technology and of optical
techniques borrowed from atomic physics. The estimated errors in the
operation of the device should be sufficiently small to allow for useful
applications in numerical simulations and quantum optics. The basic
processing units (quantum bits or qubits) are arsenic atoms implanted in
photonic-crystal silicon cavities, which are known to have very small
losses. Operations on single qubits and pairs of qubits, which are the
basic blocks for universal quantum computation, are implemented by a
combination of laser pulses and static electromagnetic fields. Reducing
the errors per operation in a silicon-based architecture is an important
step in the quest for viable quantum computers, and an indication that
silicon hardware technology may outlive Moore's law.
***
AK10597
Critical temperature of the interacting Bose gas - Case closed?
The dependence of the critical temperature for Bose-Einstein
condensation (BEC) on particle interactions has been a topic of
debate for many decades. It was not until 2003 that a consensus
emerged in the physics community, that is supported by numerical
and field theoretic considerations: Weak interactions enhance BEC,
and the dimensionless constant that controls the linear correction
of the critical temperature is c = +1.3. The flow of articles devoted
to this topic then came to an end.
Motivated by probability theory and statistical mechanics, we have
revisited this question using the model of "spatial permutations",
where quantum particles are represented by Brownian trajectories,
and where the onset of BEC is signaled by the occurrence of infinitely
long permutation cycles (see the illustration). Much to our surprise,
our calculations did not confirm the consensus, but led instead to
the constant c = -2.33. This means that particle interactions
discourage BEC! Our method involves several independent steps,
each of which was carefully justified. At the very least, our article
suggests that the case of the effects of interactions on BEC should
be reopened.
***
LL11906
Mind the Twist
Nanotubes can vary by diameter and length, but also by the angle at which
they are twisted. This later property is known as chirality and a useful
analogy is a roll of gift-wrap paper. If the roll is rewound carefully,
there is no overhang on either end. However, if the roll wound at an odd
angle, technically known as the Eshelby¹s twist, excess paper hangs off at
one end. Recent computational investigations of inorganic metal sulfide
nanotubes, published in Physical Review Letters, revealed the interesting
connection between chirality and the macroscopic phenomena of Eshelby¹s
twist. Most remarkably, the study demonstrates that varying chirality can
have a major role in engineering the mechanical and electronic properties of
these nanostructures. Responsible for this effect is the Eshelby¹s twist
present in chiral tubes.
***
LD12443
Chemistry produces fluid motion
Hydrodynamics is known to impact reactive processes, as stirring is
often used to enhance reaction rates. In this work, we
show that the reverse is also true: chemical reactions as simple as
A+B->C can strongly affect or even trigger fluid flows. By modifying
the density of a given solution, chemical reactions are indeed
strongly impacting buoyancy-driven flows. Not only are reactions
breaking the symmetry of classical hydrodynamic instabilities and
patterns, they can even induce flows in systems that would remain
quiescent in the absence of reaction.
Using a reaction as simple as the HCl-NaOH neutralization known to
every scientist, the authors demonstrate experimentally that
chemically-driven convection can develop. The beautiful patterns
appearing are asymmetric with regard to the initial contact line
between reactants. Numerical simulations of a
reaction-diffusion-convection model quantitatively explain the
instability scenarios and highlight conditions for which
chemically-driven flows can be observed.
***
LK12198
Freezing Friction – Microscopic bonds determine rubbing at cryogenic temperatures
In winter everyday experience teaches us that friction may dramatically increase if sliding surfaces are freezing together, an effect related to freezing water trapped at the sliding interface. But how does friction behave at cryogenic temperatures under perfectly dry conditions? In our research we investigated the friction of a nanoscale contact at temperatures down to -200°C in perfectly clean vacuum conditions. On different surface materials we consistently find an almost tenfold friction enhancement below temperatures of -120°C even in the absence of any water molecules. Our model simulations show that this effect is related to the dynamic behaviour of multiple atomic bonds at the sliding interface. Temperature leads to an enhanced attachment as well as rupturing rate of molecular scale bonds between the sliding surfaces, resulting in a peak-like friction increase at cryogenic temperatures, in agreement with the experiments. These results provide a new conceptual framework to describe the dynamics of dry friction.
***
LK12520
Can one see through paint?
Light propagates through a disordered scattering medium (such as a layer of paint or a biological tissue) in such a complex way that all spatial information seems lost, which shortly prevents imaging or focusing. In this paper, we present a method to measure the transmission matrix of a scattering medium, i.e. the relationship between what enters and what goes through the medium, independently of how long and complex the propagation has been. On top of demonstrating our ability to focus light through an opaque layer of paint, we show that we are able to reconstruct or “view” a simple object through it. Beyond the obvious imaging applications, this technique provides a new tool to understand and study the intriguing domain of wave propagation in complex media.
***
LJ11739
Large increase of the Curie temperature by orbital ordering control
We were able to first theoretically predict and then synthetize
superlattices
with Curie temperature (up to which the magnetic order is stable) far
above the bulk one of the building magnetic material and far above room
temperature (up to 1000K).
The stability of magnetic orders is directly related to the effective
magnetic exchange integrals between neighboring magnetic atoms.
Simultaneously, these exchange integrals are strongly dependant of weak
structural distortions due to the quasi-degenerate and highly
directional character of the magnetic d or f orbitals.
Synthesis in thin films allows a certain control over the material
structure through the in-plane coercion applied by the substrate.
Using fist principle calculations we showed that the Curie
temperature of manganites thin films can be increased by more than an
order of magnitude by applying appropriate strains. We
showed how the constraints, first applied by the substrate of the film,
need to be
maintained over the growth direction by the alternation of a manganite
layers with another appropriate material. Finally we synthesized such
super-lattices and verified the theoretical predictions. Indeed,
super-lattices with Tc up to 1000K were obtained.
The present results provide a new set of concepts for the control of
magnetic order and Curie temperature and thus open new perspectives to
design artificial materials with desired magnetic properties.
***
LF12190
Material failure classification breaks up!
Dynamic fracture drives catastrophic materials failures. Engineers and researchers generally categorize materials into three classes: (i) Ductile materials, like metals, which develop irreversible plastic deformations before breakdown (ii) Quasi-brittle materials, like rocks or mortar, which damage through the nucleation of microcracks before breaking through their coalescence (iii) Brittle materials, like glassy polymers and oxide glasses, which behave elastically up to failure, which occurs through the stretching and breaking of the atomic bonds at the tip of the propagating crack. The experiments presented in this paper challenge this classification and show that a given material can belong to two of these categories depending on the crack velocity. Beyond a critical velocity which is a well-defined fraction of the upper limit for crack speed, Plexiglas (that was considered up to now as an archetype of nominally brittle materials) damages through microcracking ahead of the main crack tip, as in quasi-brittle materials.
