egregia (Chrysomelidae: Cassidinae)
The tortoise beetle Charidotella egregia is able to modify the
structural colour of its cuticle reversibly, when disturbed by
stressful external events. After field observations, measurements of
the optical properties in the two main stable colour states and SEM
and TEM investigations, a physical mechanism is proposed to explain
the colour switching of this insect. It is shown that the gold
colouration displayed by animals at rest arises from a chirped
multilayer reflector maintained in a perfect coherent state by the
presence of humidity in the porous patches within each layer, while
the red colour displayed by disturbed animals results from the
destruction of this reflector by the expulsion of the liquid from the
porous patches, turning the multilayer into a translucent slab that
leaves an unobstructed view of the deeper-lying, pigmented red
substrate. This mechanism not only explains the change of hue but also
the change of scattering mode from specular to diffuse. Quantitative
modelling is developed in support of this analysis. ED10465
***
Corralling Nanoparticles via Directed Dewetting
Remarkably, nanoscientists have much to learn from the behaviour of liquids on what might initially be considered very mundane surfaces, including windows, table tops, and plant leaves. In particular, there are many parallels to be drawn between coffee stains on table tops and the generation of patterns in assemblies of nanoparticles. In both cases, it is the behaviour of the solvent (water in the case of coffee) in which the particles are initially dissolved that largely dictates just how the particles will be distributed across a surface. In this paper, we show that by pre-patterning a surface with microscopic and nanoscopic features, solvent evaporation and flow can be controlled with an unprecedented level of precision. This allows us to direct the organisation of nanoparticles without requiring any of the chemical modification strategies commonly used to enforce particle adsorption at specific surface regions.LE11592
***
ODD TRIPLET PAIRING INDUCED IN S-WAVE SUPERCONDUCTORS BY FERROMAGNET PROXIMITY
EFFECT
In a paper ("Odd triplet pairing in clean Superconductor/Ferromagnet
heterostructures") which will shortly appear in Physical Review Letters,
Halterman, Barsic and Valls show that triplet superconducting pairing, odd in
time, is induced in clean heterostructures consisting of an ordinary
s-wave superconductor sandwiched between ferromagnets.
The paper shows that the triplet correlations are spatially very long ranged.
They are largest at times of the order of the inverse of the cutoff frequency
(usually the Debye frequency) of the s-wave superconducting interaction. The
spatial behavior of these correlations is also discussed.
This phenomenon has implications not only for ordinary heterostructures but
also for fermionic superfluidity in atomic systems, where different mechanisms
for induced exotic superconductivity have been recently proposed (e.g. PRL
97, 020402, (2006)). ld11036
***
Cooling Raman lasers without bulky heat sinks
In our Physical Review Letters publication (N. Vermeulen, C. Debaes, P. Muys, and H. Thienpont, “Mitigating heat dissipation in Raman lasers using CARS,” PRL 2007), we introduce a novel physical mechanism to solve the important problem of unwanted heat generation in lasers. Laser sources have become very important tools in today’s society: they are used in modern telecommunication systems, in DVD-recorders, in the bar code scanners at the supermarket, in medical therapy and diagnosis, and in many more application domains. In 2004, a major breakthrough was achieved in the field of laser research: a laser based on silicon had been successfully developed, an accomplishment that could pave the way to an extended integration of laser sources in the silicon-based electronic chips of today’s computer systems. The latter technique would allow computer processors to become much more powerful than they are today. However, this newly developed silicon-based laser – its proper name is “silicon Raman laser” – suffers, like all laser sources, from internal heat generation. To remove this heat, the silicon light source is most often implemented with a conventional heat sink. The problem of this “bulky” heat disposal approach is that it is not in line with today’s tendency of continuously downscaling computer chips by minimizing the size of the chips’ components.
As a solution to this problem we have conceived for the silicon Raman laser a novel technique that intrinsically mitigates the laser heat without making use of a heat sink. Our heat mitigation approach, which can actually be applied to all lasers belonging to the Raman laser category, is based on an optical process called Coherent Anti-Stokes Raman Scattering (CARS). When the CARS-process is triggered inside the laser under appropriate conditions, the light that is generated through this optical mechanism, carries away part of the laser heat. In our Letter, we explain the basic principle of our heat mitigation approach, theoretically demonstrate the heat extracting nature of CARS, and introduce several methods to obtain a high heat mitigation efficiency. Finally, we numerically show for a silicon Raman laser that our technique can reduce the laser heat with as much as 35%. LY10439
***
Just add Light
Molecular wires are basic building blocks in the design of nanoscale
electronic devices and are natural candidates for ongoing circuit
miniaturization. This paper shows how lasers can be used to efficiently
induce, in molecular wires, ultrafast currents that last a few
femtoseconds (a millionth of a billionth of a second), and to do so
without the need for external voltage. Further, varying laser parameters
allows one control over the direction and magnitude of the resulting
current. The scenario may well lead to molecular switches in single
molecular wires with operation speeds that cannot be attained by
any conventional means. LF11007
***
Observing Spin-Polarization of a Single Atom
In recent years much effort has been aimed at reducing the size of magnetic tunnel junctions to improve the performance of magnetic memory devices, to perform fundamental studies of quantum magnetism, and to use in the area quantum information applications. The ultimate limit of such efforts is to create magnetic devices whose functionality is engineered at the level of individual magnetic atoms. An important goal in reaching this limit is to observe the spin-polarization of individual atoms placed at a surface (i.e., placed in a device-like environment). We have achieved this goal for individual Fe and Cr atoms placed at the surface of a nanostructured metal substrate.
Individual Fe and Cr atoms (adatoms) were deposited onto ferromagnetic Co nano-islands (see image), thereby coupling (and "fixing") the adatom spin to the Co island magnetization via a direct magnetic exchange interaction. We used low-temperature spin-polarized scanning tunneling spectroscopy to measure the spin-polarization of individual Fe and Cr adatoms prepared in this way and to detect the sign of their coupling to the underlying Co island (i.e., whether it is "up" or "down") over a limited energy range of the adatoms' electronic spin density. We observed clear, stationary spin-polarization for both Fe and Cr adatoms, and we find that the two types of atoms exhibit different magnetic coupling to the Co island surface: Fe atoms show parallel spin-polarization while Cr atoms show antiparallel spin-polarization. This demonstrates a new ability to engineer, fabricate, and measure spin-polarized nanostructures at the single atom limit. LC11724
***
Tornadoes align in hurricane in the desktop vortex world of electrons
Compact but strong vortices are orderly aligned spontaneously if placed
within a broad vortex. The ordered state is interrupted by intermittent
merger between the compact vortices followed by subsequent chaos until
next ordered state appears anew with fewer members. A desktop physics
experiment has revealed such human-like dynamics of struggle and peace
in the vortex motion of electrons. A new scaling here observed is that
the successive ordered states appear in a logarithmic scale of time with
a clear contrast to the power dependence so-far observed in random
motions of vortices. The temporal-relaxations in logarithmic scale may
be observed in the memory of glass and compaction states of powders. It
may also be related to the frequency of earthquakes of different
magnitudes. The observed arrays of the compact vortices exhaustively
cover the list of quantum vortices arrays in super-fluid helium as
theoretically predicted and partially observed, providing a clue to
explore more fundamental mechanisms of growth and relaxation of ordered
structures. LB10962
***
Plasmonic nanostructures pave the way to better and smaller spectrometers
Since the first experiments by Newton on light dispersion in the glass
prism, the spectral dispersion of light is ubiquitous in applications
ranging from optical spectral devices to sensing and novel optical
communication technologies. Optical spectroscopy based on light-wave
dispersion is used for studying the optical properties and the internal
structure of materials, atoms and molecules. Both prisms and diffraction
gratings can be used to disperse incident light -- separating it into
different colours. Very recently, new challenges have arisen for
ultrahigh dispersion needed in optical switching and routing of
information according to its wavelength (colour), to develop compact
devices that can be integrated into photonic circuits of the future or
into lab-on-a-chip applications. We have proposed a new principle to
realise a microscale spectral device using the properties of surface
plasmons on metallic nanostructures that can provide wavelength
separation of one or more orders of magnitude better than in other
state-of-the-art wavelength-splitting devices available to date. LC11099
***
A LITTLE SPICE MATTERS FOR FREEZING LIQUID METALS
It is hard to imagine that the weight of a bucketful of plain sand would be any different from the weight of a bucketful of spiced sand with a pinch of salt. However, this is what happened in some metallic glasses, frozen liquid metals with randomly packed atoms. A group of physicists at UNC-Chapel Hill and at the Institute of Physics, Chinese Academy of Sciences, found that the addition of a tiny amount of certain chemical element to liquid metals causes significant changes of atomic packing in the frozen liquid leading to denser packing of atoms. This makes the freezing of liquid metals a lot easier to achieve by suppressing crystallization. LD11526
***
The amazing properties of light matter-antimatter atoms on a quartz
surface
Systems of ordinary matter interacting with antimatter display
extraordinary properties. In this paper we have shown that positronium
(Ps), the bound state of an electron and a positron, can be physisorbed
on a quartz crystal surface and, when so, it is a most interesting
quantum mechanical object. The source of Ps trapping at the surface is
its attraction with the crystal atoms caused by the paradigmatic van der
Waals force, which also plays a fundamental role in chemical and
biological processes. A key step in the determination of the Ps-quartz
surface interaction potential has been a very precise first-principles
calculation of the so-called van der Waals constant. Since the Ps atom
can be considered as a very light hydrogen atom, quantum mechanics
predicts a highly fuzzy behavior, clearly seen in our results, that can
explain an enhancement of dipositronium (Ps_2) recombination near quartz
surfaces. This is a step forward in understanding important experiments
recently done with high-density Ps gas. These experiments have great
implications for the realization of the Ps Bose-Einstein condensate
predicted by A. P. Mills and P. M. Platzman and for establishing
conditions under which a gamma ray LASER can be achieved by stimulated
emission of annihilation photons.LD11807
