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Pre-suffused surfaces make them slippery (Vol. 43 No. 2)

image Two similar coffee drops after evaporation: a coffee stain is observed on a usual solid, while super-slippery oily surfaces concentrate the powder, demonstrating the absence of pinning

Liquid drops most often stick to solids, which contributes to degrade these solids and affects their transparency. Slip can be induced by coating solids with hydrophobic textures: then, liquids only contact the texture tips, which dramatically decrease adhesion. On these super-hydrophobic materials, water nicely recovers the mobility expected from its low viscosity.

Another way to make liquids mobile was proposed in this letter and by Wang et al. (Nature 2011). It uses textures in a oleophilic situation: a solid coated with posts contacting oil can be spontaneously invaded by a film of this oil, the network of pillars acting as a kind of porous medium. At the texture scale (10 µm, typically), gravity is negligible compared to surface forces, so that the film gets trapped by the pillars, even when tilted. If now a drop contacts this substrate, it lands on a substrate mostly wet, and pinning can be strongly reduced. As an example, a coffee drop evaporating on a standard substrate leaves behind a coffee stain, primarily arising from the ability of the liquid to stick, while the coffee powder gets localized on these new slippery materials - making it easy to remove afterwards.

The condition for achieving these "floating" states was explored: the pre-suffused oil must wet the substrate with air above, but also with water (or another oil) above. Apart from its potential applications, this system is one of the very first explored where four phases (instead of three, in classical wetting) meet. It also has the interesting capacity to dissolve incoming liquid contaminants, again taking advantage of the mostly-liquid nature of the substrate.

Slippery pre-suffused surfaces
A. Lafuma and D. Quéré, EPL, 96, 56001 (2011)
[Abstract]

Precise electron spin control yields faster memory storage (Vol. 50, No. 1)

Panel shows a 2D view of spin dynamics for bulk nickel

New ultra-fast laser method aims to improve control over the electron’s degree of freedom, called spins, could enhance memory storage devices

Data storage devices are not improving as fast as scientists would like. Faster and more compact memory storage devices will become a reality when physicists gain precise control of the spins of electrons. They typically rely on ultra-short lasers to control spins. However, improvement of storage devices via spin control requires first to develop ways of controlling the forces acting on these electronic spins. In a recent study published recently, the authors have developed a new theory to predict the complex dynamics of spin procession once a material is subjected to ultra-short laser pulses. The advantage of this approach, which takes into account the effect of internal spin rotation forces, is that it is predictive. The authors find that internal spin rotation forces only contribute significantly to spin dynamics when the variation in different directions of the magnetic energy—or magnetic anisotropy energy—is small. This is the case with materials which are highly symmetric such as bulk metals with a cubic structure. When such magnetic anisotropy energy is large, the spin rotation effect is too small to cause any significant precession of spins below 100 femtoseconds.

J. K. Dewhurst, A. Sanna, and S. Sharma, Effect of exchange-correlation spin-torque on spin dynamics, Eur. Phys. J. B 91, 218 (2018)
[Abstract]

Precursors to Rare Events in Stochastic Resonance (Vol. 51, No. 3)

(a)-(c) Trajectories of the system x(t) during a transition with noise realisations p(t) (crosses) and analytical predictions (solid lines) for three different simulations characterised by a Lyapunov exponent of the stable orbit, λ_s. (d) The mean standard error (MSE) of noise realisations with respect to their predicted behaviour using the same data of the box in (c) displayed over a larger time window. The solid line is the average MSE, the dashed line corresponds to the p(t) in the inset of (c) and the dotted line is the minimum value reached by the MSE if these values of p(t) are removed.

Niels Bohr has often been attributed with saying "Prediction is difficult, especially about the future."

Indeed, predicting the states of real world noisy dynamical systems continues to be a fundamental scientific challenge. In stochastic resonance a periodically forced Brownian particle jumps at rare intervals between two states. We have found precursors—predictors—to these stochastic transitions by revealing how the noise fluctuations become deterministic as the system approaches the rare event. Our path-integral method agrees with numerical simulations in extracting precursor fluctuations from data in the vast range of systems that exhibit stochastic resonance.

Reference

L. T. Giorgini et al, Precursors to rare events in stochastic resonance, EPL 129 40003 (2020)
[Abstract]

Prevention of dark currents from photocathodes (Vol. 46 No. 4)

Prevention of dark currents from photocathodes
Field emission maps of an annealed oxide-free (left) and thermally oxidized Mo surface (right). Obviously, the number of emitters providing 1 nA current at fields up to 100 MV/m is reduced by the oxide.

Alkali-based photocathodes deposited in the centre of molybdenum substrates are used as pulsed electron sources in linear particle accelerators. Operation at high electric dc or rf fields is required to obtain a low beam emittance, thus increasing the probability of unwanted dark currents from the cathode surface. Therefore, a field emission scanning microscope was used to localize parasitic electron emitters on single crystal and polycrystalline Mo plugs. In contrast to well-polished and dry-ice cleaned Mo surfaces with native oxide, strong field emission occurred after heat treatments above 400 °C (see figure), which are usually applied before the coating process. Thermal oxidation, however, partially weakened the emitters. X-ray photoelectron spectroscopy confirmed the corresponding changes of the surface oxide layer. These results suggest a selective removal of the native Mo oxide prior to the photocathode deposition to prevent the dark currents in accelerators.

S. Lagotzky, R. Barday, A. Jankowiak, T. Kamps, C. Klimm, J. Knobloch, G. Müller, B. Senkovskiy and F. Siewert, Prevention of electron field emission from molybdenum substrates for photocathodes by the native oxide layer, Eur. Phys. J. Appl. Phys. 70, 21301 (2015)
[Abstract]

Primordial curvature perturbations and the cosmological constant (Vol. 45 No.5-6)

Examples of density profiles seeded by primordial curvature perturbations modeled by gaussian curves of different width.

The standard cosmological model is based on the assumption that the Universe is homogeneous and isotropic on a sufficiently large scale. Inflation can give a natural explanation to this large scale homogeneity, through a sufficiently long period of exponential expansion of the Universe, but it also predicts the existence of perturbations of the metric, which are in good agreement with the observed anisotropy of the cosmic microwave background radiation or the large structure of the spatial distribution of galaxies.

This is the motivation to study the effects on the luminosity distance of a local inhomogeneity seeded by primordial curvature perturbations of the type predicted by the inflationary scenario. We find that a local underdensity originated from one, two or three standard deviations peaks of the primordial curvature perturbations field can induce corrections to the value of a cosmological constant of the order of 0.6%, 1%, 1.5%, respectively.

Our results can be considered an upper bound for the effect of the monopole component of the local non-linear structure which can arise from primordial curvature perturbations and requires a fully non-perturbative relativistic treatment.

A. E. Romano, S. Sanes Negrete, M. Sasaki and A. A. Starobinsky, “Non-perturbative effects of primordial curvature perturbations on the apparent value of a cosmological constant”, EPL, 106, 69002 (2014)
[Abstract]

Probing thermonuclear plasmas with atoms (Vol. 43 No. 6)

Computer simulations serve as a test of analytical line shape models. Here, we compare calculations of the angle-averaged S-matrix for a quasi-elastic atom-ion collision. The model and the simulation lead to the same line shape.

Controlling the plasma in magnetic fusion experiments remains a major challenge, in particular, with the advent of large-scale facilities such as the ITER tokamak (presently under construction in Cadarache, France). In order to support the operation of the machine, an extensive set of measurements is planned. Passive spectroscopy is a convenient diagnostic tool since it is non-intrusive and quite easy to implement experimentally. For instance, the hydrogen Balmer a line (3 ⇒ 2 transition, visible range) is considered as a way to measure fluxes of the hydrogen isotopes (H, D, T) in the divertor region (see “Progress in the ITER Physics Basis”, Nucl. Fusion, special issue, 2007).

The plasma density in the ITER divertor will be large enough to make Stark broadening observable on the spectral lines of hydrogen isotopes. Such neutral particles survive in the cold and complex edge plasma and their spectra provide invaluable information on its conditions. We have shown that the case where the Stark perturbation can be associated with a series of binary collisions with ions (impact approximation, see the work of Hans Griem, Plasma Spectroscopy) may be adapted to conditions foreseen in ITER. This model is based on an estimation of the S-matrix for atom-perturber collisions using a series expansion for large impact parameters (weak collisions) and, on the other hand, using a cut-off accounting for the oscillating behaviour of the wave-function in the case of small impact parameters (strong collisions). Confrontations with computer simulations indicate that the model is a good candidate for accurate diagnostics in the ITER plasma.

J Rosato, H Capes, L Godbert-Mouret, M Koubiti, Y Marandet and R Stamm, ‘Accuracy of impact broadening models in low-density magnetized hydrogen plasmas’, J. Phys. B: At. Mol. Opt. Phys. 45, 165701 (2012)
[Abstract]

Producing hydrogen from splitting water without splitting hairs (Vol. 49 No.5-6)

Adsorption of water molecules on the surface of copper nanoparticles could produce hydrogen faster and more efficiently

New model explains interactions between small copper clusters used as low-cost catalysts in the production of hydrogen by breaking down water molecules

Copper nanoparticles dispersed in water or in the form of coatings have a range of promising applications, including lubrication, ink jet printing, as luminescent probes, exploiting their antimicrobial and antifungal activity, and in fuel cells. Another promising application is using copper as a catalyst to split water molecules and form molecular hydrogen in gaseous form. At the heart of the reaction, copper-water complexes are synthesised in ultra-cold helium nanodroplets as part of the hydrogen production process, according to a recent paper published recently. For its authors, splitting water like this is a good way of avoiding splitting hairs. In their study, they synthesised neutral copper-water complexes by successively doping helium nanodroplets with copper atoms and water molecules. These droplets are then ionised by electrons. The authors show that the composition of the most prominent ions depends on the partial copper and water pressures in the cell where the reaction occurs. They observe ions containing several copper atoms and several dozen water molecules.

S. Raggl, N. Gitzl, P. Martini, P. Scheier, and O. Echt , Helium nanodroplets doped with copper and water, Eur. Phys. Jour. D 72, 130 (2018)
[Abstract]

Proving Einstein right using the most sensitive Earth rotation sensors ever made (Vol. 48, No. 4)

Physicists have now found a way to measure Earth's rotation in an extremely accurate way. (Photo: Fotolia, ID: #60274978 by Denis Tabler)

A new study use the most precise inertial sensor available to date to measure whether Earth partially drags inertial frames along with its rotation.

Einstein’s theory of gravity, also referred to as General Relativity, predicts that a rotating body such as the Earth partially drags inertial frames along with its rotation. In a study recently published, a group of scientists based in Italy suggests a novel approach to measuring what is referred to as frame dragging. The authors propose using the most sensitive type of inertial sensors, which incorporate ring lasers as gyroscopes, to measure the absolute rotation rate of the Earth. The experiment aims to measure the absolute rotation with respect to the local inertial frame, which is what is referred to as frame dragging. In principle, the ring laser should show one rotation around the Earth's axis every 24 hours. However, should observation by reference to fixed stars in the sky show a slightly different rate of rotation, the difference can be attributed to frame dragging. The authors’ proposed experiment, called GINGER, requires two ring lasers to provide a reference measurement. Their proposed solution can accurately test the frame dragging effect at 1%, a vast improvement compared to previous experiments, which has 19% and 5% error in their measurement.

A. D. V. Di Virgilio, J. Belfi, W.-T. Ni, N. Beverini, G. Carelli, E. Maccioni and A. Porzio, GINGER: a feasibility study, Eur. Phys. J. Plus 132, 157 (2017)
[Abstract]

Pulsating dust cloud dynamics modelled (Vol. 44 No. 6)

Different phases of a self-gravitational potential

New research outlines a new design of spatio-temporal models of astrophysical plasmas.

It is the collapse of dense molecular clouds under their own weight that offers the best sites of star formation. In the present work, the authors have proposed a new model for investigating molecular clouds fluctuations at sites of star formation and thus study their pulsational dynamics. They study the pulsating dynamics of inhomogeneous molecular clouds that periodically undergo both self-gravitational contraction due to the weight of the massive dust grains, and electrostatic expansion resulting from the interaction of dust grains of the same electric charge.

They designed a model for investigating the cloud fluctuations with charge-varying grains, as a function of weight and charge interaction (referred to as nonlinear gravito-electrostatic coupling). They then carried out a detailed shape analysis to characterize these clouds on the astrophysical scale.

P. K. Karmakar and B. Borah, ‘Nonlinear Pulsational Eigenmodes of a Planar Collisional Dust Molecular Cloud with Grain-charge Fluctuation’, Eur. Phys. J. D, 67, 187 (2013)
[Abstract]

Pushing the boundaries of magnet design (Vol. 48 No. 1)

Representation of the magnetic anisotropy of single ions contained in the rare-earth components of the magnets in the study

New method to make permanent magnets more stable over time

For physicists, loss of magnetisation in permanent magnets can be a real concern. In response, the Japanese company Sumitomo created the strongest available magnet—one offering ten times more magnetic energy than previous versions—in 1983. These magnets are a combination of materials including rare-earth metal and so-called transition metals, and are accordingly referred to as RE-TM-B magnets. The authors have now been pushing the boundaries of magnet design, as published in a recent study. They have developed methods to counter the spontaneous loss of magnetisation, based on their understanding of the underlying physical phenomenon. They have now developed a simple additive-based method for ensuring the stability of permanent magnets over time, with no loss to their main magnetic characteristics.

R. B. Morgunov, E. I. Kunitsyna, V. V. Kucheryaev, V. P. Piskorskii, O. G. Ospennikova and E. N. Kablov, Giant effect of Sm atoms on time stability of (NdDy)(FeCo)B magnet, Eur. Phys. J. Plus, 131, 344 (2016)
[Abstract]

Quantifying electrocaloric effects in multilayer capacitors (Vol. 49, No. 2)

“(a) Schematic cross-section of an MLC. (b) Photograph of three MLCs based on 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3, with active area per layer of 0.12, 0.29 and 0.42 cm2 within dashed lines. (c) Temperature change |ΔTj| measured directly using a thermocouple (solid circles) versus the ratio of active volume Vactive to total volume Vtotal. Multiplying by the ‘correction’ factor (solid squares) that would have been hitherto assumed elsewhere overestimates |ΔTj| (open circles)

Multilayer capacitors (MLCs) are now being exploited in prototype cooling devices because they show large voltage-driven changes of temperature that can be used to pump large amounts of heat. However, accurate quantification of these electrically driven temperature changes is challenging because only the core is electrocalorically active.

In a recent study, the authors investigated electrocaloric MLCs with different geometries. By increasing the active volume of the core with respect to the inactive surround, the authors were able to identify the temperature changes that could be driven in the core without thermalization due to the surround. This improves upon previous works, in which partial thermalization was assumed to be complete, leading to overestimates of temperature change.

T. Usui, S. Hirose, A. Ando, S. Crossley, B. Nair, X. Moya, and N. D. Mathur, Effect of inactive volume on thermocouple measurements of electrocaloric temperature change in multilayer capacitors of 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3, J. Phys. D: Appl. Phys. 50, 424002 (2017).
[Abstract]

Quantifying how much quantum information can be eavesdropped (Vol. 50, No. 2)

Eavesdropping. Credit: Photo by Dmitry Ratushny on Unsplash

New study yields more precise characterisation of monogamous and polygamous entanglement of quantum information units

Encrypted communication is achieved by sending quantum information in basic units called quantum bits, or qubits. The most basic type of quantum information processing is quantum entanglement. However, this process remains poorly understood. Better controlling quantum entanglement could help to improve quantum teleportation, the development of quantum computers, and quantum cryptography. Now, the authors have focused on finding ways to enhance the reliability of quantum secret sharing. In a new study published recently, they provide a much finer characterisation of the distributions of entanglement in multi-qubit systems than previously available. In the context of quantum cryptography, these findings can be used to estimate the quantity of information an eavesdropper can capture regarding the secret encryption key.

Z. Zhang, Y. Luo, and Y. Li , Tighter monogamy and polygamy relations in multiqubit systems, Eur. Phys. J. D 73, 13 (2019)
[Abstract]

Quantitative interpretation of the excitonic splittings in aluminium nitride (Vol. 42, No. 3)

image locus of the values of the hole effective masses compatible with the 38meV experimental value of the 1s-2s splitting (purple line). Similar plots for alternative values of the dielectric constant (blue and green plots). The predictions of several first principle calculations are plotted using red dots.

Properties of free exciton are used to address the first self consistent, all-optical determination of hole effective masses in aluminium nitride.

AlN is a wurtzite semiconductor, which appears to be very promising as a substrate or as a buffer layer in many heteroepitaxial growths of devices like UV light emitting diodes, solar blind light detectors, high power and/or high frequency operating field-effect transistors. Its thermal conductivity is superior to that of GaN, which is today, word widely used as a substrate for pulling forward the solid state lighting and the Blue Ray laser technologies.

The growth of AlN was only very recently achieved as bulky bowls or as hetero-epitaxial films. The band gap of AlN changes with the growth conditions, which is interpreted in terms of residual strain fields existing in hetero-epitaxies. Here, the model leading to the quantitative interpretation of the evolution of the band gap of GaN under strain (B.Gil et al., PRB, 52, R17028, (1995)) is extended to AlN.

Improved crystalline quality and residual doping now allow high-resolution optical spectroscopy. The origin of the experimental value of the 1s-2s excitonic splitting is analyzed using a model of H atom adapted to anisotropic masses and dielectric constants. This analysis permits to extract from the experimental data, the couple of relevant values of the dielectric constant, to find the locus for values for the on-axis and in-plane hole effective masses (purple line in the figure) fully compatible with the measured value of the 1s-2s splitting.

Quantitative interpretation of the excitonic splittings in aluminum nitride
B. Gil, et al., Eur. Phys. J. Appl. Phys 53, 20303 (2011)
[Abstract]

Quantitative strain mapping at the nanometer scale (Vol. 43 No. 1)

image (a) Scheme and (b) dark-field (004) electron hologram of a Si/Si0.79Ge0.21 superlattice. (c) Phase of the hologram. (d) Strain map.

As strain is now used routinely in transistor devices to increase the mobility of the charge carriers, the microelectronics industry needs ways to map the strain with nanometer resolution. Recently, a powerful TEM (transmission electron microscopy) based technique called dark-field electron holography has been invented by Martin Hÿtch at CEMES in Toulouse. To map the strain, it is necessary to thin a sample to electron transparency using a focused ion beam tool. Then, to form a dark-field electron hologram, electron beams that have been diffracted by both the region of interest (a device, a layer) and a region of reference (usually the substrate) are interfered using an electron biprism.

When grown on a Si substrate by epitaxy, SiGe layers are tensily strained in the growth direction as shown in Figure (a). Due to the presence of strain, variations of the hologram fringe spacing can be seen (b). Using Fourier space processing, the phase of the electrons can be retrieved from the hologram (c), and by taking the gradient of the phase, the strain map can then be calculated (d).

As the technique is quantitative, one can directly correlate the results with simulations to get information about the composition in the layers. As an example, we have investigated the variation of the substitutional carbon content in annealed Si/SiGeC superlattices. Carbon is used to control the strain and avoid plastic relaxation. However during annealing, the formation of Β-SiC clusters reduces the effect of the C atoms. By combining holography and finite element simulation, we have shown that after annealing at 1050°C, a SiGeC structure behaves like pure SiGe from the point of view of strain.

The reduction of the substitutional C content in annealed Si/SiGeC superlattices studied by dark-field electron holography
T. Denneulin, J. L. Rouvière, A. Béché, M. Py, J. P. Barnes, N. Rochat, J. M. Hartmann and D. Cooper, Semicond. Sci. Technol. 26, 125010 (2011) [Abstract]

Quantum holograms as atomic scale memory keepsake (Vol. 46 No. 1)

Set up of the experiment showing the orthogonal side illumination.
Credit: A. N. Vetlugin et al

A new theoretical study demonstrates for the first time that quantum holograms could be a candidate for becoming quantum information memory.

The authors have developed a theoretical model of quantum memory for light, adapting the concept of a hologram to a quantum system. They demonstrate for the first time, that it is theoretically possible to retrieve, on demand, a given portion of the stored quantised light signal of a holographic image—set in a given direction in a given position in time sequence. This is done by shaping the control field both in space and time. Ultimately, scientists aim to introduce into quantum holograms the ability not only to store quantum signals but also to perform transformations of their quantum states; an approach useful for quantum communication and computation.

A. N. Vetlugin and I. V. Sokolov, “Addressable parallel cavity-based quantum memory”, Eur. Phys. J. D 68, 269 (2014)
[Abstract]

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