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Vol. 48 No.5-6 - Highlights

Controlling negative ions in plasma using tailored voltage (Vol. 48, No. 5-6)

Calculated profile of negative ions for different voltage waveforms

Plasma processing of materials has wide applications in science and industry. In a capacitively coupled plasma, a feedstock gas, often diluted with hydrogen, is partially ionized with an electric discharge. In the most basic configuration, the key plasma parameters of ion flux and ion energy are strongly linked: increasing the applied sinusoidal voltage amplitude increases both. With a multiple harmonics waveform, it is possible to decouple ion flux and energy, obtaining increased processing speed while maintaining the ideal ion energy for surface reactions. This concept has been formulated and exploited for positive ions. Hydrogen plasmas, however, produce also negative ions which are important in other applications like heating systems in nuclear fusion.

The authors investigated, using a comprehensive numerical model, the effect of tailored voltage waveforms on the location, peak density and dynamics of negative hydrogen ions and the influence of molecular physics on their production rate. They found that under appropriate conditions it is possible to concentrate negative ions in a desired position. This is due to a complex interplay between plasma electrical quantities, particle energy distributions and transport phenomena.

P. Diomede, B. Bruneau, S. Longo, E. Johnson and J.-P. Booth, Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: vibrational kinetics and negative ions control, Plasma Sources Sci. Technol. 26, 075007 (2017)
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A Fokker-Planck Model for Wealth Inequality Dynamics (Vol. 48, No. 5-6)

Model comparison with data. Overall trend is well captured as well as some short-term details.

The growing wealth inequality in most western countries during the past several decades led to an increased interest in the nature of wealth inequality dynamics – particularly, what has driven wealth inequality upwards? Statistical mechanics can be used for addressing this question. We present a simple stochastic model for wealth and income and derive from it a Fokker-Planck equation – a standard tool in non-equilibrium statistical mechanics for studying the evolution of a distribution. Using this equation we are able to calculate the joint wealth-income distribution and its dynamics. Our analysis supports empirical findings on the dynamics of wealth inequality. We find that wealth inequality inevitably tends to increase in the long run. However, even if inequality eventually goes up, we find a criterion for its possible short run decrease. This criterion is most likely to be fulfilled if the correlation between wealth and income is very low. The conditions for such a decrease are found to be met at several points during the 20th century, coinciding indeed with an observed decrease in wealth inequality.

Y. Berman, Y. Shapira and M. Schwartz, A Fokker-Planck model for wealth inequality dynamics, EPL 118, 38004 (2017)
[Abstract]

Magnetite or maghemite? There is a simple answer. (Vol. 48, No. 5-6)

Correlation between δRT, the ‘centre of gravity’ of the room temperature 57Fe Mössbauer spectrum of a magnetite/maghemite sample vs. the fraction of 57Fe atoms present in magnetite, α.

The composition or stoichiometry of magnetite (Fe3O4) and maghemite (γ-Fe2O3) mixtures or solid solutions is important for the physical, geological and material sciences. It is also significant in biomedical science, where magnetic nanoparticles are used both in vitro and clinically, and where both ferrous and ferric iron ions play active roles in the production of reactive oxygen species. However, the accurate determination of the composition/stoichiometry can be tricky, as it requires either well-crystallised samples suitable for x-ray diffraction, or it relies on chemical dissolution methods that, depending on the nature of the sample, are often either unfeasible or inappropriate.

However, there is a simple answer, in the form of the recently proposed 57Fe Mössbauer spectroscopic ‘centre of gravity method’. The COG method is non-destructive and determines the composition/stoichiometry from the mean isomer shift, ¬δRT. It is well suited to nanomaterials, is simple and straightforward, and as long as appropriate measures and protocols are observed – all of which are explained in the paper – even inexperienced users will find little difficulty in its implementation.

J. Fock, L. K. Bogart, D. González-Alonso, J. I. Espeso, M. F. Hansen, M. Varón, C. Frandsen and Q. A. Pankhurst, On the ‘centre of gravity’ method for measuring the composition of magnetite/ maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via 57Fe Mössbauer spectroscopy, J. Phys. D: Appl. Phys. 50, 265005 (2017)
[Abstract]

Best tactical approach to handling patients with simultaneous parasitic and HIV infection (Vol. 48, No. 5-6)

Cryptosporidiosis

New mathematical model for cryptosporidiosis - HIV co-infection explores their synergistic relationship in connection with prevention and treatment

One of the most common waterborne diseases worldwide is cryptosporidiosis, a parasitic disease affecting the small intestine and possibly our airways. It is a common cause of diarrhoea in HIV-positive patients, who are known to have lower immunity. Now the authors have developed a new model and numerical simulations to determine the optimal combination of prevention and treatment strategies for controlling both diseases in patients who have been co-infected. Their results, recently published, show a positive impact on the treatment and prevention for cryptosporidiosis alone, for HIV-AIDS alone, or for both together. They found that cryptosporidiosis preventions and treatment alone had no significant impact on reducing HIV-AIDS-related problems. By contrast, the prevention and treatment strategy for HIV-AIDS had a significant positive impact on the co-infected patients. Finally, applying both strategies at the same time resulted in reduction in all cases.

K.O. Okosun, M.A. Khan, E. Bonyah and S.T. Ogunlade, On the dynamics of HIV-AIDS and cryptosporidiosis, Eur. Phys. J. Plus 132, 363 (2017)
[Abstract]

Highway traffic fluctuations impact congestion durations (Vol. 48, No. 5-6)

Traffic flow time series (vehicles per minute) during one day on a highway in Germany, with two rush hours and strong fluctuations

Many highways around large cities are running above their capacity, leading to congested traffic. A useful statistical description of congestion distinguishes three phases: Free flow, synchronized traffic and wide moving jams. Traffic breakdown from free flow becomes increasingly likely around a critical flow (a certain number of vehicles per minute, specific for the highway section). Here we investigate the influence of flow fluctuations on congestion durations. As can be seen in the figure, traffic flow is antipersistent: It shows large fluctuations on short time scales which quickly trend back to the mean value, i. e. they reverse fast. Therefore, the duration of times with a flow above a critical value (here 60 vehicles per minute) is most oftenly a few minutes, but sometimes it extends over longer intervals up to several hours. We find that durations of congested traffic behave in the same way, and we conclude that traffic flow fluctuations are an important factor in congestion dynamics. The large number of short-lasting traffic jams implies a large risk for rear-end collisions.

S. M. Krause, L. Habel, Th. Guhr and M. Schreckenberg, The importance of antipersistence for traffic jams, EPL 118, 38005 (2017)
[Abstract]

Like a game of 'spot the difference' for disease-prone versus healthy people (Vol. 48, No. 5-6)

Dynamical behaviour of different low-density lipoproteins as a function of temperature and pressure

The change in behaviour of natural nanoparticles, called lipoproteins, under pressure could provide new insights to better understand the genesis of high cholesterol and atherosclerosis

Understanding common diseases sometimes boils down to grasping some of their basic mechanisms. For instance, a specific kind of natural nanoparticles, called low-density lipoproteins (LDL), are fascinating scientists because their modification plays a key role in people affected by high cholesterol. They are also known for their role in the formation of atherosclerosis. The authors mimicked variations of LDL found in people affected by such diseases. They then compared their responses to temperature variations and increased pressure with those of lipoproteins found in healthy people. Their findings, recently published, show that the LDL from healthy people behaved differently when subjected to high pressure compared to LDL affected by the common diseases studied. The authors found that when LDL particles were subjected to variations in temperature, their behaviour was very similar. In fact, a rise in temperature increased their dynamics at the molecular level. However, when the authors increased the pressure on LDL particles, they found that their flexibility actually increased under pressure in healthy people. By contrast, their flexibility clearly decreased for the two modified forms mimicking disease states. This difference, the authors believe, could stem from a slightly different lipid composition.

J. Peters, N. Martinez, B. Lehofer and R. Prassl, Low-density lipoproteins investigated under high hydrostatic pressure by elastic incoherent neutron scattering, Eur. Phys. J. E 40, 68 (2017)
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Robustness of states at topological insulator interfaces (Vol. 48, No. 5-6)

Interface states emerging between two topological insulators with opposite spin Chern numbers

Topological phases of matter are characterized by invariant numbers. In two-dimensional time-reversal symmetric electronic systems, a Z2 valued (0 or 1) invariant distinguishes trivial insulators from non-trivial ones. Interfaces between trivial and non-trivial topological insulators are known to host conductive channels protected against disorder. The protection of these states originates from the necessity of a gap closure in order to change topology. However, if the two regions are of the same topological phase, there is no such requirement.

Using a multi-orbital model, it is shown in this study that conductive states can also emerge at the interface between two non-trivial topological insulators characterized by opposite spin Chern numbers, another invariant. In general, these states are sensitive to disorder. However, it is possible under some conditions to reduce the effect of disorder, or even to cancel it. These conditions are clarified. Since analogues of topological insulators can be presently made with polaritons, ultracold atomic gases, phononic or photonic materials, these conclusions should motivate experimental studies in many directions.

A. Tadjine and Ch. Delerue, Robustness of states at the interface between topological insulators of opposite spin Chern number, EPL 118, 67003 (2017)
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The secret to improving liquid crystal's mechanical performance (Vol. 48, No. 5-6)

3D plot of the concentration of nanoparticles around a moving edge dislocation in a smectic A liquid crystal

Better lubricating properties of lamellar liquid crystals could stem from changing the mobility of their structural dislocations by adding nanoparticles

By deliberately interrupting the order of materials—by introducing different atoms in metal or nanoparticles in liquid crystals—we can induce new qualities. For example, metallic alloys like duralumin, which is composed of 95% of aluminium and 5% copper, are usually harder than the pure metals. This is due to an elastic interaction between the defects of the crystal, called dislocations, and the solute atoms, which form what are referred to as Cottrell clouds around them. In such clouds, the concentration of solute atoms is higher than the mean concentration in the material. In a paper published recently, the authors have now theoretically calculated the static and dynamical properties of the Cottrell clouds, which form around edge dislocations in lamellar liquid crystals of the smectic A variety decorated with nanoparticles. In this study, they demonstrate a formula previously used to approximate the mobility of dislocations in the presence of Cottrell clouds. They then perform a numerical simulation of the problem to study how the Cottrell cloud erodes when the dislocation moves at high speed. This work could be important, for example, in the context of improving the lubricating performance of such liquid crystals.

P. Oswald and L. Lejček, Drag of a Cottrell atmosphere by an edge dislocation in a smectic-A liquid crystal, Eur. Phys. J. E 40, 84 (2017)
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Information stored in quantum states of water fragments (Vol. 48, No. 5-6)

Typical example of a Boltzmann plot. The magenta curved line is a two temperature fit, where the blue and red straight lines are its components.

Does water have memory? Well, not in the usual sense. But it is known, that if you tear water molecules apart, the remaining fragments can tell you a story about how it happened. To investigate this phenomenon, a plasma reactor producing miniature lightnings in direct contact with water level was constructed. The electrical discharges are powerful enough to cause dissociation of water molecules in various ways. To facilitate the electrical breakdown, the atmosphere in the reactor was replaced by argon.

The water molecule can be broken by impact of sufficiently fast electron, absorption of deep UV photon or previously excited argon atom. Each of these processes has a different energy balance and the remaining energy is partially conserved in quantum states of the water fragments. By careful analysis of the light emitted by the relaxing OH radicals, we can disentangle the respective contributions to the total spectrum and calculate the portion of water molecules undergoing various dissociation mechanisms.

The water fragments really remember what preceded their creation and they let us know by emitting photons. The time scale for "forgetting" depends on the collisional rate, i.e. the pressure. At atmospheric pressure, the information can be kept for several nanoseconds.

J. Voráč, P. Synek, V. Procházka and T. Hoder, State-by-state emission spectra fitting for non-equilibrium plasmas: OH spectra of surface barrier discharge at argon/water interface, J. Phys. D: Appl. Phys. 50, 294002 (2017)
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Astronauts to bring asteroid back into lunar orbit (Vol. 48, No. 5-6)

How to bring an asteroid back nearer to lunar orbit

Italian Space Agency presents plans to develop a robotic solar-powered spacecraft capable of displacing a near-Earth asteroid towards lunar orbit for ease of study

Future space exploration aims to fly further from Earth than ever before. Now, Italian Space Agency scientists have expressed an interest in contributing to the development of robotic technologies to bring an asteroid from beyond lunar orbit back into closer reach in order to better study it. In a paper published recently, the authors make the case for taking part in the robotic phase of the Asteroid Redirect Mission (ARM). In addition to taking manned spaceflights deeper into space than ever before, the proposed mission would also bring some benefit for planetary science. Further, the mission has potential implications for a field called planetary defence. The next step for human space exploration after the International Space Station is to send astronauts on a Near Earth Asteroid by 2025, as planned by NASA. This constitutes an intermediate step towards future manned missions to Mars. The planned ARM mission has been part of the NASA program since 2013.The robotic spacecraft would cruise in deep space towards a near-Earth asteroid, using a technology called advanced Solar Electric Propulsion. Under the proposed plan, Italy would contribute by enhancing the carrying capacity of that spacecraft.

M. Tantardini and E. Flamini, Synergies between human space exploration and science in the asteroid redirect mission and the potential Italian participation in the asteroid redirect robotic mission phase, Eur. Phys. J. Plus 132, 314 (2017)
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Monodisperse magnetic nanoparticles prepared from block copolymer template (Vol. 48, No. 5-6)

SEM images of superparamagnetic nanoparticles made from self-assembled block copolymer template (a) on Si substrate, (b) after their release in solution

Magnetic nanoparticles are playing an increasing role in biomedical applications, both for diagnosis (e.g. contrast agent in MRI (Magnetic resonance imaging) or for MPI (magnetic particles imaging)) and for therapy thanks to their ability to exert forces and torques on biological species allowing for instance cancer cells destruction or oriented growth of biological tissue.

In order to fabricate magnetic nanoparticles with high monodispersity, required in particular in biomedical imaging, we have developed a new preparation method based on the use of self-assembled block copolymer template.

Such techniques have already been explored for the preparation of patterned media for ultra-high density magnetic recording. However, our requirements substantially differ from those for storage media. A sacrificial layer has to be introduced between the substrate and the diblock copolymer to allow the release of the nanoparticles in solution. For that purpose, an optimized germanium oxide layer was used. The obtained superparamagnetic particles do not agglomerate in solution. They can be made of biocompatible material (magnetite) and exhibit very narrow size dispersion (≈7%). They can be good contrast agents for medical imaging.

M. Morcrette, G. Ortiz, S. Tallegas, H. Joisten, R. Tiron, T. Baron, Y. Hou, S. Lequien, A. Bsiesy and B. Dieny, Fabrication of monodisperse magnetic nanoparticles released in solution using a block copolymer template, J. Phys. D: Appl. Phys. 50, 295001 (2017)
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First aid kit in some living organisms helps fix DNA after lengthy sun exposure (Vol. 48, No. 5-6)

Important photolyase residues for DNA binding.”

New study unveils the binding mechanisms of enzymes capable of repairing DNA damaged by UV light before any risk of cellular malfunction sets in

Sunburn in living organisms is caused by ultraviolet (UV) light from the sun damaging the DNA in the cells. Many organisms, however, have an in-built mechanism for repairing the sun damage. This is possible thanks to an enzyme called DNA photolyase, which is so specialised that cryptochrome, a structurally similar molecule, is unable to do the same job. By comparing both types of molecule, physicists can understand precisely how the ability of our enzymes to repair DNA boils down to the most minute structural details. In a study published recently, the authors pinpoint the mechanism by which repair enzymes bind to the damaged site.

K. Aalbæk Jepsen and I. A. Solov'yov, On binding specificity of (6-4) photolyase to a T(6-4)T DNA photoproduct, Eur. Phys. J. D 71, 155 (2017)
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