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Nanoscale light trapping in graphene/h-BN by sound waves (Vol. 49 No.4)

SAW-driven generation of polaritons in graphene/h-BN on a piezoelectric; Insets: Electric field profile for surface and hyperbolic plasmon-phonon polaritons (SPPP and HPPP)

Graphene plasmonics is a rapidly emerging field exploiting the extreme light confinement provided by the 2D nature of graphene that results in enhanced light-matter interaction. However, a large momentum mismatch needs to be overcome by a photon to couple with a plasmon into a plasmon polariton. In this work, the authors have demonstrated a unique way for exciting polaritons in graphene/h-BN systems on a piezoelectric material by using a surface acoustic wave (SAW) generated by an integrated transducer. The SAW acts as a virtual diffraction grating providing the extra momentum needed to generate propagating polaritons using a simple laser. h-BN is widely used as an ideal substrate for graphene providing it with very high mobility. Moreover, h-BN is a natural hyperbolic insulator (i.e. the dielectric function has opposite sign in the two reststrahlen bands), where graphene plasmons couple strongly to both surface and hyperbolic (waveguided) phonons in h-BN, leading to hybridized surface and hyperbolic plasmon-phonon polaritons. These results pave the way for engineering SAW-driven graphene/h-BN plasmonic devices and metamaterials covering the mid-IR to THz range.

R. Fandan, J. Pedrós, J. Schiefele, A. Boscá, J. Martínez and F. Calle, Acoustically-driven surface and hyperbolic plasmon-phonon polaritons in graphene/h-BN heterostructures on piezoelectric substrates, J. Phys. D: Appl. Phys. 51, 204004 (2018)
[Abstract]

Nanosecond high-voltage pulses for air purification (Vol. 49, No. 2)

NO removal (and by-product formation) with fast high voltage pulses

Transient plasmas generated by high-voltage pulses have been widely studied and used for industrial and environmental applications for more than 100 years. The highly reactive species that are generated in these plasmas can react with particles in polluted gas and water streams. We focus on plasma for environmental applications and developed a new, very fast high-voltage pulse source for this purpose (0.5-10 ns pulse duration, 200 ps rise time and 50 kV amplitude). We showed that with this pulse source, we can achieve extremely high energy yields in ozone generation (typically used for water decontamination) and nitrogen oxide (NO) removal (a typical exhaust gas for diesel engines). Interestingly, the pulse duration, a figure of merit that has long been claimed as the key success in high-yield plasma processing (the shorter the better), had no significant influence on our yields. It appears that the key to these high yields is the very fast rise time of our high-voltage pulses. They are so fast that the complete electric field is applied to the gas while the plasma is still developing, which results in higher electron densities and ultimately in more reactive species.

T. Huiskamp, W. F. L. M. Hoeben, F. J. C. M. Beckers, E. J. M. van Heesch and A. J. M. Pemen , (Sub)nanosecond transient plasma for atmospheric plasma processing experiments: application to ozone generation and NO removal, J. Phys. D: Appl. Phys. 50, 405201 (2017)
[Abstract]

Negative refraction without negative-index materials (Vol. 47 No. 3)

Simulation of the electric field of a beam incident on a transformed lens (left) and object (right).

Transformation optics enables engineered metamaterials to manipulate spatial transformations, which can redirect the propagation paths of electromagnetic waves. Such a technology opens up a novel approach to control electromagnetic fields, and hence provides many possibilities to explore new physical phenomena and develop new devices. It can be employed to design unconventional and versatile devices based on metamaterials, such as free-space invisibility cloaks.

The authors have demonstrated a transformation-optics device to generate negative refraction and negative reflection without negative-index materials. In their contribution, they presented two kinds of transformation lenses with anisotropic material parameters: one inhomogeneous lens and one homogeneous lens. Their results verified the unusual properties of the transformation optical devices, and good wave-controlling performance was demonstrated.

W. X. Jiang, D. Bao and T. J. Cui, Designing novel anisotropic lenses with transformation optics, J. Opt. 18, 044022 (2016)
[Abstract]

Nernst effect and diamagnetic response in superconducting cuprates (Vol. 41, No. 6)

One of the central puzzles of the high-temperature cuprate superconductors is the nature of the pseudo-gap regime that extends from the parent Mott insulator to at least optimal doping and up to several hundred degrees Kelvin. Though it is unlikely that the entire pseudo-gap region is directly related to superconductivity, there is compelling experimental evidence of anomalously strong superconducting fluctuations within the pseudo-gap.

Two phenomena have particularly high sensitivities to the presence of superconductivity, even local or fluctuating one: the diamagnetism and the Nernst effect. The diamagnetic part of field-induced magnetization can reveal the presence of even isolated superconductivity inclusions, as they expel magnetic field. The Nernst effect is a thermal analogue of the conventional Hall effect – the transverse voltage generation in response to the heat current flow in a magnetic field. The Nernst effect in metals is typically very weak; however, in the presence of superconductivity it is greatly enhanced due to the thermal drift of vortices, which induces voltage via the Josephson relationship.

We examine the possibility that the experimentally observed enhancement of superconducting fluctuations above the superconducting transition temperature in the underdoped cuprates is caused by stripes -- an intrinsic electronic inhomogeneity, common to hole-doped cuprates. By evaluating the strengths of the diamagnetic response and the Nernst effect within a striped SC model, we find results that are qualitatively consistent with the experimental observations. We make a prediction for anisotropic thermopower in detwinned samples that can be used to further test the proposed scenario.

Nernst effect and diamagnetic response in a stripe model of superconducting cuprates
I. Martin and C. Panagopoulos, EPL, 91, 67001 (2010)
[Abstract]

Neural networks beyond the mean-field paradigm (Vol. 46 No. 2)

Schematic of a hierarchical neural network (center): four stored patterns (top), with two retrieved (bottom).

The seminal paper "Neural Networks and physical systems with emergent collective computational abilities" by Hopfield (1982) and its statistical mechanical treatment by Amit, Gutfreund and Sompolinsky (1985) still play as "harmonic oscillators" in Artificial Intelligence: crucially, in their picture, "associative memory" emerges as a collective feature of neurons. Due to mathematical constraints, this paradigmatic formalisation relies on the so-called "mean-field" approximation: each neuron interacts with all others in the network, regardless of their reciprocal distance. As a (non-obvious) consequence, the network performs “serial processing'': it is able to retrieve one pattern of information per time. Here we show a way, based on a hierarchical underlying topology (see figure), to overcome mean-field limitations, thus accounting for neuronal distance in the network (this also allows for dilution as neurons too far away do not interact). Remarkably, simply introducing a metric (that is a biological must) enables the network to spontaneously switch from serial processing to parallel processing: it can retrieve several patterns of information simultaneously. These emergent multitasking features characterize a novel generation of neural networks, which better capture real brain behavior.

E. Agliari, A. Barra, A. Galluzzi, F. Guerra, D. Tantari and F. Tavani, “Metastable states in the hierarchical Dyson model drive parallel processing in the hierarchical Hopfield network”, J. Phys. A: Math. Theor. 48, 015001 (2015)
[Abstract]

Neutron-induced capture and fission reactions together (Vol. 43 No. 3)

image Minor actinide part of the table of isotopes indicating for which isotopes n_TOF has carried out measurements (TOF-Ph1) and for which measurements are intended (n TOF-Ph2), differentiating between fissile and non-fissile isotopes.

The accurate knowledge of (n,gamma) neutron-capture cross-sections for fissile isotopes is highly relevant for next-generation applications of nuclear technology. However, accurate measurements are difficult due to the gamma-ray background generated in competing (n,f) fission reactions. Scientists at the CERN n_TOF facility have developed a new experimental setup that is capable of simultaneously measuring and identifying the capture and fission reactions.

The setup combines the existing 4pi BaF2 Total Absorption Calorimeter (TAC) with a set of three MicroMegas detectors (MGAS). A successful test experiment was performed using moderated spallation neutrons in an energy region of 6-22 eV with a 235U sample.

Neutron captures were measured in the TAC while fission reactions were simultaneously detected in the TAC and MGAS. Coincident events recorded in the TAC and MGAS were selected and the specific TAC response to capture and fission events was exploited to select the two components by imposing conditions on sum energy and event multiplicity to disentangle them.

It was important to precisely determine the different detector efficiencies for both reaction types, which in turn allowed to derive capture and fission cross-sections, as well as their ratios. The comparison of the experimentally determined values with evaluated cross-sections showed good agreement between measurement and evaluation for both types of reaction.

Thus the experimental method was validated for an unambiguous extraction of cross-sections from a simultaneous measurement of the capture and fission reaction.

With the successful commissioning of the combined experimental setup, the n_TOF collaboration has now developed a tool that will enable them to target the accurate measurement of neutron-induced capture cross-sections of fissile isotopes.

This new method brings interesting perspective for the necessary improvement of nuclear data for next-generation nuclear technology applications. The figure shows the neutron capture measurements of actinides performed at n_TOF (233,234U, 237Np, 240Pu and 243Am) and those intended for future experiments (236,238U and 241Am)

Simultaneous measurement of neutron-induced capture and fission reactions at CERN
C. Guerrero et al. (85 co-authors), Eur. Phys. J. A, 48, 29 (2012)
[Abstract]

Neutrons escaping to a parallel world? (Vol. 43 No. 5)

Anomalous loss of neutrons observed experimentally.

An anomaly in the behaviour of ordinary particles may point to the existence of mirror particles that could be candidates for dark matter responsible for the missing mass of the universe.

In the present paper, the authors hypothesised the existence of mirror particles to explain the anomalous loss of neutrons observed experimentally. The existence of such mirror matter had been suggested in various scientific contexts some time ago, including in the search for dark matter.

The authors re-analysed the experimental data obtained at the Institut Laue-Langevin, France. It showed that the loss rate of very slow free neutrons appeared to depend on the direction and strength of the magnetic field applied.

The authors believe it could be interpreted in the light of a hypothetical parallel world consisting of mirror particles. Each neutron would have the ability to transition into its invisible mirror twin, and back, oscillating from one world to the other. The probability of such a transition happening was predicted to be sensitive to the presence of magnetic fields, and could be detected experimentally.

This oscillation could occur within a timescale of a few seconds, according to the paper. The possibility of such a fast disappearance of neutrons—much faster than the ten minute long neutron decay—is subject to the condition that the earth possesses a mirror magnetic field that could be induced by mirror particles floating around in the galaxy as dark matter. Hypothetically, the earth could capture the mirror matter via some feeble interactions between ordinary particles and those from parallel worlds.

Z. Berezhiani and F. Nesti, ‘Magnetic anomaly in UCN trapping: signal for neutron oscillations to parallel world?’, Eur. Phys. J. C (2012) 72, 1974
[Abstract]

New complex may offers safer alternative for gene therapy (Vol. 42, No. 6)

image Self-organized DNA fragments in a non-cationic Lα lipid phase

The authors have created a complex system designed to hold DNA fragments in solution between the hydrophilic layers of a matrix of fatty substances (also known as lipids) combined with a surfactant (used to soften the layers' rigidity). One possible application that has yet to be tested is gene therapy.

Although gene therapy was initially delivered using viral vectors, recent attempts at devising alternative vectors have exploited positively charged lipids to form complex structures holding DNA fragments with electrostatic forces. However the positively charged ions (cations) used in this type of vector have proven toxic for human cells.

Until now, only positively charged fatty substance were thought capable of holding DNA in a complex vector. The authors of this study have proved otherwise by creating an electrically neutral matrix, structured like a multi-layered cake, which holds the DNA fragments at a high concentration in solution between the layers.

It appears that DNA fragments within the complex self-organise over time. These fragments spontaneously align parallel to one another and form rectangular and hexagonal structures across the layers. This ordering in a special matrix holds the key to creating non-toxic gene therapy delivery vectors. The change of atomic-level interactions within the layers and the appearance of interactions at the interface between the layers and the DNA molecules may explain the emergence of ordered structures at high DNA concentrations.

The next step of this research involves elucidating the precise physical forces that hold the complex together. Applications of such technology go beyond gene therapy vector design, as the same principle can be applied for the delivery of other particles such as chemical drugs.

Supramolecular polymorphism of DNA in non-cationic Lα lipid phases
E.R. Teixeira da Silva, E. Andreoli de Oliveira, A. Février, F. Nallet and L. Navailles, Eur. Phys. J. E, 34, 83 (2011)
[Abstract]

New experimental approach for near–edge ultrafast EUV absorption spectroscopy (Vol. 46 No. 2)

Ultrafast absorption spectroscopy at the L(2,3)-edge of silicon

Time resolved pump-probe spectroscopy in the extreme ultraviolet (EUV) spectral range offers the opportunity to study properties and structure of the electron subsystem in condensed materials under non-equilibrium conditions rapidly changing in the sub-ps time scale. New frontiers studies can be accomplished thanks to the availability of new generation sources such as free electron lasers (FEL). The breakthrough research interest in the EUV radiation–matter interactions requires the development of pivotal optical elements able to manipulate short wavelength beams. Conventional single layers coated mirrors provide negligible reflectance in the extreme ultraviolet spectral range, therefore knowledge coming from other disciplines is required to overcome such technological limits. The development of the multilayer coated mirrors has been intensively driven by the microelectronics industry in view of their application in EUV lithographic apparatus. The same technology has been used to develop a novel broadband multilayer coated mirror conceived specifically for near-edge ultrafast absorption spectroscopy. Such an optical element has been inserted in the EIS-TIMEX end-station at FERMI@ELETTRA FEL. The design of the device has been optimised in order to manipulate the FEL pulses preserving their temporal and wavefront properties in the wavelength range required by the ultrafast absorption spectroscopy at L(2,3)-edge of silicon (see figure).

A. J. Corso, P. Zuppella, E. Principi, E. Giangrisostomi, F. Bencivenga, A. Gessini, S. Zuccon, C. Masciovecchio, A. Giglia, S. Nannarone and M. G. Pelizzo, "Broadband multilayer optics for ultrafast EUV absorption spectroscopy with free electron laser radiation", J. Opt. 17, 025505 (2015)
[Abstract]

New insights into the early stages of creep deformation (Vol. 50, No. 5-6)

New insights into the early stages of creep deformation
Varying strain patterns during creep deformation.

Computer simulations show that the evolution of material structures during creep deformation can modify material properties.

The properties of many materials can change permanently when they are pushed beyond their limits. When a given material is subjected to a force, or ‘load’, which is stronger than a certain limit, it can become so deformed that it won’t return to its original shape, even after the load is removed. However, heavy loads aren’t strictly necessary to deform materials irreversibly; this can also occur if they are subjected to lighter loads over long periods of time, allowing a slow process called ‘creep’ to take place. Physicists have understood for some time that this behaviour involves sequences of small, sudden deformations, but until now, they have lacked a full understanding of how creep deformation affects material properties over time. In new research published recently, the authors analysed the characteristic ways in which material structures evolve during the early stages of creep deformation.

D.Fernandez Castellanos, and M. Zaiser, Statistical dynamics of early creep stages in disordered materials, Eur. Phys. J. B 92, 139 (2019)
[Article]

New method helps stabilise materials with elusive magnetism (Vol. 47 No. 5-6)

Visualisation of itinerant ferromagnetic domains

Stabilising materials with transient magnetic characteristics makes it easier to study them

Magnetic materials displaying what is referred to as itinerant ferromagnetism are in an elusive physical state that is not yet fully understood. They behave like a magnets under very specific conditions, such as at ultracold temperatures near absolute zero. Realising the itinerant ferromagnetic state experimentally using ultracold gas is a challenging undertaking. This is because when three atoms - one with the opposite spin of the other two - come close to each other two atoms with opposite spin will form molecules and the other one carries the binding energy away; a phenomenon called rapid three-body recombination. Now, the authors, have introduced two new theoretical approaches to stabilise the ferromagnetic state in quantum gases to help study the characteristics of itinerant ferromagnetic materials. The first approach involves imposing a moderate optical lattice. There, the three-body recombination is small enough to permit experimental detection of the phase. In a second approach, they suggest to prepare two initially separated clouds and study their time evolution. The ferromagnetic domains has longer life time because of the reduced overlap region between the two spins. These results were recently published.

I. Zintchenko, L. Wang and M. Troyer,, Ferromagnetism of the repulsive atomic Fermi gas: three-body recombination and domain formation, Eur. Phys. J. B 89, 180 (2016)
[Abstract]

New model for epidemic contagion (Vol. 43 No. 3)

image Star-shaped network representing human mobility

Improved estimates on the geographical spread of infectious diseases are achieved by studying human mobility networks. Given that humans are considered as the hosts spreading the epidemics, the speed at which an epidemic spreads is now better understood thanks to a new model accounting for the provincial nature of human mobility.

The authors modelled human mobility by accounting for the recurrent bi-directional travels of individuals around a central node representing their home location and forming a star-shaped network. Previous models are based on diffusion and would imply that people travel randomly in space, not necessarily returning to their home location again and again.

The researchers found that older diffusion-based models overestimated the speed at which epidemics spread. The speed of epidemics spreading through bi-directional travel, which is dependent on the travel rate, is significantly lower than the speed of epidemics spreading by diffusion. In addition, the authors discovered that it is the time an individual spend outside their home location and not, as diffusion models suggests, the rate of travel between locations, which influences the speed of epidemics spreading and whether an outbreak goes global. This model has yet to be tested against real data on human mobility before it can be used as a risk analysis and decision making tool for epidemics or in population dynamics and evolutionary biology.

Recurrent hostmobility in spatial epidemics: beyond reaction-diffusion
V. Belik, T. Geisel and D. Brockmann, Eur. Phys. J. B 84, 579 (2011)
[Abstract]

New neuron dynamics model better fitted to the biological reality (Vol. 48 No. 1)

The phase trajectories of the solutions for the simplified biologically relevant HH model and the scaled theoretical FHN model for neuron dynamics (black curve).

Scientists have now adopted a qualitative theoretical neuroscience model commensurate with actual measurements of neurons' dynamics

Neuroscientists are currently working diligently to understand the dynamics of thousands of coupled neurons. Understanding how they operate requires accurate models. The trouble is that each of the existing neuroscience models has its own shortcomings. The authors have, for the first time, developed an effective method for solving the equations of a well-known theoretical neuroscience dynamic model and make it more biologically relevant. These findings have just been published. They could not only help resolve problems in the neurosciences, but could also provide a deeper understanding of neuronal activity in the emerging sector of neurovascular dynamics, which describes the interplay between the brain's neurons and the blood flow.

E.B. Postnikov and O.V. Titkova, A correspondence between the models of Hodgkin-Huxley and FitzHugh-Nagumo revisited, Eur. Phys. J. Plus 131, 411 (2016
[Abstract]

New precision coating method for industrial granular material (Vol. 48 No. 1)

Ignited plasma – starfish shaped racetracks

Deposition of a thin film catalyst of a predicted thickness on the surface of novel hydrogen storage microbeads helps release hydrogen.

As anyone who eats their cereal with milk in the morning knows: coating large volumes of granular material homogeneously is no mean feat. In a recent paper published by the authors, they have developed a new method, based on physical vapour deposition, to upscale the quantity of coating without affecting the quality and homogeneity of the film. In this study, they also developed a model capable of predicting the film thickness. This represents a major step forward for industrial materials, as previous approaches relied on optical measurement after the coating had been deposited. Because this coating system is capable of implementing a plasma close to the granular substrate, it opens the door to new surface treatment and modification possibilities. Applications are expected for the many granular materials used in industry, including, for example, a novel hydrogen-storage system, which stores hydrogen in hollow glass spheres. Hydrogen stored in microbeads can be released by applying heat to the spheres. The new method helps meet the challenge of applying heat to the beads, thanks to a chemical reaction triggered by a catalyst, which is applied to the sphere's surface.

A. Eder, G.H.S. Schmid, H. Mahr and C. Eisenmenger-Sittner, Aspects of thin film deposition on granulates by physical vapor deposition, Eur. Phys. J. D 70, 247 (2016)
[Abstract]

New university ranking system includes the cultural perspective (Vol. 50, No. 2)

Network of friends of top 20 PageRank universities from the French Wikipedia edition.

A new study proposes a new way of ranking universities, using a more balanced cultural view and based on 24 international editions of Wikipedia

Scientists in France have developed a new way of generating a ranking of the world’s universities that places more emphasis on the cultural perspective. In a study published recently, the authors perform an analysis of Wikipedia editions in 24 languages, collected in May 2017—previous studies pursuing a similar approach focused on data from 2013. Employing well-known ranking algorithms, they establish a Wikipedia Ranking of World Universities based on the relative cultural views of each of the 24 language-specific Wikipedia editions. Thus, they provide a more balanced view that reflects the standpoints of different cultures. Specifically, the authors use (for the first time for this purpose) a new tool for the analysis of online networks, which is based on the PageRank algorithm and known as reduced Google Matrix analysis. In this study, they determine the interactions between leading universities on a scale of ten centuries, which provides insights into the relative influence of specific universities in each country.

C.Coquidé, J.Lages, and D. L. Shepelyansky, World influence and interactions of universities from Wikipedia networks, Eur. Phys. J. B 92, 3(2019)
[Abstract]

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