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May the 5th force be with you (Vol. 47 No. 1)

The author revisits the wealth of research emerging from the quest for the fifth force, which he hypothesised in the 1980s as being a new fundamental force in nature.

Discovering possible new forces in nature is no mean task. The discovery of gravity linked to Newton’s arguably apocryphal apple experiment has remained anchored in popular culture. In January 1986, the author had his own chance to leave his mark on collective memory. His work made the front page of the New York Times after he and his co-authors published a study uncovering the tantalising possibility of the existence of a fifth force in the universe. In an article published recently, he gives a personal account of how the existence of the gravity-style fifth force has stimulated an unprecedented amount of research in gravitational physics - even though its existence, as initially formulated, has not been confirmed by experiment.

E. Fischbach, The fifth force: A personal history, Eur. Phys. J. H 40, 385 (2015)
[Abstract]

Mean-field theory and stochastic evolution (Vol. 42, No. 2)

image Opposing pairs (A-C, B-D) of equal strength evolve deterministically on a closed orbit (blue loop), oscillating endlessly (upper left). An example of stochastic trajectories is shown in red (dashed line). Lower panels display systems with identical initial conditions but different strengths, ending at opposite edges.

Population dynamics is a venerable and widely applicable subject. Over the last two centuries, many studies provided valuable insights into various phenomena, e.g., the emergence of biodiversity and fitness/extinction, while novelties are continually being discovered. Specifically, recent investigations of three species competing cyclically (e.g., rock-paper-scissors game) revealed rich and complex behaviours, whether the populations are well-mixed or dwelling on one- or two-dimensional lattices. Indeed, the well-mixed system displayed surprising survival probabilities: The species with the slowest consumption rate wins, leading to a popularized headline "Survival of the Weakest." Fascinating properties were also found in systems with spatial structure, including formation of complex patterns and mobility effects. Many aspects can be understood by exploiting techniques from statistical physics and non-linear dynamics.

Our work focuses on four cyclically competing species. Unlike the 3-species case, ours allows final states with coexisting pairs. The reason is simple: Resembling the game of Bridge, the four form two opposing teams of ally-pairs. For each pair, the product of their consumption rates determines if it wins or loses. From a master equation for the full stochastic problem, we derive an approximate set of rate equations (ODE's). Predictions from the latter typically agree well with simulations. Instead of the weakest surviving, our observations support a different maxim: "The prey of the prey of the weakest is the least likely to survive." Intuitively reasonable, this principle also applies to the special 3-species case! Meanwhile, a variety of intriguing extinction probabilities, discovered through simulations, provides numerous challenges for future research.

Cyclic competition of four species: Mean-field theory and stochastic evolution
S. O. Case, C. H. Durney, M. Pleimling and R. K. P. Zia, EPL, 92, 58003 (2010)
[Abstract] | [PDF]

Measuring Higgs couplings at a linear collider (Vol. 44 No. 3)

Expected precision for Higgs coupling measurements for the HL-LHC, ILC and the combination of the HL-LHC and the ILC.

In 2012 the experiments ATLAS and CMS discovered a new particle in proton-proton collisions at CERN's Large Hadron Collider (LHC). The measurements show that the properties of the particle are compatible with those predicted for the Higgs boson of the Standard Model. In the article we estimate the precision with which some of the fundamental properties of the particle, its couplings to other particles, can be measured including theoretical errors, at a high-luminosity LHC (HL-LHC), a linear electron-positron collider and the combination of the two. The uncertainties are expected to be better than 1% for a single parameter modifying all Higgs couplings simultaneously, and at the percent level if all relevant couplings are left free and independent of each other. The combination of the measurements at the two machines improves on the uncertainty of each one of these. Thus a HL-LHC and a linear collider form a dream team to study the properties of the Higgs boson with high precision.

M. Klute, R. Lafaye, T. Plehn, M. Rauch and D. Zerwas, ‘Measuring Higgs couplings at a linear collider’, EPL, 101, 51001 (2013)
[Abstract]

Mechanisms of two-proton emission seen in three-body correlations (Vol. 46 No. 2)

Sequential two-proton decay of the 16Ne Er=7.57 MeV state. The fractional energy distribution (left) gives resonance energy in 15F while the angular distribution (right) determines Iπ of the initial state.

Hitherto three-body correlations between decay products of nuclear resonances, unstable to the emission of two neutrons have been a very effective tool in the analysis of GSI-experiments on 5H, 10He, 13Li, and 14Be. Here the first report is given about the mechanisms for two-proton emission from states in 16Ne, representing the presently most complete study of this nucleus. One-neutron knockout from 17Ne populated the 16Ne(g.s.) (Er=1.39 MeV, Γ=0.08 MeV) above the 14O+p+p threshold, and resonances at Er=3.22 MeV and 7.57 MeV. The decay mechanisms were revealed analysing three-body energy correlations in the 14O+p+p system. It was found that the 16Ne(g.s.) undergoes a democratic three-body decay. In contrast to this, the 16Ne(21+) state emits protons through the 15F(g.s.) sequentially. The decay of 7.57 MeV state is well-described assuming emission of a proton from the d5/2 shell to 15F(5/2+), which decays by d5/2 proton emission to 14O(g.s.). By using R-matrix analysis and mirror symmetry this state was unambiguously identified as the third 2+ state in 16Ne.

J. Marganiec et al. (+ 58 co-authors), “Studies of continuum states in 16Ne using three-body correlation techniques”, Eur. Phys. J. A 51, 9 (2015)
[Abstract]

Metal contacts in terahertz quantum cascade lasers (Vol. 42, No. 6)

image (a) Threshold current densities of four THz quantum cascade lasers with different metal contacts. (b) Current-voltage curves of the devices at 10 K.

Schottky contacts should be avoided in electrically-pumped semiconductor laser devices because they cause an extra voltage drop at metal-semiconductor interfaces - wasting power, overheating device active region and degrading performance. Metal stacks of Ni/Ge/Au and Ti/Pt/Au are commonly employed to form ohmic contacts with n-type and p-type III-V semiconductors, respectively. The optical loss of these ohmic metal contacts is negligible in the visible-light/near-infrared range (~hundreds of THz) due to their much lower Plasmon frequencies (1-10 THz). The optical properties of these metals are therefore not a concern in conventional semiconductor diode lasers.

This drastically changes if the lasing frequency approaches the terahertz range (1012 Hz), i.e., THz quantum cascade lasers (QCLs) that are based on GaAs/AlGaAs multiple quantum-well structures. The high tangent loss of the commonly-used metals in this frequency range could become a substantial part of the total waveguide loss of the lasers, however metal stacks (such as Ti/Au and Ta/Cu/Au) that exhibit low optical loss in the terahertz frequency range from non-ohmic contacts with III-V semiconductors. Researchers often face a dilemma when picking the metals - ohmic or non-ohmic contacts?

We experimentally investigated the electrical and optical behaviours of THz QCLs with four different Au- and Cu-based metal contacts. The QCL device with non-alloyed Ta/Cu/Au exhibits the lowest threshold current density and the highest lasing temperature in pulsed mode. The better performance is attributed to the lower optical loss of the device waveguide in spite of the formation of a Schottky contact. The findings clarify an important issue that will help researchers design and fabricate THz QCLs operating at higher temperatures and eventually at room temperature.

On metal contacts of terahertz quantum cascade lasers with a metal-metal waveguide
S. Fathololoumi, E. Dupont, S. G. Razavipour, S. R. Laframboise, G. Parent, Z. Wasilewski, H. C. Liu and D. Ban, Semicond. Sci. Technol. 26, 105021 (2011) [Abstract]

Metallic nanoparticles for plasmonic absorption enhancement (Vol. 45 No. 1)

Expected optical generation rate for a solar cell with integrated random nanoparticles assemblies as visible from the overlapped topography.

Random arrangements of nanoparticles are easy to fabricate and therefore find widespread application. But how do these random assemblies influence the local optical generation rate? The authors investigated Ag nanoparticle assemblies with scanning near-field optical microscopy (SNOM) detecting the optical response to local near-field excitation through an aperture tip while simultaneously recording the topography with atomic force microscopy. 3D simulations in finite-difference time domain confirmed that areas in between irregularly arranged nanoparticles show the strongest response and that no direct correlation of hot spots to particularly sized nanoparticles is possible. An overall highly non-uniform distribution of the electric field is found around the nanoparticles for various wavelengths both in experiment and theory. These variations in local electric field are expected to translate directly to the optical generation rate, which therefore will equally suffer from inhomogeneities (see figure), and will thus crucially determine the effectiveness of plasmonic absorption enhancement. Therefore mapping the local field distributions as possible with SNOM is expected to be highly advisory to optimize nanoparticle geometries.

M. Schmid, J. Grandidier and H. A. Atwater, “Scanning near-field optical microscopy on dense random assemblies of metal nanoparticles“, J. Opt., 15, 125001 (2013)
[Abstract]

Metering the plasma dosage into the physiological environment (Vol. 47 No. 5-6)

Plasma therapy

There is significant optimism that cold atmospheric (ionised gas) plasma could play a role in the treatment of life-threatening diseases, such as non-healing chronic wounds and cancers. The medical benefits from plasma are thought to arise from the reactive oxygen and nitrogen species (RONS) generated by plasma upon interaction with air and liquids. However, it is unclear what RONS are delivered by plasma into tissue fluid and tissue, and their rate of delivery. This knowledge is needed to develop safe and effective plasma therapies.

In this investigation, a simple approach was proposed to monitor the dynamic changes in the concentrations of RONS and dissolved oxygen within tissue-like fluid and tissue during plasma treatment. A plasma “jet” device was shown to non-invasively transport RONS and oxygen deep within tissue (to millimetre depths). However, tissue fluid directly treated with the plasma jet was deoxygenated due to the gas flow purging oxygen out of the fluid.

Monitoring and controlling the plasma delivery of both RONS and oxygen into tissue fluid and tissue is necessary to avoid hypoxia in open wound treatment, to achieve targeted destruction of cancerous cells within solid tumours and to oxygenate oxygen-starved tissue to stimulate tissue regeneration.

J.-S. Oh, E. J. Szili, N. Gaur, S.-H. Hong, H. Furuta, H. Kurita, A. Mizuno, A. Hatta and R. D. Short, How to assess the plasma delivery of RONS into tissue fluid and tissue, J. Phys. D 49, 304005 (2016)
[Abstract]

Micro-pattern formation of extracellular-matrix (ECM) layers (Vol. 43 No. 3)

image Micrograph of HEK 293 cell arrangement over patterned ECM strips on a Si substrate. The ECM shown here is Poly-L-Lysine and was patterned by application of low-temperature APPs through thin slits of a metal stencil mask placed firmly on the substrate. After plasma application and removal of the mask, HEK 293 cells were cultured on the substrate. The cells adhered to and proliferated on the remaining ECM strips. The white dashed lines here delineate the mask slit patterns.

Cell-based biochips/biosensors may advance various scientific and technological fields. For example, a neuronal network chip that simulates how our brains function, may be used to detect the cause of brain diseases such as Alzheimer's. Since living cells typically do not survive in direct contact with semiconductor surfaces, one of the major challenges for the development of cell-based biochips/biosensors is the formation of bio-interfaces that maintain living cells in an environment of microelectronics. Especially desirable is inexpensive technologies for attachment and arrangement of living cells on large areas of substrate surfaces according to one's design. In this study, we have developed a new and simple micro-patterning technique for extracellular matrices (ECMs) deposited on Si substrates by low-temperature atmospheric-pressure plasmas (APPs) and a metal stencil mask. Low-temperature APPs are suitable for such patterning because of their ability to produce highly reactive species efficiently without causing thermal damages to ECMs. In this study, it is shown that, with a short-period application of APP, ECM layers of 1 cm2 area deposited on Si substrates can be patterned for lines and intervals whose typical dimensions are in the range of 100 µm.

Micro-pattern formation of extracellular-matrix (ECM) layers by atmospheric-pressure plasmas and cell culture on the patterned ECMs
A. Ando, T. Asano, T. Urisu and S. Hamaguchi, J. Phys. D: Appl. Phys. 44, 482002 (2011)
[Abstract]

Microgels behaviour under scrutiny (Vol. 44 No. 4)

Set of measured velocity profiles with Carbopol microgel

A new study explores the counter-intuitive behaviour of a microgel composed of soft polymer blobs with the capability of behaving like dense emulsions or granular materials The authors have studied the flow of a microgel known for their intermediate behaviour between fluid and solid, confined in microchannels. They have shown, in the present study, that its behaviour under confinement differs from predictions based on standard theories. Indeed, its molecules are not only subjected to local forces, but also to neighbouring forces that affect its flow.

The authors chose to study the influence of confinement on the flow of a polymer microgel named Carbopol. It is made of jammed acrylic acid polymer blobs, typically a few microns in size, dispersed in water. They confirmed, for the first time in a microgel, that the flow properties at a local point do not depend only on the local force, but also on the dynamics of its vicinity. This has previously been shown in concentrated emulsions, granular materials and foams under confinement.

B. Geraud, L. Bocquet and C. Barentin, ‘Confined flows of a polymer microgel’, Eur. Phys. J. E, 36, 30 (2013)
[Abstract]

Microgravity and aqueous wet foams (Vol. 45 No.4)

Foam made in the device elaborated by Astrium for the future foam studies in the ISS.

Foams and foaming processes pose interesting questions for both fundamental research and practical applications. Although foams are a familiar thing, both in our everyday lives and in industry, many aspects of foam physics and chemistry still remain unclear.

This work comprehensively reviews the studies of foams under microgravity, including studies conducted in parabolic flights, in sounding rockets and in the International Space Station.

Experiments on foams performed under microgravity can be extended far beyond the conditions of experiments carried out on Earth. In particular, when gravity is minimized, it is possible to observe the behaviour of wet foams obtained during the foaming process. On Earth, foams at this stage evolve too quickly due to gravity drainage and cannot be studied.

D. Langevin and M. Vignes-Adler, “Microgravity studies of aqueous wet foams”, Eur.Phys. J. E, 37, 16 (2014)
[Abstract]

MINOS: A vertex tracker for in-beam spectroscopy of exotic nuclei (Vol. 45 No.2)

View of the MINOS device inside the DALI2 gamma array at the RIKEN Radioactive Isotope Beam Factory

MINOS is a new apparatus dedicated to in-beam nuclear structure experiments with low-intensity exotic beams at energies above 150 MeV/nucleon. It is intended to provide increased luminosity compared to standard solid-target experiments in hydrogen-induced studies, while simultaneously improving experimental resolution. This work exposes the concept of the device developed at the CEA in France and reviews in detail the associated recent technical advances. MINOS is composed of a thick finger-shaped liquid hydrogen target, from 50 to 200 mm thick, combined with a compact time projection chamber serving as a vertex tracker, the first of its kind in low-energy nuclear physics. This innovative setup offers access to the first spectroscopy of a new range of very exotic nuclei beyond our current reach. An exciting program on the search for new 21+ states in neutron-rich even-even nuclei, spectroscopy of unbound oxygen nuclei and di-neutron correlations in Borromean nuclei will be performed with MINOS at the RIKEN Radioactive Isotope Beam Factory in Japan over the next few years. MINOS is funded by the European Research Council.

A. Obertelli et al., “MINOS: A vertex tracker coupled to a thick liquid-hydrogen target for in-beam spectroscopy of exotic nuclei”, Eur. Phys. J. A, 50, 8 (2014)
[Abstract]

Minutest absolute magnetic field measurement (Vol. 47 No. 1)

Geometry of the experiment.

High-precision and high-accuracy magnetic field measurement to support quest for missing antimatter in the universe.

Every measurement is potentially prone to systematic error. The more sensitive the measurement method, the more important it is to make sure it is also accurate. This is key for example in measuring magnetic fields in state-of-the-art fundamental physics experiments. Now, an international team of physicists has developed an extremely high-precision method for the determination of magnetic fields. The resulting device, they found, has an intrinsic sensitivity that makes it ideal for fundamental physics and cosmology experiments attempting to explain the missing antimatter of the universe. The findings of the authors have been published recently. They calculated the sensitivity of the magnetometer in the envisioned application in an experiment searching for the electric dipole moment of neutrons (nEDM), which are basic constituents of ordinary matter. Observing an nEDM would imply a broken symmetry of the laws of physics, called CP-violation. Such a finding could help to account for the primordial matter-antimatter imbalance at Big Bang stage, leading to the current abundance of matter.

H.-C. Koch, G. Bison, Z. D. Grujić, W. Heil, M. Kasprzak, P. Knowles, A. Kraft, A. Pazgalev, A. Schnabel, J. Voigt and A. Weis, Design and performance of an absolute 3He/Cs magnetometer, Eur. Phys. J. D 69, 202 (2015)
[Abstract]

Mitigating disasters by hunting down Dragon Kings (Vol. 43 No. 4)

image Dragons Kings stem from a combination between the supernatural powers of dragons and the anomaly of a king’s wealth relative to that of his subjects. © Anek Suwannaphoom /photos.com.

Scientists aim at forecasting natural or economic disasters by identifying statistical anomalies in complex systems that deviate from behaviour obeying traditional power laws. This article and the special issue it comes from, present the many facets of Dragon Kings in a review alongside nineteen other contributions exploring to which extent this emerging field of statistical analysis could become further established.

Dragon Kings are events akin to catastrophes. They don’t belong to the same power law regime as the more standard events. For example, they can be found in financial market bubbles ending in crashes and neuron-firing cascades leading to epileptic seizures.

This review focuses on elucidating how Dragon Kings are created and can be detected. It also gives an overview of their empirical evidence in abnormal rainfall, hurricanes, and sudden events such as landslides and snow avalanches. The authors also outline the limitations of this sort of statistical analysis. For example, despite being sometimes interpreted as featuring characteristic events of Dragon Kings, great earthquakes may not be formally confirmed as such.

Finally, the authors share their views on the importance of devising prediction models that could become the basis for Dragon Kings simulators. These could be designed to help interpret the warning signs of complex systems evolving out of their safe equilibrium into extreme events such as the subprime crisis, and to steer them into sustainability and ultimately avoid such crisis.

Dragon-kings: mechanisms, statistical methods and empirical evidence
D. Sornette and G. Ouillon, Eur. Phys. J. Special topics 205 1-26 (2012, Discussion and debate issue: from black swans to dragon-kings - Is there life beyond power laws?)
[Abstract]

mm Universe @ NIKA2 (Vol. 51, No. 2)

The NIKA2 camera opens a new area for millimeter observations of the Universe.

Conference Proceedings

NIKA2 is a millimetre camera recently installed at the 30-m telescope of IRAM. It can survey large areas of the sky at a high-angular resolution, with a high sensitivity and a large field of view. It allows observers to address questions, such as the environment impact on dust properties, the star formation processes at low and high redshifts, the evolution of the large-scale structures and the use of galaxy clusters for precision cosmology. In June 2019, the mm Universe conference in Grenoble brought together the scientific community working with the NIKA2 camera. It was the first edition in a series of conferences that will accompany the scientific exploitation of NIKA2.

F. Mayet, A. Catalano, J.F. Macías-Pérez and L. Perotto (Eds.), mm Universe @ NIKA2 - Observing the mm Universe with the NIKA2 Camera, Grenoble, France, June 3-7, 2019, EPJ Web of Conferences 228 (2020), ISBN: 978-2-7598-9097-2
[Proceedings]

Modelling of Exchange Spring Media magnetic properties (Vol. 44 No. 2)

Left: TEM cross-section image of the FePt ESM media. White dotted line marks the interface between hard and soft phase. Right: Based on experimental findings numerical models used for simulation of the magnetic reversal behaviour. All dimensions are written in nanometres.

The authors analyze the magnetic properties of strongly interacting hard and soft magnetic thin films: so called Exchange Spring Media (ESM). Such materials are characterized by a good thermal stability, while keeping the coercive field of the total structure below the value of the hard magnetic layer. FePt hard/soft bilayers are used, of which crystallographic nanostructure is very complex, as shown by Transmission Electron Microscopy (TEM). For instance, a rough interface between hard and soft layers is created with obstacles of one phase embedded in the second phase (Fig. left).

To understand how such a complex nanostructure influences magnetic properties of the ESM, the experimental phase distributions were transferred into numerical models (Fig. right) with which the magnetization reversal processes were studied using finite element micro-magnetic simulations. This analysis was supported by the analytical consideration of the magnetic pinning field (damping of the magnetization reversal). The combination of analytical and numerical approaches showed that observed nanostructures could significantly alter the coercive field of the media. For each distribution the coercive field minimum was found when nucleation field was equal to the maximal pinning field within the ESM structure. Moreover, the influence of the obstacles on the magnetic properties of the media was strongly restricted by their size and position within the material.

The present model of magnetic properties analysis is general enough to be applied to other ESM materials, such as CoPt or FePt/FeAu hard/soft structures.

B. Dymerska, J. Lee, J. Fidler and D. Suess, ‘Micromagnetic study of exchange spring media with a rough interface on an example of FePt films’, J. Phys. D: Appl. Phys. 45, 495001 (2012)
[Abstract]

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