Vol. 50 No. 4 - Highlights

How to stop diseases and forest fires from spreading (Vol. 50, No. 4)

How to stop diseases and forest fires from spreading
When the population approaches a certain level of heterogeneity, the infection slows

A new model, exploring how epidemics spread, could help prevent infections and forest fires from getting out of hand

Recently, epidemics like measles have been spreading due to the lack of vaccinations, and forest fires have become increasingly frequent due to climate change. Understanding how both these things spread, and how to stop them, is more important than ever. Now, the authors, have studied the way epidemics spread in heterogeneous populations. Their findings were recently published in EPJ B.

A. B. Kolton and K. Laneri, Rough infection fronts in a random medium, Eur. Phys. J. B 92 ,126 (2019)

XXI International Conference on Ultrafast Phenomena 2018 (UP 2018) (Vol. 50, No. 4)

XXI International Conference on Ultrafast Phenomena 2018 (UP 2018)
Welcome laser beam from Elbphilharmonie to European XFEL.

The International Conference on Ultrafast Phenomena is the premier international forum for gathering the community of scientists and engineers in research and technology related to ultrafast phenomena covering the time scales ranging from picoseconds (1 ps = 10-12 s) to attoseconds (1 as = 10-18 s).

In the past decade, this field has moved ahead rapidly due to new laser- as well as accelerator-based sources of electron and light pulses, such as high harmonic generation, few-cycle optical pulses, sources of short wavelength radiation such as x-ray free electron lasers. Together with new methodologies, e.g. multidimensional spectroscopies, THz spectroscopy, electron-based techniques (EELS, PINEM, UED, etc.) and x-ray based techniques such as serial femtosecond coherent diffractive imaging, these great leaps forward are delivering an impressive degree of insight into phenomena both within atoms and between atoms and up in scale to macromolecular systems. These developments open up new perspectives for major applications in the fields of solar energy, molecular electronics, optoelectronic devices, biomimetic devices, etc... Last but not least, all this is accompanied by an improvement in theoretical models, which are indispensable for our understanding of phenomena on such ultra-short time scales.

G. Cerullo, J. Ogilvie, F. Kärtner, M. Khalil and R. Li (Eds.), XXI International Conference on Ultrafast Phenomena 2018, EPJ Web of Conferences 205, (2019)

Collagen fibres grow like a sunflower (Vol. 50, No. 4)

Collagen fibres grow like a sunflower
Phyllotactic pattern with concentric circles

Collagen fibrils are a major component of the connective tissues found throughout the animal kingdom. The cable-like assemblies of long biological molecules combine to form tissues as varied as skin, corneas, tendons or bones. The development of these complex tissues is the subject of a variety of research efforts, focusing on the steps involved and the respective contributions of genetics and physical chemistry to their development. Now, the authors have shed new light on how complex collagen fibrils form. In a new study published recently, they focus on one of the hierarchical steps, in which molecules spontaneously associate in long and dense axisymmetric fibres, known as type I collagen fibrils. In this study, the spontaneous association step under scrutiny is unique because the diameter of the fibre remains constant throughout its growth, while the end of growth manifests a characteristic parabolic profile. After studying several possible models, the researchers concluded the most likely explanation is that the fibres spread out from the fibre axis, along a stem, similar to how a sunflower’s florets grow.

J. Charvolin, and J-F. Sadoc, Type I collagen fibrils: from growth morphology to local order, Eur. Phys. J. E 42, 49 (2019)

Topological Insulator in an Atomic Liquid (Vol. 50, No. 4)

Topological Insulator in an Atomic Liquid
Phase diagram of the model liquid system with topologically nontrivial electronic structure

Topological insulators are a new class of materials whose electronic states are characterized by global topological invariants. Due to their nontrivial topology, these materials are able to conduct electricity on the surface despite being insulating in the bulk. Moreover, the metallic surface is robust against disorder and other imperfections, making topological insulators strong candidates for the building blocks of next-generation electronics technology. Although almost all known topological insulators are crystals, it has recently been shown that topological insulators and superconductors can also exist in the amorphous or glass states, as long as the relevant symmetries are maintained.

This work further generalizes the notion of topological materials by theoretically demonstrating an atomic liquid phase that supports topologically nontrivial electronic structure. Using quantum molecular dynamics simulations, it is shown that by melting a topological crystalline state with elevated temperatures, the resultant liquid phase inherits the nontrivial topology that is characterized by a nonzero Bott index, named after the famous topologist Raoul Bott. This work points to a new systematic approach for searching topological phases in amorphous and liquid systems.

G.-W. Chern, Topological insulator in an atomic liquid, EPL 126, 37002 (2019)

Inhibitory neurons have two types of impact on brain oscillations (Vol. 50, No. 4)

Inhibitory neurons have two types of impact on brain oscillations
The emergence of synchronization with excitatory and inhibitory neurons

A certain type of neuron, called inhibitory neurons, can have two types of overall effect on oscillations in the brain

Studying the brain involves measuring the activity of billions of individual brain cells called neurons. Consequently, many brain measurement techniques produce data that is averaged to reflect the activity of large populations of these neurons. If all of the neurons are behaving differently, this will average out. But, when the behaviour of individual neurons is synchronized, it produces clearly visible oscillations. Synchronisation is important to understanding how neurons behave, which is particularly relevant with regard to brain diseases like Alzheimer’s, epilepsy and Parkinson’s. Now, a group of researchers has used a combination of two computer models to study the ways different kinds of neurons can impact synchronisation. The study has been published recently. To study the effects on synchronisation, the authors examined neurons called inhibitory neurons – which work to slow down or stop the activity of other neurons. Moreover, they explored the likelihood of these inhibitory neurons firing either spontaneously or not at all within the network.

P.-X. Lin, Ch.-Y. Wang and Z.-Xi Wu (2019), Two-fold effects of inhibitory neurons on the onset of synchronization in Izhikevich neuronal networks, Eur. Phys. J. B 92, 113 (2019)

Graphene plasmonics for semiconductor photonic crystals (Vol. 50, No. 4)

Graphene plasmonics for semiconductor photonic crystals
Side view of semi-infinite semiconductor photonic crystal.

The new area called graphene plasmonics is an emerging field that addresses the study and application of effects of the light interaction with the surface electron gas in the graphene sheet for the development of new devices. These types of light-matter interactions are called plasmon-polaritons. Specifically, surface polaritons are mixed excitations (where one of the components is the photon) that can propagate near the interface between two dielectric (or semiconductor) media; the associated electric and magnetic fields rapidly decay away from this interface. For this type of system, one of the most important basic questions with a view to device applications is to determine the polaritons’ dispersion relation (or an equivalent energy-momentum diagram), which will be strongly dependent on the surroundings media. In a recent paper, the authors have studied a graphene system where they have a semi-infinite semiconductor photonic crystal with graphene sheets interlayers, whose external medium is typically vacuum (see the figure). They apply their results to doped GaAs and they have found that the graphene sheets play an important role in modifying the surface (and bulk) plasmon-polariton properties, mainly for the frequency region around 1 THz.

M. S. Vasconcelos and M. G. Cottam, Surface and bulk plasmon-polaritons in semiconductor photonic crystals with embedded graphene sheets, J.Phys. D: Appl. Phys. 52, 285104 (2019)

How red blood cells behave in crowded vessels (Vol. 50, No. 4)

How red blood cells behave in crowded vessels
Red blood cells flowing through a blood vessel.

A new model of red blood flowing through narrow capillaries shows that the cells change shape and alignment, allowing plasma to flow down the sides

Blood consists of a suspension of cells and other components in plasma, including red blood cells, which give it its red colour. When blood flows through the narrowest vessels in the body, known as the capillaries, the interactions between the cells become much more important. In a new study published recently, a team of researchers has now developed a mathematical model of how red blood cells flow in narrow, crowded vessels. This could help design more precise methods for intravenous drug delivery, as well as 'microfluidic chips' incorporating artificial capillaries, which could offer faster, simpler and more precise blood-based diagnoses.

G. R. Lázaro, A. Hernandez-Machado and I. Pagonabarraga, Collective behavior of red blood cells in confined channels, Eur. Phys. J. E 42, 46 (2019)

Chip-Based Frequency Shift Super-Resolution Imaging (Vol. 50, No. 4)

Chip-Based Frequency Shift Super-Resolution Imaging
Schematic and mechanism of the Si3N4 waveguide based super-resolution chip.

Current label-free super-resolution methods suffer from either limited resolution improvements, small field-of-views or complex implementations, and a method with high-resolution, high-throughput and easy configuration is desirable for the practical applications in materials, biology and medicine, etc. To break this limitation, the authors propose a Si3N4 waveguide platform design for multi-wavelength illuminated label-free super-resolution microscopy imaging. The deep-subwavelength resolution was enabled by large wave-vector evanescent illumination induced spatial frequency shift effect, which also provides the high throughput for its wide field implementation.

In the method, chip-based waveguide with high refractive index produced evanescent wave illumination on the upper surface, the high tangential wave vector shifted the high spatial frequency components into the passband of the detection system from the perspective of the reciprocal domain. Multi-wavelength illuminated method was employed to cover the complete detection in the frequency domain. This enlarged the detectable broad-band frequency spectrum range, and high-resolution image with high signal-to-noise-ratio could be achieved using reconstruction algorithm. The optimized waveguide design to realize single mode behaviour and low attenuation guaranteed uniform evanescent wave illumination for the low-noise wide-field imaging. The integrated device is cost-effective, mass-producible and can conveniently give super resolvability to conventional microscopy as a compact module.

X. Xu, X. Liu, Ch. Pang, Y. Ma, Ch. Meng, J. Zhang, X. Liu and Q. Yang, Si3N4 waveguide platform for label-free super-resolution imaging: simulation and analysis, J. Phys. D: Appl. Phys. 52, 284002 (2019)

Inner electrons behave differently in aromatic hydrocarbons (Vol. 50, No. 4)

Inner electrons behave differently in aromatic hydrocarbons
Coincidence spectrum for benzene and other hydrocarbons

When an electron from one of the lower energy levels in an atom is knocked out of the atom, it creates a space which can be filled by one of the higher-energy electrons, also releasing excess energy. This energy is released in an electron called an Auger electron - and produces an effect known as Auger decay. Now, the authors have studied the Auger effect in four hydrocarbon molecules: benzene, cyclohexane, hexatriene and hexadiene. These molecules were chosen because they exhibit different characteristics of aromaticity. They found that molecules containing pi bonds have a lower threshold for Auger decay. Potential applications of this decay effect include a treatment called Auger therapy, which is used to help cancer patients.

G. Zhao, T. Miteva, and N. Sisourat, Inner-valence Auger decay in hydrocarbon molecules, Eur. Phys. J. D 73, 69 (2019)

Is there memory for the memoryless? (Vol. 50, No. 4)

Is there memory for the memoryless?
Overdamped Brownian particle under the influence of non-Markovian stochastic force

Inertia plays a role on the evolution of Brownian particles. Nevertheless, the interplay of inertial time-scale contributions and an overdamped dynamics with non-Markovian stochastic forces leads to contradictions that make equilibration impossible. This is due to assuming memory correlations for the dissipation, which seems to be inconsistent with the overdamped approximation, where thermal fluctuations adjust instantaneously to the state of the particle. Effectively, by taking the noise correlation time-scale to be zero (no memory) we certainly recover the expected physical behaviour of the problem, e.g., the equilibrium distribution. On the other hand, we can deal with the contradiction by inserting another source of noise, of Markovian type, and with “effective temperature” different from the non-Markovian noise. As a result, the stationary state may be regularized and the equilibrium recovered if both noises have same temperatures, even for finite memory time-scales. The additional white noise brings the system back to equilibrium, no matter how small the new noise intensity is.

E. S. Nascimento and W. A. M. Morgado, Non-Markovian effects on overdamped systems, EPL 126, 10002 (2019)