Vol. 47 No.5-6 - Highlights

Better material insights with gentle e-beams (Vol. 47 No. 5-6)

An early 2-D EELS of nitrogen

Great potential for a new, more accurate, tool for using electron collisions to probe matter

There are several ways to change a molecule, chemically or physically. One way is to heat it; another is to bombard it with light particles, or photons. A lesser known method relies on electron collision, or e-beam technology, which is becoming increasingly popular in industry. In a review outlining new research avenues based on electron scattering, the authors explain the subtle intricacies of the extremely brief electron-molecule encounter, in particular with gentle, i.e., very low energy electrons. In this paper, which was recently published, the authors describe how the use of very low energy electrons and a number of other performance criteria, make the approach with the so-called Fribourg instrument a more appealing candidate than previously available tools used to study electron collisions. One of the potential applications of this approach is in the quest to find a replacement for a molecule called sulfur hexafluoride (SF6), a greenhouse gas stored in high voltage electricity distributing devices, such as switches and transformers. Electron collision could help identify a more suitable gas.

M. Allan, K. Regeta, J. D. Gorfinkiel, Z. Mašín, S. Grimme and C. Bannwarth, Recent research directions in Fribourg: nuclear dynamics in resonances revealed by 2-dimensional EEL spectra, electron collisions with ionic liquids and electronic excitation of pyrimidine, Eur. Phys. J. D 70, 123 (2016)

Better defining the signals left by as-yet-undefined dark matter at the LHC (Vol. 47 No. 5-6)

Schematic of an Effective Field Theory interaction between dark matter and the standard model

New theoretical models that better describe the interaction between dark matter and ordinary particles advance the quest for dark matter

In the quest for dark matter, physicists rely on particle colliders such as the LHC in CERN, located near Geneva, Switzerland. The trouble is: physicists still don't know exactly what dark matter is. Indeed, they can only see its effect in the form of gravity. Until now, theoretical physicists have used models based on a simple, abstract description of the interaction between dark matter and ordinary particles, such as the Effective Field Theories (EFTs). However, until we observe dark matter, it is impossible to know whether or not these models neglect some key signals. Now, the high energy physics community has come together to develop a set of simplified models, which retain the elegance of EFT-style models yet provide a better description of the signals of dark matter, at the LHC. These developments are described in a review published by the authors.

A. De Simone and T. Jacques, Simplified models vs. effective field theory approaches in dark matter searches, Eur. Phys. J. C 76, 367 (2016)

A New High for Magnetically Doped Topological Insulators (Vol. 47 No. 5-6)

Temperature dependence of the magnetization, M(T), of CrxSb2-xTe3 thin film samples with varying Cr concentration, x. The most highly doped and structurally uncompromised film shows a transition temperature of 125 K

Topological insulators (TIs) are a new phase of quantum matter whose conducting surface states are a result of the topology of their bulk band structure. Their spin-momentum locked topological surface states are resilient to backscattering owing to their protection by time-reversal symmetry (TRS). These properties make them intriguing candidates for low-power devices, spintronics and quantum computation. Breaking TRS by introducing magnetic dopants, and introducing a gap in the topological surface states, unlocks exotic quantum phenomena such as the quantum anomalous Hall state. Doping TIs with magnetic impurities is an experimentally challenging process and most TI materials only exhibit magnetic ordering at low temperatures.

In this study, using a variety of complementary structural, electronic and magnetic characterisation techniques, we demonstrate the synthesis of magnetically doped TI thin films with high structural quality. The Cr-doped Sb2Te3 thin films were grown on sapphire using low-temperature molecular beam epitaxy. We show that this particular system exhibits uniform ferromagnetic ordering up to ~125 K – a step forward towards device-friendly TI materials.

L. J. Collins-McYntire + 13 co-authors, Structural, electronic, and magnetic investigation of magnetic ordering in MBE-grown CrxSb2-xTe3 thin films, EPL 115, 27006 (2016)

Arbitrarily slow, non-quasistatic, isothermal transformations (Vol. 47 No. 5-6)

A thermodynamically irreversible cycle for single-particle and classical engines

Joule or free expansion of an ideal gas into a volume at lower pressure is an example of an irreversible isothermal process. This nonequilibrium example is often used in thermodynamics texts to demonstrate that an arbitrarily slow process need not be reversible. Cyclic operation of engines that involve a free expansion therefore requires work.

Here, the authors explore experimentally the origin of thermodynamic irreversibility at the level of a single-particle “gas”. A feedback trap confines a silica particle in a virtual bistable potential, creating a system analogous to two vessels connected by a valve, where the volume of one vessel is adjustable via piston. The authors operate two types of cyclic transformations; both start and end in the same equilibrium state, and both use the same basic operations—but in different order. One transformation required no work, while the other required work, no matter how slowly it was carried out.

Why the difference? As the illustration shows, the result of carrying out a protocol backwards in time may not match the initial state. This property is not possible to notice in a single repetition, unlike in a macroscopic system where free expansion is followed by a “whoosh”.

M. Gavrilov and J. Bechhoefer, Arbitrarily slow, non-quasistatic, isothermal transformations, EPL 114, 50002 (2016)

Germanium detectors get position sensitive (Vol. 47 No. 5-6)

Interaction positions determined by the pulse shape analysis of AGATA and the AGATA spectrometer at GANIL (picture by P. Lecomte)

High purity germanium detectors have grown into very popular devices within the field of gamma ray spectroscopy. The sensitive part of these detectors consists of the largest, purest and monocrystalline semi-conductors used on earth. Ge detectors are famous for their outstanding energy resolution for electromagnetic radiation, especially in the field of nuclear physics and astrophysics. Recently technical advances and the segmentation of the Ge crystals opened up new opportunities. In this way, the Ge detector becomes a position sensitive device and allows for the novel gamma-ray tracking technique.

New gamma ray spectrometers are currently under construction and implement the new method. The article describes all the theoretical concepts, which are needed for a precise understanding of all detector properties. Moreover, an elaborate computer code, named ADL, was developed and yielded a huge set of hundred thousands of detector pulses. These pulses are compared to measured pulses from individual gamma rays in order to extract the position where the radiation interacted with the detector material and created charges. ADL utilizes all relevant aspects of signal creation and formation with the Ge detector and the subsequent electronics. Meanwhile the code is successfully used for position sensitive spectroscopy within the AGATA project.

B. Bruyneel, B. Birkenbach and P. Reiter, Pulse shape analysis and position determination in segmented HPGe detectors: The AGATA detector library, Eur. Phys. J. A 52, 70 (2016)

Improving safety of neutron sources (Vol. 47 No. 5-6)

Sampling of Lead-Bismuth-eutectic material/cover gas-interface sample consisting of solid material forming a powdery crust onto the steel wall

Testing liquid metals as target material bombarded by high-energy particles

There is a growing interest in the scientific community in a type of high-power neutron source that is created via a process referred to as spallation. This process involves accelerating high-energy protons towards a liquid metal target made of material with a heavy nucleus. The issue here is that scientists do not always understand the mechanism of residue nuclei production, which can only be identified using spectrometry methods to detect their radioactive emissions. In a new study examining the radionuclide content of lead-bismuth-eutectic (LBE) targets, the authors found that some of the radionuclides do not necessarily remain dissolved in the irradiated targets. Instead, they can be depleted in the bulk LBE material and accumulate on the target's internal surfaces. These findings have recently been published. The results improve our understanding of nuclear data related to the radionuclides stemming from high-power targets in spallation neutron sources. They contribute to improving the risk assessment of future high-power spallation neutron beam facilities --including, among others, the risk of erroneous evaluation of radiation dose rates.

B. Hammer-Rotzler, J. Neuhausen, V. Boutellier, M. Wohlmuther, L. Zanini, J.-C. David, A. Türler and D. Schumann, Distribution and surface enrichment of radionuclides in lead-bismuth eutectic from spallation target, Eur. Phys. J. Plus 131, 233 (2016)

Surprising neutrino decoherence inside supernovae (Vol. 47 No. 5-6)

Illustration of the shift of two wave packets with large spread. Loss of coherence occurs even if the packets overlap due to the spatial energy redistribution within the whole wave packets.

Theory to explain collective effects of neutrinos inside supernovae strengthened

Neutrinos are elementary particles known for displaying weak interactions. As a result, neutrinos passing each other in the same place hardly notice one another. Yet, neutrinos inside a supernova collectively behave differently because of their extremely high density. A new study reveals that neutrinos produced in the core of a supernova are highly localised compared to neutrinos from all other known sources. This result stems from a fresh estimate for an entity characterising these neutrinos, known as wave packets, which provide information on both their position and their momentum. These findings have just been published by the authors. The study suggests that the wave packet size is irrelevant in simpler cases. This means that the standard theory for explaining neutrino behaviour, which does not rely on wavepackets, now enjoys a more sound theoretical foundation.

J. Kersten and A. Yu. Smirnov, Decoherence and oscillations of supernova neutrinos, Eur. Phys. J. C 76, 339 (2016)

How cooperation emerges in competing populations (Vol. 47 No. 5-6)

The fraction of cooperative players as a function of the site-occupancy probability ρ obtained using numerical simulations.

New theoretical approach to understand the dynamics of populations reaching consensus votes or of spreading epidemics

Social behaviour like reaching a consensus is a matter of cooperation. However, individuals in populations often spontaneously compete and only cooperate under certain conditions. These problems are so ubiquitous that physicists have now developed models to understand the underlying logic that drives competition. A new study published recently shows the dynamics of competing agents with an evolving tendency to collaborate that are linked through a network modelled as a disordered square lattice. These results are the work of the authors. They believe that their theoretical framework can be applied to many other problems related to understanding the dynamical processes in complex systems and networked populations, such as the voter dynamics involved in reaching a consensus and spreading dynamics in epidemic models and in social networks.

C. Xu, W. Zhang, P. Du, C.W. Choi and P.M. Hui, Understanding cooperative behavior in structurally disordered populations, Eur. Phys. J. B 89, 152 (2016)

Electron scavenging to mimic radiation damage (Vol. 47 No. 5-6)

Molecule of trifluoroacetamide (TFAA)

New study could help unveil negative effect of radiation on biological tissues due to better understanding of low energy electron-induced reactions

High energy radiation affects biological tissues, leading to short-term reactions. These generate, as a secondary product, electrons with low energy, referred to as LEEs, which are ultimately involved in radiation damage. In a new study, scientists study the effect of LEEs on a material called trifluoroacetamide (TFAA). This material was selected because it is suitable for electron scavenging using a process known as dissociative electron attachment (DEA). These findings were recently published, as part of a topical issue on Advances in Positron and Electron Scattering. Experiments confirm that DEA reactions occur due to electrons entering unoccupied molecular orbitals, at an energy level located near one electronvolt. This means that low-energy electrons can be exploited with solid materials like TFAA to trigger selective reactions, resulting in multiple bond cleavages inside the material. Ultimately, this leads to the creation of specific negative ions and stable molecules of interest.

J. Kopyra, C. König-Lehmann, E. Illenberger, J. Warneke and P. Swiderek, , Low energy electron induced reactions in fluorinated acetamide – probing negative ions and neutral stable counterparts, Eur. Phys. J. D 70, 140 (2016)

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)

Polychromatic cylindrically polarized beams (Vol. 47 No. 5-6)

Various polarization patterns (arrows) and intensity distributions (underlying doughnut) of a co-rotating radially polarized X-wave

Cylindrically polarized beams represent a class of solutions, where the polarization can be radially or azimuthally distributed across the intensity profile. These beams have very intriguing properties, both from a fundamental and an applied perspective. Despite their great success, they have been almost exclusively studied and realized within the monochromatic regime.

An open question is if non-monochromatic cylindrically polarized solutions of Maxwell equations exist. New research answers to this question by employing X waves with orbital angular momentum (the polychromatic counterpart of Bessel beams) as building blocks to generate optical pulses with radial and azimuthal polarization. This approach is different from the monochromatic case where Hermite-Gaussian beams are typically used. Solutions are investigated in the paraxial and the nonparaxial regime and the role of the pulse’s spectrum in the polarization properties of the pulse itself is pointed out. Analysis shows that the generalization of the concept of non-uniform polarization to the domain of optical pulses leads to new intriguing applications, such as spatially resolved Raman spectroscopy. Cylindrically polarized X-waves with orbital angular momentum could also open new intriguing scenarios for fundamental research and quantum optics.

M. Ornigotti, C. Conti and A. Szameit, Cylindrically polarized nondiffracting optical pulses, J. Opt. 18, 075605 (2016)

Asymmetrical magnetic microbeads transform into micro-robots (Vol. 47 No. 5-6)

Transformation of particle clusters while exposed to an oscillating external magnetic field

Thanks to the ordering effects of two-faced magnetic beads, they can be turned into useful tools controlled by a changing external magnetic field

Janus was a Roman god with two distinct faces. Thousands of years later, he inspired material scientists working on asymmetrical microscopic spheres—with both a magnetic and a non-magnetic half—called Janus particles. Instead of behaving like normal magnetic beads, with opposite poles attracting, Janus particle assemblies look as if poles of the same type attract each other. A new study reveals that the dynamics of such assemblies can be predicted by modelling the interaction of only two particles and simply taking into account their magnetic asymmetry. These findings were recently published by the authors. It is part of a topical issue entitled "Nonequilibrium Collective Dynamics in Condensed and Biological Matter." The observed effects were exploited in a lab-on-a-chip application in which microscopic systems perform tasks in response to a changing external magnetic field, such as, for instance, to create a zipper-style micro-muscle on a chip.

G. Steinbach, S. Gemming and A. Erbe, Non-equilibrium dynamics of magnetically anisotropic particles under oscillating fields, Eur. Phys. J. E, 39 69 (2016)

The effect of spatiality on multiplex networks (Vol. 47 No. 5-6)

The multiplex structure arising from beginning with nodes on a lattice and connecting them through two layers of links (gray and black) with the length of each link following an exponential distribution

When a node can only form a link to its nearest neighbour, the topology is entirely determined by the spatial locations of the nodes. But when near and far links can form, the influence of the spatial embedding of the topology is much less. In this paper, we use this to modulate the strength of spatial effects on network topology. This allows us to consider the question: Does increasing the allowed geometric length of links in a network improve its robustness? In single-layer networks, the answer is generally that it does. However, in multiplex networks, we find that increasing the link lengths actually makes the network vulnerable to more severe cascade behaviours. This is because in multiplex networks, longer links allow for a discontinuous percolation transition which is characterized by a nucleation process. Our model and results demonstrate the surprising effects of spatial embedding and provide a simple new framework for assessing spatial networks of one or more layers.

M. M. Danziger, L. M. Shekhtman, Y. Berezin and S. Havlin, The effect of spatiality on multiplex networks, EPL 115, 36002 (2016)

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)

Versatile method yields synthetic biology building blocks (Vol. 47 No. 5-6)

Fluorescence microscopy image of polymersomes, taken 3 days after production

New high-throughput method to produce both liposomes and polymersomes on the same microfluidic chip Synthetic biology involves creating artificial replica that mimic the building blocks of living systems. It aims at recreating biological phenomena in the laboratory following a bottom-up approach. Today scientists routinely create micro-compartments, so called vesicles, such as liposomes and polymersomes. Their membranes can host biochemical processes and are made of self-assembled lipids or a particular type of polymers, called block copolymers, respectively. In a new study, researchers have developed a high-throughput method--based on an approach known as microfluidics--for creating stable vesicles of controlled size. The method is novel in that it works for both liposomes and polymersomes, without having to change the design of the microfluidic device or the combination of liquids. The authors recently published these findings. Typical applications in synthetic biology include the encapsulation of biological agents and creation of artificial cell membranes with a specific biochemical function. They anticipate that their method might also be applicable for the controlled fabrication of hybrid vesicles used in bio-targeting and drug-delivery.

J. Petit, I. Polenz, J.- C. Baret, S. Herminghaus and O. Bäumchen, Vesicles-on-a-chip: A universal microfluidic platform for the assembly of liposomes and polymersomes, Eur. Phys. J. E 39, 59 (2016)