Bacteria-inspired motility could power a new generation of mini-robots (Vol. 50, No. 1)

Diagram depicting the orientational configuration of the helix in the laboratory

Physicists develop a model to explain how deforming a helix could generate additional locomotion for some microorganisms and mini-robots

Many microorganisms rely on helices to move. For example, some bacteria rotate a helical tail, called a flagellar filament, for thrust and deform these tails during rotation. In addition, some types of bacteria, named Spirochaetes, rely on the deformation of a helical body for their motion. To better understand such locomotion mechanisms, scientists have created mathematical models of mini-robots with helical structures, referred to as swimmers. In a study published recently, the authors identify the factors enhancing the agility of deforming helix swimmers. They examine what happens when these swimmers placed in a fluid uniformly change the radius, the helical pitch and the wavelength of the helix across their body. They identify swimming strokes that allow rotation and motion in a given direction and thus explain how the helix’s deformation influences the direction in which the swimmers move.

L. Koens, H. Zhang, M. Moeller, A. Mourran, and E. Lauga, The swimming of a deforming helix, Eur. Phys. J. E 41, 119 (2018)

P2 – The weak charge of the proton (Vol. 50, No. 1)

The experimental setup of the P2-experiment to measure the weak mixing angle at the new electron accelerator MESA in Mainz

The P2-experiment at the new electron accelerator MESA in Mainz aims at a high-precision determination of the weak mixing angle at the permille level at low Q2. This accuracy is comparable to existing measurements at the Z-pole but allows for sensitive tests of the Standard Model up to a mass scale of 50 TeV. The weak mixing angle will be extracted from a measurement of the parity violating asymmetry in elastic electron-proton scattering. The asymmetry measured at P2 is smaller than any asymmetry measured so far in electron scattering, with an unprecedented accuracy. This review describes the underlying physics and the innovative experimental techniques, such as the Cherenkov detector, beam control, polarimetry, and the construction of a novel liquid hydrogen high-power target. The physics program of the MESA facility comprises indirect, high-precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclei, transverse single-spin asymmetries, and a possible future extension to the measurement of hadronic parity violation.

D. Becker and 45 co-authors, The P2-Experiment - A future high-precision measurement of the weak mixing angle at low momentum transfer, Eur. Phys. J. A 54, 208 (2018)

How to optimise an interface in spin-orbitronics? (Vol. 50, No. 1)

Tuning α and Ds through % Pt.

Spintronics is a rapidly developing field of applied physics seeking to exploit electron spins as a further degree of freedom, which is extremely appealing to numerous applications related to magnetic information processing and data storage. Creation of energy saving spintronic devices based on spin currents operated without magnetic fields is currently a key challenge in this domain. This fundamental problem can be resolved by making use of the effects related to Spin-Orbit Coupling (SOC), the approach adopted in a novel direction referred to as spin-orbitronics.

Such effects are typically of interfacial nature that take place in ferromagnetic metal/heavy metal bilayers, Pt being the most promising heavy metal candidate. In this paper the authors have investigated major tendencies in the behaviour of three of them, the Gilbert damping α, the magnetic anisotropy and the interfacial Dzyaloshinskii–Moriya interaction Ds, as a function of Pt concentration in Py (5 nm)/Cu1−xPtx bilayers. Although they demonstrate correlated general features as Pt is replaced by Cu, confirming their common physical nature, their behaviour is not identical. This opens up the possibility of creation of optimised interfaces with SOC-related parameters tuned independently for a specific application.

H. Bouloussa, R. Ramaswamy, Y. Roussigné, A. Stashkevich, H. Yang, M. Belmeguenai and S. M. Chérif, Pt concentration dependence of the interfacial Dzyaloshinskii–Moriya interaction, the Gilbert damping parameter and the magnetic anisotropy in Py/Cu1-xPtx systems, J. Phys. D: Appl. Phys. 52, 055001 (2019)

Precise electron spin control yields faster memory storage (Vol. 50, No. 1)

Panel shows a 2D view of spin dynamics for bulk nickel

New ultra-fast laser method aims to improve control over the electron’s degree of freedom, called spins, could enhance memory storage devices

Data storage devices are not improving as fast as scientists would like. Faster and more compact memory storage devices will become a reality when physicists gain precise control of the spins of electrons. They typically rely on ultra-short lasers to control spins. However, improvement of storage devices via spin control requires first to develop ways of controlling the forces acting on these electronic spins. In a recent study published recently, the authors have developed a new theory to predict the complex dynamics of spin procession once a material is subjected to ultra-short laser pulses. The advantage of this approach, which takes into account the effect of internal spin rotation forces, is that it is predictive. The authors find that internal spin rotation forces only contribute significantly to spin dynamics when the variation in different directions of the magnetic energy—or magnetic anisotropy energy—is small. This is the case with materials which are highly symmetric such as bulk metals with a cubic structure. When such magnetic anisotropy energy is large, the spin rotation effect is too small to cause any significant precession of spins below 100 femtoseconds.

J. K. Dewhurst, A. Sanna, and S. Sharma, Effect of exchange-correlation spin-torque on spin dynamics, Eur. Phys. J. B 91, 218 (2018)

The power of resolution in charge-exchange reactions (Vol. 50, No. 1)

Spectra of the 76Ge(3He,t)76As reaction unveiling an enormous number of resolved states at low excitation energies. Five colour-coded spectra are stacked on top of each other showing the angular dependences. The isobaric analog state (IAS), GT resonance (GTR) and spin-dipole resonance (SDR) are indicated

This review highlights the extraordinary power of the hadronic charge-exchange reactions at intermediate energies and at highest spectral resolution, as exemplified by the (n,p)-type (d,2He) and the (p,n)-type (3He,t) reactions. The review shows how areas of nuclear physics, astrophysics and particle physics alike benefit from such enhanced resolution. A major part of this review focuses on weak interaction processes in nuclei, especially on those, where neutrinos play a pivotal role like in solar neutrino induced reactions or in ββ decay. Unexpected and even surprising new features of nuclear structure are being unveiled as a result of high resolution. (See figure).

High resolution proves to be of equally high importance in areas where this would not immediately be expected. This is outlined in the chapters dealing with the neutron-neutron scattering length, with the properties of halo-nuclei or with the explosion dynamics of supernovae. Finally, the review portrays how high-resolution charge-exchange reactions connect to the symmetry energy and the nuclear equation-of-state or to processes involving ordinary capture of muons by nuclei. Clearly, the insight into the physics, which is made possible with high-resolution charge-exchange reactions, could not possibly be more diverse.

D. Frekers and M. Alanssari, Charge-exchange reactions and the quest for resolution, Eur. Phys. J. A 54, 177 (2018)

Factors affecting turbulence scaling (Vol. 50, No. 1)

Stability regions calculated by a model for a given compressibility

Study focuses on hydrodynamic effects of external disturbances on fluids at critical points, including inconsistent turbulence in all directions, or anisotropy, and varying degrees of compressibility

Fluids exhibiting scaling behaviour can be found in diverse physical phenomena occurring both in the laboratory and in real-world conditions. For instance, they occur at the critical point when a liquid becomes a vapour, at the phase transition of superfluids, and at the phase separation of binary liquids whose components exhibit two different types of behaviour. Until now, models have not fully taken the effect of external turbulences into account. In a recent study published recently, the authors investigate the influence of ambient turbulent speed fluctuations in physical systems when they reach a critical point. These fluctuations are found to be the result of a lack of spatial regularity in these systems, or anisotropy, and of the compressibility of fluids. What is unique about this study is that the turbulence introduced in the model is novel and helps to elucidate the extent to which the speed of these fluctuations affects their scaling behaviour.

M. Hnatič, G. Kalagov, and T. Lučivjanský, Scaling behavior in interacting systems: joint effect of anisotropy and compressibility, Eur. Phys. J. B, 91, 269 (2018)

Making plasma medicine available for in-body applications (Vol. 50, No. 1)

Endoscope tip without and with Neon plasma

Ever since non-thermal plasmas showed efficacy in decontamination and wound healing, the idea of deploying plasma medical therapy within the human body emerges. Besides the need for flexibility, small dimensions and biological effectiveness, also a minimal plasma-caused applicator erosion as well as an electrically safe operation mode are necessary. Of course, the endoscopic plasma source must also operate inside hollow cavities independent of the environmental conditions present. Since all requirements need to be fulfilled at the same time, the development task is quite complex.

The present paper tackles those requirements and sets special focus on new approaches for reducing leakage current, increasing the bactericidal efficacy and avoiding material erosion simultaneously. The jet-like plasma at the tube tip is maintained by a capacitively coupled discharge configuration. An additional shielding gas surrounds the jet in order to assure reproducible environmental conditions inside the body. Finally, it is found that a combination of Neon feed gas, CO2 shielding gas and a current limited high voltage supply gives the best bactericidal results and, at the same time, reduces material erosion as well as patient leakage current.

J. Winter, Th. M. C. Nishime, R. Bansemer , M. Balazinski, K. Wende and K.-D. Weltmann, Enhanced atmospheric pressure plasma jet setup for endoscopic applications, J. Phys. D: Appl. Phys. 52, 024005 (2019)

Wetting routes of droplet upon patterned hydrophilic surface (Vol. 50, No. 1)

Different wetting routes of droplet upon patterned hydrophilic surface

The wetting transition of the droplet on the patterned hydrophilic surface can occur spontaneously and may further lead to superwetting that has the potential to develop novel technologies in the field of anti-fogging, printing and heat transfer. However, it is still unknown how the wetting transition occurs on such a patterned surface. In contrast to the conventional view that wetting occurs immediately in the vertical direction upon the contact of the droplet with the solid surface due to the capillary force, we find that the droplet spreads first in the horizontal direction if the patterned surface has a large enough roughness. Then, the wetting transition occurs at the periphery rather than at the middle part of droplet, which is termed as “one-dimensional wetting”. We ascribe such an interesting phenomenon to the competition between the horizontal force arising from the non-equilibrium surface tension and the vertical capillary force as well as to the different pressure under the droplet, which lead to three different wetting routes (one-dimension wetting (One), two dimension wetting (Two), Between one and two dimension wetting (BOT)).

T. Li, X. Liu, H. Zhao, B. Zhang and L. Wang, Counterintuitive wetting route of droplet on patterned hydrophilic surface, EPL 123, 36003 (2018)

Conjugate coupling-induced spontaneous symmetry breaking (Vol. 50, No. 1)

Analytical and numerical representations of dynamical regimes in the system

Spontaneous symmetry breaking (SSB) is a phenomenon that can facilitate the onset of a rich variety of complex patterns observed in several natural systems. In SSB, asymmetric states arise from symmetric systems spontaneously as a control parameter is varied. This study reveals the existence of spontaneous symmetry breaking state induced by conjugate coupling which corresponds to coupling in paradigmatic Stuart-Landau oscillators. The system exhibits distinct dynamical states, namely in-phase synchronized (IPS), out-of-phase synchronized (OPS), nontrivial amplitude death (NAD) and oscillation death (OD) states. We have deduced the explicit analytical solutions of these states and have studied their stability. The system also exhibits multistabilities among the dynamical states including IPS-OPS (R1), OPS-NAD (R2), SSB-NAD (R3), NAD-OD (R4) and SSB-NAD-OPS (R5). It is known that feedback is a useful control mechanism in many biological systems. While introducing the feedback factor in a conjugately coupled system it completely suppresses the SSB and OD states but does not influence the NAD state. These results will shed light on the dynamics of SSB and the control of such dynamical states.

K. Ponrasu, K. Sathiyadevi, V. K. Chandrasekar and M. Lakshmanan, Conjugate coupling-induced symmetry breaking and quenched oscillations, EPL 124, 20007 (2018)