Vol. 48 No. 3 - Highlights

Smart multi-layered magnetic material acts as an electric switch (Vol. 48, No. 3)

The structure of ultrathin films of a cobalt nickel alloy

New study reveals characteristic of islands of magnetic metals between vacuum gaps, displaying tunnelling electric current.

The nanometric-size islands of magnetic metal sporadically spread between vacuum gaps display unique conductive properties under a magnetic field. In a study recently published, the authors found that the vacuum gaps impede the direct magnetic alignment between the adjacent islands — which depends on the external magnetic field — while allowing electron tunneling between them. Such externally controlled conducting behaviour opens the door for applications in electronics with magnetic field sensors – which are used to read data on hard disk drives – , biosensors and microelectromechanical systems (MEMS), as well as in spintronics with magnetic devices used to increase memory density. They found that the maximum values of the electric conductivity under an external magnetic field are obtained when the islands have a width of between 3 nm and 5 nm, with vacuum barriers of between 1 nm and 3 nm between them. However, they also observed that the tunnelling of electrons between the islands depends on the relative orientation of the direction of magnetisation in the adjacent islands and on the external magnetic field.

A.M. Chornous, Yu.O. Shkurdoda, V.B. Loboda, Yu.M. Shabelnyk and V.O. Kravchenko, Influence of the surface morphology on the magnetoresistance of ultrathin films of ferromagnetic metals and their alloys, Eur. Phys. J. Plus 132, 58 (2017)

Optical detection of low frequency NQR signals (Vol. 48, No. 3)

Optical detection of NQR signal

Nuclear quadrupole resonance (NQR) is a branch of radiofrequency (RF) spectroscopy. It became a promising tool in detecting illicit substances like explosives, narcotics and counterfeit medicines. Many of these substances contain 14N nuclei and are detectable by the NQR spectrometer. Practically all 14N NQR frequencies are in the range below 5 MHz and correspondingly the spectrometer sensitivity is low. One of possible improvements is a combination of the very sensitive potassium (K) pumped optical magnetometer (KPOM) and the pulsed NQR spectrometer. The linearly polarized probe laser beam detects the magnetic part of the low frequency 14N RF signal. This results in a rotation of the probe beam polarization plane after the beam leaves the K-cell. This rotation is measured and is proportional to the NQR signal. Combination of the classic RF excitation of the sample 14N nuclei and a subsequent optical detection of the sample response leads to a S/N improvement of up to a factor of 10 as it was demonstrated in the case study of some difficult-to-detect illicit substances. An efficient magnetic shielding may be necessary.

S. Begus, J. Pirnat, V. Jazbinsek and Z Trontelj, Optical detection of low frequency NQR signals: a step forward from conventional NQR, J. Phys. D: Appl. Phys. 50, 095601 (2017)

Does the universe have a rest frame? (Vol. 48, No. 3)

A simplified diagram showing the basic idea of the experimental design

Experiment aims at resolving divergence between special relativity and standard model of cosmology.

Physics is sometimes closer to philosophy when it comes to understanding the universe. The author attempts to elucidate whether the universe has a resting frame. The results have recently been published. To answer this tricky question, he has developed an experiment to precisely evaluate particle mass. This is designed to test the special theory of relativity that assumes the absence of a rest frame, otherwise it would be possible to determine which inertial frame is stationary and which frame is moving. This assumption, however, appears to diverge from the standard model of cosmology, which assumes that what we see as a vacuum is not an empty space. The assumption is that the energy of our universe comes from the quantum fluctuation in the vacuum. In this study, the author set out to precisely measure the masses of two charged particles moving in opposite directions. The conventional thinking assumes that the inertial frame applies equally to both particles. If that’s the case, no detectable mass difference between these two particles is likely to arise. However, if the contrary is true, and there is a rest frame in the universe, the author expects to see mass difference that is dependent on the orientation of the laboratory frame.

D. C. Chang, Is there a rest frame in the universe? A proposed experimental test based on a precise measurement of particle mass, Eur. Phys. J. Plus 132, 140 (2017)

Nonlinear scattering of atomic bright solitons in disorder (Vol. 48, No. 3)

Histogram of the atom reflected fraction when a soliton is scattered in a disordered potential. The atoms tend to be collectively either transmitted or reflected in contrast to the observation for non-interacting atoms. The non-linear scattering behaviour appears as a double peak structure in the histogram

Atomic bright solitons are self-trapping Bose-Einstein condensates. They exist in one-dimension because of attractive interactions. We observe nonlinear scattering of 39K atomic bright solitons launched in a one-dimensional disordered potential. The atoms from solitons behave collectively, i.e. are either mostly reflected or transmitted in contrast to non interacting atoms, which behave as independent quantum particles. This is the first observation of a non-linear behaviour with atomic bright solitons beyond their self-trapped nature. It requires the soliton interaction energy to be of the order of its center-of-mass kinetic energy. Our observations are reproduced in a mean-field framework by Gross-Pitaevskii simulations, while mesoscopic quantum superpositions of the soliton being fully reflected and fully transmitted are not expected for our parameters. We discuss the conditions for observing such superpositions, which would find applications in atom interferometry beyond the standard quantum limit.

A. Boissé, G. Berthet, L. Fouché, G. Salomon, A. Aspect, S. Lepoutre and T. Bourdel, Nonlinear scattering of atomic bright solitons in disorder, EPL 117, 10007 (2017)

Flat inductive plasma for large area plasma processing (Vol. 48, No. 3)

Schematic of the 1.2 x 1.2 m2 planar antenna. Capacitors join the ends of copper leg inductors to form a LC resonant network.”

Low temperature plasma generated by a resonant network antenna.

1 m2) is of fundamental importance for the industrial production of solar cells, flat panel displays, packaging, surface treatment, large area electronics, etc. Magnetic induction by RF oscillating currents in parallel legs is often used to drive the plasma in large inductive sources. In this work, the novel plasma source is a multiple LC resonant network antenna as shown in the figure. An electromagnetic model describes the antenna-plasma coupled system as a multi-conductor transmission line. Inspired by the “complex image” model for power transmission lines, this theory is used for the first time to calculate the induced image currents in the plasma. This approach could be applied generally to ICP antennas for large area plasma processing.

Ph. Guittienne, R. Jacquier, A. A. Howling and I. Furno, Electromagnetic, complex image model of a large area RF resonant antenna as inductive plasma source, Plasma Sources Sci. Technol. 26, 035010 (2017).

Ionisation mechanisms of captive atoms struck by light matter (Vol. 48, No. 3)

Ejected electron spectrum of the bare and caged hydrogen atom subjected to an external light pulse

Physicists elucidate the effects of light rays falling onto hydrogen atoms trapped in a carbon atom cage.

Light interacting with hydrogen atoms enclosed in hollow cages composed of carbon atoms—referred to as fullerene material—produces ionisation. This phenomenon, which has been the subject of intense theoretical scrutiny, is particularly interesting because the light rays can have dramatic effects in inducing small external energy potentials. Specifically, they alter the structural and dynamic properties of the atoms confined within the fullerene molecule. The authors have just published a study explaining the theory behind the ionisation. Applications of this process include drug delivery, quantum computation, photovoltaics and hydrogen storage.

A. L. Frapiccini, G. Gasaneo and D. M. Mitnik, Generalized Sturmians in the time-dependent frame: effect of a fullerene confining potential, Eur. Phys. J. D 71, 40 (2017)

Molecular scale transporter with a twist, powered by liquid crystal defects (Vol. 48, No. 3)

Twisting effect, called chirogyral, dictated by the handedness of the fibre in a vertical magnetic field

Delivery of biochemical substances is now possible using a novel application of liquid crystal defects, forming a loop enclosing the substance travelling alongside twisted fibres.

Defects that break the symmetry of otherwise orderly material are called topological defects. In solid crystals, they are called dislocations because they interrupt the regularly structured atom lattice. In contrast, topological defects called disclinations take the form of loops in liquid crystals of the nematic variety, whose elongated molecules look like a shoal of fish. New experiments supported by a theoretical model show how defects forming loops around twisted plastic fibres dipped in liquid crystal could be used for the transport of biochemical substances, when controlled by electric and magnetic fields. These findings, published recently, have potential applications in electro-optical micromechanical and microfluidic systems. The loops have the ability to move alongside a translational motion when a magnetic field is applied in a direction oblique to the fibre. This means that by applying such a field, it is possible to control the transport of molecules trapped inside the loops, moving alongside the fibres.

M. Dazza, R. Cabeça, S. Čopar, M. H. Godinho and P. Pieranski, Action of fields on captive disclination loops, Eur. Phys. J. E 40, 28 (2017)

Speed-dependent attraction governs what goes on at the heart of midge swarms (Vol. 48, No. 3)

Trajectories of individual midges within a swarm recorded using high-speed cameras

New study reveals swarm cohesion stems from an adaptive behaviour, where the faster individual midges fly, the stronger the gravitational-like force they experience.

Ever wondered what makes the collective behaviour in insect swarms possible? The authors modelled the effect of the attraction force, which resembles Newton’s gravity force, acting towards the centre of a midge swarm to give cohesion to their group movement. In a recently published work, their model reveals that the gravity-like attraction towards the heart of the swarm increases with an individual’s flight speed. The authors confirmed the existence of such an attractive force with experimental data. They chose to focus on insect swarms, rather than bird flocks or fish shoals, because interactions between neighbouring individuals appear not to play a key role. This makes insect swarms easier to model. Instead of building a model describing the microscale movement of individuals and confronting it with experimental data, the authors built a model of swarm behaviour that is consistent with experimental observations, in terms of swarm density, of individual midges’ speed and acceleration.

A. M. Reynolds, M. Sinhuber and N. T. Ouellette, Are midge swarms bound together by an effective velocity-dependent gravity?, Eur. Phys. J. E 40, 46 (2017)

Twisted waves in a magnetoplasma (Vol. 48, No. 3)

Twisted wave propagating in a magnetized plasma along an arbitrary direction

In recent years, the properties of twisted light beams have been widely explored. In particular, it was realized that twisted laser beams are able to excite twisted density perturbations in a plasma, and that these density perturbations are indeed new forms of twisted waves. Each twisted wave solution is characterized by a topological charge. A further step in the understanding of twisted light was recently made, by studying twisted wave solutions in a magnetized plasma. This leads to a variety of twisted wave solutions, both electrostatic and electromagnetic, depending on the angle of propagation with respect to the static magnetic field. These waves can also be seen as quasi-particles, carrying an intrinsic angular momentum, which is determined by the value of their topological charge.

Furthermore, the kinetic description of a gas of such quasi-particles can also be established. This leads to a generalized concept of plasma turbulence, made of a gas of several types of twisted quasi-particles. An example of application was considered, where two twisted modes with different topological charge interact with each other, exchanging energy and angular momentum inside the plasma.

J. T. Mendonça and J. P. S. Bizarro, Twisted waves in a magnetized plasma, Plasma Phys. Control. Fusion 59, 054003 (2017)

Appearance of deformation in the yttrium isotopic chain (Vol. 48, No. 3)

Evolution of deformation across Y isotopic chain.

In the isotopes of rubidium (Rb), strontium (Sr), ittrium (Y), zirconium (Zr) and niobium (Nb) (i.e., with Z=37-41), a sudden change of the nuclear structure occurs when the number of neutrons reaches N=60. While the nuclei with N<60 exhibit spherical shape in their ground states, the isotopes with N≥60 are significantly deformed. This phenomenon is considered the most dramatic shape change in the nuclear chart. A question was raised of whether the deformed structures appear just at N=60 or they reside also in the lighter isotopes. Indeed, deformed rotational bands built on the excited isomeric states are placed in 95Rb, 96Sr, 98Y, 98-99Zr, i.e., at N=58 and 59, however, nothing was known about location of such collective excitations at N<<58. In our work, it was possible to significantly develop the level scheme of 96Y57 via gamma-coincidence spectroscopy technique. During the analysis, a new 201(30)-ns isomeric state at 1655 keV excitation energy was located and the existence of a rotational band built on it was suggested. This result points to the presence of deformed structures already at N=57 which, with the increasing number of neutrons, gradually decrease in energy, to become dominant at N≥60.

L. W. Iskra and 35 co-authors, New isomer in 96Y marking the onset of deformation at N = 57, EPL 117, 12001 (2017)

Reading between the lines of highly turbulent plasmas (Vol. 48, No. 3)

A short sequence of solitons.

Study shows how to identify highly turbulent plasma signatures in the broadening of the shapes of lines emitted by ions and atoms within.

Plasma, the ionised state of matter found in stars, is still not fully understood, largely due to its instability. Astrophysicists have long-since sought to develop models that can account for the turbulent motions inside plasma, based on observing line shapes emitted by atoms and ions in the plasma. Turbulences are typically detected through the observation of broadened lines due to the Doppler effect, similar to the principle behind radar. In a new study published recently, the authors develop an iterative simulation model that accurately predicts, for the first time, the changes to the line shape in the presence of strong plasma turbulence. Ultimately, the authors aim to provide a system for assessing plasma turbulence that is valid for both a stellar atmosphere and the ITER tokamak designed to generate fusion energy.

R. Stamm, I. Hannachi, M. Meireni, H. Capes, L. Godbert-Mouret, M. Koubiti, J. Rosato, Y. Marandet, M. Dimitrijević and Z. Simić, Line shapes in turbulent plasmas, Eur. Phys. J. D 71, 68 (2017)