Vol. 49 No. 5-6 - Highlights

Is the relation between mass and energy universal? (Vol. 49 No.5-6)

The energy of ordinary particles is related to their mass through the famous relation E=mC2, where m is both the inertial and the gravitational mass of the particle. This energy is minimum when the momentum p of the particle is p=0. Things are completely different if the energy is minimum for a momentum p=po≠0. The inertial mass density of a gas of such particles is then ρ=npo2/(3kBT), where n is the density of particles, and T the temperature of the gas. It is not related to the energy density.

Condensed matter gives an example of such particles. Rotons, which are excitations of superfluid 4He, have their energy minimum at a finite momentum. They largely contribute to the inertial mass density of the “normal fluid” in the two fluid model of superfluid 4He. Nothing similar has been evidenced, up to now, within the cosmological particles, but one can raise the question: would the gravitational mass be related to the energy or the inertial mass? Assuming that gravitational and inertial mass densities are the same gives for the gas of such particles properties close to those expected for Dark Energy. This work is a discussion about these questions.

B. Castaing, What is the gravitational mass when energy and inertial mass are not equivalent?, EPL 123, 20003 (2018)

Antimatter study to benefit from recipe for ten-fold spatial compression of plasma (Vol. 49 No.5-6)

Example of raw images from the detector for identical particle operations with antiproton detection (left) and electron detection (right)

Improving the spatial compression of a mixed matter-antimatter trapped plasma brings us one step closer to grasping the acceleration of antimatter due to Earth’s gravity.

An international team of physicists studying antimatter have now derived an improved way of spatially compressing a state of matter called non-neutral plasma, which is made up of a type of antimatter particles, called antiprotons, trapped together with matter particles, like electrons. The new compression solution, which is based on rotating the plasma in a trapped cavity using centrifugal forces like a salad spinner, is more effective than all previous approaches. In this study published recently, the team shows that — under specific conditions — a ten-fold compression of the size of the antiproton cloud, down to a radius of only 0.17 millimetres, is possible. These findings can be applied in the field of low-energy antimatter research, charged particle traps and plasma physics. Further, this work is part of a larger research project, called AEgIS, which is intended to achieve the first direct measurement of the gravitational effect on an antimatter system. The ultimate goal of the project, which is being pursued at CERN, the Particle Physics Laboratory in Geneva, Switzerland, is to measure the acceleration of antimatter — namely antihydrogen — due to Earth’s gravity with a precision of 1%.

S. Aghion and 61 co-authors, Compression of a mixed antiproton and electron non-neutral plasma to high densities, Eur. Phys. Jour. D 72, 76 (2018)

Clearer vision of the biochemical reaction that allows us to see (Vol. 49 No.5-6)

Probability of the nuclei crossing the energy barrier over time

Physicists develop improved algorithms for simulating how complex molecules respond to excitation by photons, and explaining what happens when photons hit our eyes.

What makes it possible for our eyes to see? It stems from a reaction that occurs when photons come into contact with a protein in our eyes, called rhodopsin, which adsorbs the photons making up light. In a paper published recently the authors propose a refined approximation of the equation that describes the effect of this photo-excitation on the building blocks of molecules. Their findings also have implications for other molecules, such as azobenzene, a chemical used in dyes. The incoming photon triggers certain reactions, which can result, over time, in dramatic changes in the properties of the molecule itself. This study was included in a special anniversary issue of EPJB in honour of Hardy Gross.

F. Agostini, I.Tavernelli, and G. Ciccotti, Nuclear Quantum Effects in Electronic (Non)Adiabatic Dynamics, Eur. Phys. Jour. B 91, 139 (2018)

Solid deuterium surface degradation at ultracold neutron sources (Vol. 49 No.5-6)

As shown in the figure for the PSI UCN source, the intensity decrease can be fully recovered by a conditioning process, removing the frost from the sD2 and restoring good surface quality.

Highest intensities of ultracold neutrons (UCN) are in worldwide demand for fundamental physics experiments. Tests of the Standard Model of particle physics and searches for physics beyond it are performed with UCN.

Two of the leading UCN sources, at Paul Scherrer Institute (PSI) and at Los Alamos National Laboratory (LANL), are based on solid deuterium (sD2) at temperatures around 5 K. Here, together with NCSU they joined forces to understand UCN intensity decreases observed during pulsed neutron production. The study shows that the decrease can be completely explained by the build-up of frost on the sD2 surface during operation. Pulsed proton beams hitting the spallation targets generate heat pulses causing cycles of D2 sublimation and subsequent resublimation on the sD2 surface. Even very small frost flakes can act as total reflectors for UCN and cause an intensity decrease. Optical observation of the sD2 surface at NCSU – not possible at the operating spallation neutron sources – confirmed a severe surface degradation due to heat pulsing with an external heater in strong support of the frost model.

A. Anghel and 29 co-authors, Solid deuterium surface degradation at ultracold neutron sources, Eur. Phys. J. A 54, 148 (2018)

Physical properties of solids elucidated by zooming in and out of high resolution (Vol. 49 No.5-6)

A new study shows how to couple highly accurate and simplified models of the same system to extract thermodynamics information using simulations

Computer simulations are used to understand the properties of soft matter—such as liquids, polymers and biomolecules like DNA –which are too complicated to be described by equations. They are often too expensive to simulate in full, given the intensive computational power required. Instead, a helpful strategy is to couple an accurate model—applied in the areas of the system that require greater attention—with a simpler, idealised model. In a paper published recently, the authors make the accurate model in high-resolution coincide seamlessly with an exactly solvable representation at lower resolution.

M. Heidari, R. Cortes-Huerto, K. Kremer, and R. Potestio , Concurrent coupling of realistic and ideal models of liquids and solids in Hamiltonian adaptive resolution simulations, Eur. Phys. J. E 41, 64 (2018)

Turning graphene into light nanosensors (Vol. 49 No.5-6)

Absorption on the plane of incident angle and wavelength

Tuning the graphene embedded in a photonic crystal by varying the external temperature can transform it into a light-sensitive sensor

Graphene has many properties; it is e.g. an extremely good conductor. But it does not absorb light very well. To remedy this limiting aspect of what is an otherwise amazing material, physicists resort to embedding a sheet of graphene in a flat photonic crystal, which is excellent for controlling the flow of light. The combination endows graphene with substantially enhanced light-absorbing capabilities. In a new study published recently, the authors demonstrate that, by altering the temperature in such a hybrid cavity structure, they can tune its capacity for optical absorption. They explain that it is the thermal expansion and thermo-optical effects which give the graphene these optical characteristics. Potential applications include light sensors, ultra-fast lasers, and systems capable of modulating incoming optical beams.

A. Rashidi and A. Namdar, Tunability of temperature-dependent absorption in a graphene-based hybrid nanostructure cavity, Eur. Phys. J. B 91, 68 (2018)

Signature of Fermi arc surface states in Andreev reflection (Vol. 49 No.5-6)

Examples of dV/dI(V) characteristics with subgap resonances for two Nb-WTe2 junctions.

Weyl semimetals are conductors which are characterized by topologically protected conducting surface states. In contrast to three-dimensional topological insulators described by Z2 invariant, Weyl surface states inherit the chiral property of the Chern insulator edge states, similarly to the quantum Hall effect regime. To observe this difference in symmetry, we experimentally investigate charge transport through the junction between a niobium superconductor and a three-dimensional WTe2 Weyl semimetal. In addition to classical Andreev reflection, we observe sharp non-periodic subgap resistance resonances. From an analysis of their positions, magnetic field and temperature dependencies, we can interpret them as an analog of Tomasch geometrical oscillations for transport along the topological surface state across the region of proximity-induced superconductivity at the Nb-WTe2 interface. The crucial point is that observation of distinct geometrical resonances implies a specific transmission direction for carriers, which is impossible for trivial two-dimensional surface states in planar junctions without strict axial symmetry. In contrast, for Weyl chiral surface states the preferable direction is present, forming a specific transmission direction for surface carriers. Thus, observation of distinct geometrical resonances is a hallmark of the Fermi arc Weyl surface states.

A. Kononov, O. O. Shvetsov, S. V. Egorov, A. V. Timonina, N.N.Kolesnikov and E. V. Deviatov, Signature of Fermi arc surface states in Andreev reflection at the WTe2 Weyl semimetal surface,

Producing hydrogen from splitting water without splitting hairs (Vol. 49 No.5-6)

Adsorption of water molecules on the surface of copper nanoparticles could produce hydrogen faster and more efficiently

New model explains interactions between small copper clusters used as low-cost catalysts in the production of hydrogen by breaking down water molecules

Copper nanoparticles dispersed in water or in the form of coatings have a range of promising applications, including lubrication, ink jet printing, as luminescent probes, exploiting their antimicrobial and antifungal activity, and in fuel cells. Another promising application is using copper as a catalyst to split water molecules and form molecular hydrogen in gaseous form. At the heart of the reaction, copper-water complexes are synthesised in ultra-cold helium nanodroplets as part of the hydrogen production process, according to a recent paper published recently. For its authors, splitting water like this is a good way of avoiding splitting hairs. In their study, they synthesised neutral copper-water complexes by successively doping helium nanodroplets with copper atoms and water molecules. These droplets are then ionised by electrons. The authors show that the composition of the most prominent ions depends on the partial copper and water pressures in the cell where the reaction occurs. They observe ions containing several copper atoms and several dozen water molecules.

S. Raggl, N. Gitzl, P. Martini, P. Scheier, and O. Echt , Helium nanodroplets doped with copper and water, Eur. Phys. Jour. D 72, 130 (2018)

Image-guided restricted drug release in friendly implanted therapeutics (Vol. 49 No.5-6)

This review exposes a possible therapeutics scheme of using active implants for restricted drug release, accounting for friendly wellbeing and security of patient. Friendly therapeutics ought to use controlled drug release with minimally-invasive and non-ionizing techniques. The review of different issues elucidates that the strategy sought may use non-ionizing image-guided drug release embedded implant, which is powered and controlled wirelessly by an external source. The analysis of the principal biomedical imagers indicates the MR (magnetic resonance) imager as the most adequate non-ionizing solution.

The review of MRI (magnetic resonance imaging) technology suggests an optimization of performance versus biological effects, as well as of compatibility with hosting materials in its environment. For the sake of such compatibility, an EMC (electromagnetic compatibility) analysis has been performed considering the nature of different MRI fields and their conventional protections and corrections.

A possible future strategy would consist of an interactive system operating autonomously. Such a system is composed of the imager, the implant and its external wireless powering/control device. It integrates an AI (artificial intelligence) algorithm and has to operate under the supervision of the health-care team.

A. Razek, Towards an image-guided restricted drug release in friendly implanted therapeutics, Eur. Phys. J. Appl. Phys. 82, 31401 (2018)

Better chemo drug adsorption onto targeted delivery capsules (Vol. 49 No.5-6)

Adsorption of chemo drug onto active carbon with aluminium inclusion

New study demonstrates adsorption of chemotherapy drugs onto active carbon delivery capsule can be enhanced with aluminium atom inclusions

The efficacy of chemotherapy treatment depends on how effectively it reaches cancerous cells. Increasing targeted delivery could mean decreasing side effects. Scientists are enhancing methods of selectively transmitting active chemotherapy agents and reducing their toxicity by encapsulating chemo drugs into active carbon used as the targeted delivery device. In a new study published recently, the authors have demonstrated that adding minute amounts of aluminium atoms onto activated carbon atoms helps increase the adsorption onto the delivery carbon capsule of a standard chemotherapy drug, called 5-Fluorouracil (5-FU). This drug is typically used for stomach, colorectal, neck and head cancer treatments. This model could lead to more effective and convenient cancer treatments with fewer side effects by encapsulating the chemo drug into the active carbon, so that it can be taken orally.

G. Román, E. Noseda Grau, A. Diaz Compañy, G. Brizuela, A. Juan, and S. Simonetti, A first-principles study of pristine and Al-doped activated carbon interacting with 5-Fluorouracil anticancer drug, Eur. Phys. J. E 41, 107 (2018)

Nonexistence of PT-symmetric gain-loss photonic quantum systems (Vol. 49 No.5-6)

Realizing non-Hermitian systems by coupling to a reservoir

Our common understanding of quantum mechanics relies on the Hamiltonian operator describing any quantum mechanical system to be Hermitian. This has been challenged 20 years ago by the discovery that, for an operator to possess real eigenvalues, it only needs to be invariant under combined parity-time (PT) symmetry operations. This had profound impact on photonics where potential landscapes with tailored gain and loss for electromagnetic waves can easily be implemented.

However, this is as far as the analogy to quantum mechanics can be taken. A straightforward implementation of gain-loss structures for quantum states of light - even for the most classical ones, coherent states - fails. As the authors show in this article, concatenating lossy and amplifying media turn coherent states into thermally broadened quantum states, whose first moments (i.e. their amplitudes) are retained, but whose variances are increased proportional to the amount of gain.

This shows that PT-symmetric quantum optics cannot be implemented within the prevailing paradigm of using distributed gain and loss. The wider consequence of this simple result hints at the limits of simulating quantum physics beyond wave mechanics using photonic quantum systems.

S. Scheel and A. Szameit, PT-symmetric photonic quantum systems with gain and loss do not exist, EPL 122, 34001 (2018)

Self-extension model of slime mold’s allorecognition behaviour (Vol. 49 No.5-6)

Two typical encounter cases of Physarum rigidum (plasmodium). They can recognize an encounter as self or non-self and decide to fuse (a) or avoid (b) allogeneic individuals.

When slime molds encounter an allogeneic individual, they judge whether to fuse or avoid it. This decision can be made without coming in contact with each other.

Slime molds move, feed, and grow during single-celled amoeboid stage—plasmodium. They can divide into multiple individuals and fuse. In this study on Physarum rigidum, an interesting behaviour was observed when they encountered an allogeneic individual. The plasmodia stopped their movement, came in contact with each other at the cell membrane surface, and then decided their actions. If they judge the encounter can become “self”, they fused, and if they recognize it as “non-self”, they avoided each other. This allorecognition behaviour can sometimes take several hours. More importantly, this behaviour can occur without contact between cell membranes. It is impressive to observe plasmodia stay apart from each other and decide their behaviour. In our study, we clarified that this behaviour, i.e., non-contact allorecognition, occurs with the spread of slime sheath, which is hyaline mucus secreted by plasmodium. Plasmodium diffuses slime sheath as an information substance of “self” to the environment, and it can be called "self-extension".

M. Masui, S. Satoh and K. Seto, Allorecognition behaviour of slime mold plasmodium—Physarum rigidum slime sheath-mediated self-extension model, J. Phys. D: Appl. Phys. 51, 284001 (2018)

Scoping magnetic fields out for prevention (Vol. 49 No.5-6)

Magnetic field generated by a current (I) through a finite wire

A new study reveals how to best evaluate the circulation of magnetic fields around closed loops

Concerns about the effects of magnetic fields on human health require careful monitoring of our exposure to them. Mandatory exposure limits have been defined for electric and hybrid vehicle architectures, in domestic and work environments, or simply to shelter sensitive devices from unintended sources of magnetic disturbance. In a new study published recently, the authors develop a method for deriving an approximate value of the circulation around a loop of the magnetic field generated by the flow of electric current in an arbitrarily-shaped wire of a given length. In this study, the authors set out to adapt Biot-Savart’s law, which describes the magnetic field generated by finite wires, to evaluate the circulation of such fields around a closed path or loop. This led the authors to a mathematical formula that, as the finite wire thickness decreases to zero, becomes identical to one of their recent research results expressing the magnetic field circulation as a function of the wire current and of the solid angles between the circulation path and each of the conducting wire’s endpoints.

J. M. Ferreira and J. Anacleto, The magnetic field circulation counterpart to Biot-Savart’s law, Eur. Phys. J. Plus 133, 234 (2018)