Vol. 47 No.3 - Highlights

Back to basics with thermoelectric power (Vol. 47 No. 3)

Determination of the Seebeck coefficient for a circuit composed of two dissimilar materials

New study highlights the role of electron diffusivity when turning waste heat into electricity

Many phenomena in physics, though well-known, are not necessarily widely understood. That’s the case with thermoelectricity, which harnesses waste heat by coupling heat flux and electric current. However, understanding such phenomena is important in order to leave the door open for discovering novel manifestations of them. Thus, even today, physicists working in the area of thermoelectricity continue to ask fundamental questions about the underlying physical process. For example, in a recent study, the authors questioned the nature of the force that puts electrons to work when a temperature difference is applied across a thermoelectric material. Now, they have published a study showing that the force that puts electrons to work to harness the waste heat is linked to the ability of electrons to diffuse through the material. Potential applications in the field of electrical power production from waste heat include thermoelectric devices designed to boost power over a range spanning ten orders of magnitude: typically from microwatts to several kilowatts.

Y. Apertet, H. Ouerdane, C. Goupil and Ph. Lecoeur, A note on the electrochemical nature of thermoelectric power, Eur. Phys. J. Plus 131, 76 (2016)

First measurement of 60Ge β decay (Vol. 47 No. 3)

A sample image recorded by the OTPC detector’s CCD camera showing the particle trajectories. The vertical track corresponds to the 60Ge ion entering the detector and the other track corresponds to the (β-delayed) proton.

60Ge, with its 28 neutrons and 32 protons, is an extremely exotic nucleus, discovered about 10 years ago when only three ions were produced. Its decay properties were measured for the first time in this work. In this experiment, performed at the National Superconducting Cyclotron Laboratory (MSU, USA), the 60Ge ions were produced in 78Kr beam fragmentation reactions and separated from the other reaction products in the A1900 separator. The ions were detected in the active volume of the gaseous time-projection chamber with optical readout (OTPC), where they later decayed. This detector allows exotic decay modes to be identified, even with very small statistics present. The decay of about 20 60Ge ions was observed by β-delayed proton emission yielding a branching ratio of ~100% and a half-life of 20+7-5 ms. This value agrees well with theoretical predictions.

A. A. Ciemny +25 co-authors, First measurement of 60Ge β decay, Eur. Phys. J. A 52, 89 (2016)

Jamming meets Constraint Satisfaction (Vol. 47 No. 3)

A jammed configuration of a binary mixture of spheres on the surface of a larger sphere

Amorphous packing of spheres is also known as jamming. Jamming points are reached through a-thermal compression, when the space where particles can move shrinks to zero. This system is mechanically marginally stable and critical. Differently from usual phase transitions, the critical exponents –which are nontrivial- do not appear to depend on space dimension and have been computed in mean field theory. Our paper connects jamming to the Constraint Satisfaction Problems (CSP) of optimization theory, in the case of continuous variables. We study a neural network, the Perceptron, close to the capacity limit for storing random patterns: the jamming transition of the model. Parameters dependent, one has a convex or a non-convex optimization problem: jamming is non-critical in the former but critical in the latter. Physically in the convex case jamming is approached from a liquid phase while in the non-convex case it occurs from a marginal glass phase. Surprisingly we find the same exponents as in spheres. We conjecture a unique super-universality class for continuous non-convex CSP depending on the glassy nature of the configurations in the vicinity of jamming.

S. Franz and G. Parisi, The simplest model of jamming, J. Phys. A: Math. Theor. 49, 145001 (2016)

Electronic counterpart to ecological models revealed (Vol. 47 No. 3)

Two coupled logistic maps

Peering into the future of populations with the help of complex networks of predictive maps.

Predicting the future from the present—that’s what logistic maps can do. For example, they can be used to predict the evolution of a population in the near future based on its present situation. They are relevant when studing systems such as entire populations, where the behaviour of the separate units—which have the ability to self-organise—cannot explain the behaviour of the system as a whole. The authors have now developed an electronic version of a logistic map that is capable of interacting with many other maps, making the model scalable. As a benchmark to explain new emerging behaviours of entire complex systems, they have studied networks of logistic maps coupled together at various levels. Their findings were recently published and make it possible to more easily compare previous computer simulations with experimental results obtained using this state-of-the art electronic model.

A. L'Her, P. Amil, N. Rubido, A. C. Marti and C. Cabeza, Electronically implemented coupled logistic maps, Eur. Phys. J. B 89, 81 (2016)

Tumble-proof cargo transporter in biological cells (Vol. 47 No. 3)

The average number density field of particles in the vicinity of the motor.

New model shows how collective transport by synthetic nanomotors along biopolymer filaments can be effectively directed

Ever wondered how a molecular nanomotor works when repairing DNA or transporting material such as organelles in the cell? Typically, nanomotors move along biopolymer filaments to go about their duties in the cell. To do so, they use the energy of chemical reactions derived from their surroundings to propel themselves. In a new study published recently, the authors show that small synthetic motors can attach to polymeric filaments and--unlike what previous studies showed--move along without changing either their shape or the direction in which they set out to move. The authors studied the motions of these nanomotors on a filament surrounded by solvent by creating a coarse-grained level biomimetic model featuring all chemical species as particles. They found that the local concentration of catalytic product helping fuel their movement leads to a reversal of the direction of the collective movement of nanomotors, provided that they are in high enough concentration. The work promises to stimulate further research on directed cargo transport to effectively deliver the likes of anti-cancer drugs or anti-pollutants.

M-J. Huang and R. Kapral, Collective dynamics of diffusiophoretic motors on a filament, Eur. Phys. J. E 39, 36 (2016)

Negative refraction without negative-index materials (Vol. 47 No. 3)

Simulation of the electric field of a beam incident on a transformed lens (left) and object (right).

Transformation optics enables engineered metamaterials to manipulate spatial transformations, which can redirect the propagation paths of electromagnetic waves. Such a technology opens up a novel approach to control electromagnetic fields, and hence provides many possibilities to explore new physical phenomena and develop new devices. It can be employed to design unconventional and versatile devices based on metamaterials, such as free-space invisibility cloaks.

The authors have demonstrated a transformation-optics device to generate negative refraction and negative reflection without negative-index materials. In their contribution, they presented two kinds of transformation lenses with anisotropic material parameters: one inhomogeneous lens and one homogeneous lens. Their results verified the unusual properties of the transformation optical devices, and good wave-controlling performance was demonstrated.

W. X. Jiang, D. Bao and T. J. Cui, Designing novel anisotropic lenses with transformation optics, J. Opt. 18, 044022 (2016)

Repulsive Casimir forces at quantum criticality (Vol. 47 No. 3)

Casimir forces act between macroscopic objects immersed in a fluctuating entity, which may be the quantum vacuum or material medium in a state hosting sizable fluctuations. These forces were first discussed in 1948 as an observable manifestation of the quantum nature of the vacuum, lying at the heart of quantum electrodynamics. At a somewhat later stage it was realized that a material medium brought to the vicinity of a critical state induces analogous interactions once some macroscopic bodies become immersed therein.

In most of the known cases the Casimir force is attractive in situations where the bodies in question are identical. This however turns out not to be a general rule.

In our theoretical work we addressed a system of bosonic particles in the vicinity of a quantum critical state, where both thermal and quantum fluctuations are strong. As our exact analysis indicates, the sign of the Casimir force between two bodies immersed in such a medium may be changed by varying the ratio between their separation D and the thermal de Broglie length λ. In the thermal regime D>>λ the force in question is attractive, however, by varying the system setup so that D<<λ one crosses over to a regime admitting repulsive Casimir interactions.

P. Jakubczyk, M. Napiórkowski and T. Sęk, Repulsive Casimir forces at quantum criticality, EPL 113, 30006 (2016)

Timeless thoughts on the definition of time (Vol. 47 No. 3)

The solar year was often determined as the interval between consecutive spring equinoxes when the sun is directly over the equator.

On the evolution of how we have defined time, time interval and frequency since antiquity.

The earliest definitions of time and time-interval quantities were based on observed astronomical phenomena, such as apparent solar or lunar time, and as such, time as measured by clocks, and frequency, as measured by devices were derived quantities. In contrast, time is now based on the properties of atoms, making time and time intervals themselves derived quantities. Today’s definition of time uses a combination of atomic and astronomical time. However, their connection could be modified in the future to reconcile the divergence between the astronomic and atomic definitions. These are some of the observations made by the author of this riveting work published recently, which provides unprecedented insights into the nature of time and its historical evolution.

J. Levine, The history of time and frequency from antiquity to the present day, Eur. Phys. J. H 41, 1 (2016)

Analysing Coulomb-excitation experiments with exotic beams (Vol. 47 No. 3)

Dedicated detection arrays for particle-γ-ray coincidences are now routinely in use at radioactive-ion beam facilities around the world.

This paper presents a number of novel and alternative analysis techniques to extract transition strengths and quadrupole moments from Coulomb excitation data with Radioactive Ion Beams (RIBs) using the GOSIA code. It is anticipated that related approaches and techniques will gain an even greater importance as a wider range of post-accelerated RIBs becomes available at the next generation of ISOL facilities.

Indeed, recent advances in RIB technology, in particular the increasing range of species and post-acceleration energies available from ISOL facilities, have led to a resurgence of the use of nuclear reactions to study the structure of nuclei. Specifically, Coulomb excitation at safe energies, where the surfaces of the colliding nuclei are kept apart (typically 2-5 MeV/A), is now giving unrivalled information on the electromagnetic properties of exotic nuclei, leading to knowledge of the nuclear shape or, more precisely, the charge distribution of individual states. The usefulness of the technique to extract key nuclear-structure information has been demonstrated since the 1960's with stable beam and target combinations.

The paper addresses the situation where new challenges present themselves when studying exotic nuclei, including dealing with low statistics or number of data points, absolute and relative normalization of the measured cross sections and a lack of complementary experimental data, such as excited-state lifetimes, mixing and branching ratios.

M. Zielińska + 9 co-authors, Analysis methods of safe Coulomb-excitation experiments with radioactive ion beams using the GOSIA code, Eur. Phys. J. A 52, 99 (2016)

Charge and spin density in helical Luttinger liquids (Vol. 47 No. 3)

Planar spin helix, pinned by an impurity

Often an electron confined to one spatial dimension does not have many options. It can have spin up or down and it can go right or left. In helical systems the possibilities are further reduced: the spin projection is tied to the direction of motion. This phenomenon is called spin-momentum locking and takes place at the edges of two-dimensional topological insulators. Its consequences are exciting for spintronics: since spin up and down electrons counter-propagate, spin transport can be easily generated by charge currents. In our work, we identify peculiar properties of the weakly interacting helical Luttinger liquid. We address the interesting question: Are the charge density and spin correlations influenced by spin-momentum locking? While for strong interactions the system becomes a Wigner crystal of fractional charges e/2 built on a strongly anisotropic spin wave (which we have shown in a previous publication), in this work, we demonstrate that, for weak interactions, density correlations are featureless, i.e. the density is not affected by impurities. However, spin correlations are well represented by a planar spin helix that can be pinned by magnetic impurities.

N. Traverso Ziani, C. Fleckenstein, F. Crépin and B. Trauzettel, Charge and spin density in the helical Luttinger liquid, EPL 113, 37002 (2016)