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Vol. 46 No.4 - Highlights

How supercooled water is prevented from turning into ice (Vol. 46 No. 4)

How supercooled water is prevented from turning into ice
Representative configurations of ice in its Ic format, which is a simulation in a cube with 216 molecules, Ih format, which is in a rectangular cell with 432 molecules, and in liquid water

Calculating the energy barrier that keeps liquid water below zero from immediately turning into ice provides the key to understanding its ability to be compressed as temperature drops.

Water behaves in mysterious ways. Especially below zero, where it is dubbed supercooled water, before it turns into ice. Physicists have recently observed the spontaneous first steps of the ice formation process, as tiny crystal clusters as small as 15 molecules start to exhibit the recognisable structural pattern of crystalline ice. This is part of a new study, which shows that liquid water does not become completely unstable as it becomes supercooled, prior to turning into ice crystals. The team reached this conclusion by proving that an energy barrier for crystal formation exists throughout the region in which supercooled water’s compressibility continues to rise. Previous work argued that this barrier vanished as the liquid gets colder.

C. R. C. Buhariwalla, R. K. Bowles, I. Saika-Voivod, F. Sciortino and P. H. Poole, Free energy of formation of small ice nuclei near the Widom line in simulations of supercooled water, Eur. Phys. J. E, 38, 39 (2015)
[Abstract]

Topology and Onsager symmetry: How Linear Networks become Exciting (Vol. 46 No. 4)

Topology and Onsager symmetry: How Linear Networks become Exciting
Left: Exemplary linear network containing a loop. Blue dots represent oscillators, red dots represent magnetic couplings that break time-reversal symmetry, e.g., Hall elements. Tuning the system parameters allows to force destructive interference of all signals that propagate from the right port around the two sides of the network. Thus, the system acts as a frequency-independent linear diode. Right: In contrast to systems without magnetic couplings, maximum power can be transferred here with an efficiency that is higher than the fundamental limit of 0.5.

Oscillator networks are omnipresent; they occur, e.g., in electric devices, in mechanical systems, and in biochemistry. However, purely linear networks have limited capabilities. Diode-like, monodirectional transport is usually impossible. Also, maximum power can not be transmitted with an efficiency larger than 1/2. Therefore, engineered networks often contain active or non-linear elements such as transistors. As a possible alternative, the authors investigated purely linear networks containing Lorentz-force-like couplings that break time-reversal symmetry. These networks allow to construct linear diodes with frequency-independent isolation properties. Also, the efficiency at maximum power can approach unity. We show that this surprising system behaviour requires a combination of network loops with magnetic time-reversal symmetry breaking.

B. Sabass, Network topology with broken Onsager symmetry allows directional and highly efficient energy transfer, EPL, 110, 20002 (2015)
[Abstract]

A new generation of chiral nuclear forces (Vol. 46 No. 4)

A new generation of chiral nuclear forces
Neutron-proton differential cross section at Elab=143 MeV up to fourth order in the chiral expansion.

Chiral effective field theory provides a systematically improvable perturbative approach to deriving nuclear forces in harmony with the symmetries of Quantum Chromodynamics. Combined with modern few- and many-body methods, this framework represents a commonly accepted procedure for ab initio studies of nuclear structure and reactions.

In this work, the authors introduce a new generation of nucleon-nucleon forces up to fourth order in the chiral expansion. By employing an appropriate regularization in coordinate space, which maintains the analytic structure of the amplitude, the authors succeed in significantly reducing the amount of finite-cutoff artefacts. In addition, a simple approach to estimating the theoretical uncertainty in few- and many-nucleon calculations from the truncation of the chiral expansion is formulated. By calculating various two-nucleon scattering and bound-state observables, the authors verify that the results at different chiral orders and for different values of the regulator are indeed consistent with each other and with the experimental data. The new generation of chiral nuclear forces is expected to provide an excellent starting point for applications in nuclear physics.

E. Epelbaum, H. Krebs and U.-G. Meißner, Improved chiral nucleon-nucleon potential up to next-to-next-to-next-to-leading order, Eur. Phys. J. A, 51, 53 (2015)
[Abstract]

Prevention of dark currents from photocathodes (Vol. 46 No. 4)

Prevention of dark currents from photocathodes
Field emission maps of an annealed oxide-free (left) and thermally oxidized Mo surface (right). Obviously, the number of emitters providing 1 nA current at fields up to 100 MV/m is reduced by the oxide.

Alkali-based photocathodes deposited in the centre of molybdenum substrates are used as pulsed electron sources in linear particle accelerators. Operation at high electric dc or rf fields is required to obtain a low beam emittance, thus increasing the probability of unwanted dark currents from the cathode surface. Therefore, a field emission scanning microscope was used to localize parasitic electron emitters on single crystal and polycrystalline Mo plugs. In contrast to well-polished and dry-ice cleaned Mo surfaces with native oxide, strong field emission occurred after heat treatments above 400 °C (see figure), which are usually applied before the coating process. Thermal oxidation, however, partially weakened the emitters. X-ray photoelectron spectroscopy confirmed the corresponding changes of the surface oxide layer. These results suggest a selective removal of the native Mo oxide prior to the photocathode deposition to prevent the dark currents in accelerators.

S. Lagotzky, R. Barday, A. Jankowiak, T. Kamps, C. Klimm, J. Knobloch, G. Müller, B. Senkovskiy and F. Siewert, Prevention of electron field emission from molybdenum substrates for photocathodes by the native oxide layer, Eur. Phys. J. Appl. Phys. 70, 21301 (2015)
[Abstract]

Noise produces volcanic seismicity, akin to a drumbeat (Vol. 46 No. 4)

Noise produces volcanic seismicity, akin to a drumbeat
A diagram showing the plug dynamics and the various friction forces at work

A new study shows that relatively small external disturbances play a crucial role in chaotic phenomena like the recent Calbuco volcanic eruption in Chile, leading to drum-beat-like seismicity.

Volcanoes are considered chaotic systems. They are difficult to model because the geophysical and chemical parameters in volcanic eruptions exhibit high levels of uncertainty. Now, the authors have further extended an eruption model—previously developed by other scientists—to the friction force at work between the volcanic plug and volcanic conduit surface. The results provide evidence that volcanic activity can be induced by external noises that would not otherwise have been predicted by the model. The authors show that the external noise is also linked to the appearance of large-amplitude oscillations in the volcanic plug and high seismicity. An increase in noise intensity leads to drumbeat-type plug movement, exhibiting irregular periodicity dependent on noise. Such beat-type behaviour is a building block for understanding the physical mechanisms of volcanic drumbeat seismicity.

D. V. Alexandrov, I. A. Bashkirtseva and L. B. Ryashko, How a small noise generates large-amplitude oscillations in the volcanic plug and produces high seismicity, Eur. Phys. J. B 88, 106 (2015)
[Abstract]

Does knowing the opponent's strategy guarantee optimal play? (Vol. 46 No. 4)

Does knowing the opponent's strategy guarantee optimal play?
Reentrant phase transition between two cyclically dominant strategy triplets who form alliances against the fourth strategy. The triples are indicated by curved arrows in the inset.

Methods of statistical physics are proving indispensable for the study of evolutionary games in structured populations. The evolution of cooperation and the phase transitions leading to favorable evolutionary outcomes depend sensitively on the structure of the interaction network and the type of interactions, as well as on the number and type of competing strategies. Now, physicists have solved the puzzle of the availability of information in evolutionary games. In a new theoretical model, the authors answer whether knowing the strategy of an opponent is indeed the holy grail of optimal play in social dilemmas, or whether the situation is in fact more complex. It is indeed the latter, as final evolutionary outcomes depend sensitively not just on individual relations between the competitors as determined by payoff elements, but equally strongly on the spatiotemporal dynamics of defensive alliances that emerge spontaneously as a result of strategic complexity. Reentrant phase transitions highlight the fact that the viability of an alliance depends sharply on the invasion speeds between group members who cyclically dominate each other.

A. Szolnoki and M. Perc, Reentrant phase transitions and defensive alliances in social dilemmas with informed strategies, EPL, 110, 38003 (2015)
[Abstract]

Novel plasma diagnostics method (Vol. 46 No. 4)

Novel plasma diagnostics method
Sketch of the momentum fluxes across the sheath edge

Could the mundane action of switching on an energy saving light bulb still hold secrets? It does, at least for physicists. These bulbs are interesting because they contain low-temperature plasma—a gas containing charges from ions and electrons. Now, the authors have developed a method that could be used for measuring the increase in the plasma force on the inner side of such a light bulb when the light is switched on. These findings have implications for plasma diagnostics concerning plasma-wall interactions used in surface modification and the production of thin film solar cells and microchips. This could lead to a promising new kind of plasma diagnostics, providing insights into processes that conventional electrical probes can’t detect.

T. Trottenberg, T. Richter and H. Kersten,, Measurement of the force exerted on the surface of an object immersed in a plasma, Eur. Phys. J. D 69, 91 (2015)
[Abstract]

The importance of rheology in tissue development (Vol. 46 No. 4)

The importance of rheology in tissue development
The dissipation function formalism helps construct constitutive equations relating the mechanical stresses with intracellular variables such as cell deformations and intercellular variables such as cell velocities and cell neighbour changes. Experiments where cells are forced to flow in a narrow geometry and where these variables are measured help to infer and/or test such equations.

Our understanding of biomechanics increasingly improves through the use of physics models. There are some intriguing biological questions regarding the interplay between the behaviour of cells and the mechanics at the level of tissues. For example, how does a collective behaviour, not apparent at the cell scale, emerge at the tissue level? Or how can the mechanical state of a tissue affect the cell division rate or the orientation of cells undergoing division?

The authors think that the interplay between genes and mechanics is key to understanding how the adult shape emerges from a developing tissue.

They construct rheological diagrams based on insights concerning the mechanics of the biological tissue. One of the main insights is a distinction between intra-cellular and inter-cellular mechanism. The local rheological equations obtained allow to generate a complete spatial model expressed as a set of partial differential equations. This procedure is conducted not only in the case of small elastic deformations, but also in the relevant, less discussed, case of large elastic deformations. The authors provide a functional and versatile toolbox for tissue modelling and propose a framework for a tensorial treatment of heterogeneous tissues. Although the simplest applications concern in vitro experiments, the same approach may be used for many other living tissues including animal tissues during development, wound healing, or carcinogenesis.

S. Tlili, C. Gay, F. Graner, Ph. Marcq, F. Molino and P. Saramito, Colloquium: Mechanical formalisms for tissue dynamics, Eur. Phys. J. E, 38, 33 (2015)
[Abstract]

Self-imaging process at the near field of cylindrical convex gratings (Vol. 46 No. 4)

Self-imaging process at the near field of cylindrical convex gratings
Experimental set-up used to obtain the near field diffraction pattern.

Diffraction gratings have become one of the most used optical elements. Their behaviour has been extensively analysed from many diverse points of view. From a general sight, diffraction gratings produce diffraction orders at the far field and self-images at the near field. The applicability of diffraction gratings is quite extensive. They can be found as fundamental parts of many different devices such as telescopes, spectrometers, optical encoders, etc.

One particular kind of optical encoder uses cylindrical convex gratings. The authors show the near-field diffraction of cylindrical convex gratings illuminated by a general source that can be punctual or finite, monochromatic or polychromatic. They analyse how the size and polychromatism of the source affect the self-imaging process of cylindrical convex gratings. A decrease in the self-images contrast is produced for finite non-punctual sources. On the other hand, polychromaticity of the source produces quasi-continuous diffraction fringes from a certain distance forward.

All the results have been proven by experiments and could be helpful in applications that include convex diffraction gratings.

F. J. Torcal-Milla, L. M. Sanchez-Brea and E. Bernabeu,, Near field diffraction of cylindrical convex gratings, J. Opt., 17, 035601 (2015)
[Abstract]

Fragmentation of random trees (Vol. 46 No. 4)

Fragmentation of random trees
The random forest

Networks are ubiquitous, appearing in the study of subjects as diverse as gene-protein interactions, power grids, and algorithms.

The function of a network is closely linked to its structure. For instance, in biochemical reaction networks, removal of a species or reaction can dramatically change the output of the system. Evolving networks often undergo degradation, making it important to understand how the structure breaks apart when components are randomly removed, also revealing how resilient a network is to attacks.

We studied the fragmentation of a random tree, a network formed by repeatedly attaching new nodes to an existing node chosen uniformly randomly.

We present exact equations governing the evolution of fragment sizes after a fraction of the nodes are removed at random, along with asymptotic solutions. For very large trees, fragment size distribution decays as a power law, with an exponent of 1+1/m, m being the fraction of remaining nodes. This implies that a few very large fragments coexist with many small ones (see figure).

Our findings reveal unusual fragmentation kinetics, where the fragment size distribution is characterized by a time-dependent exponent, and can provide insight into other fragmentation processes where dynamic parameters are observed.

Z. Kalay and E. Ben-Naim, Fragmentation of random trees, J. Phys. A, Math. Theor., 48, 045001 (2015)
[Abstract]