epn

Vol. 43 No.4 - Highlights

Electric charge disorder: A key to biological order? (Vol. 43 No. 4)

image Sphere rotating next to a planar substrate, both carrying random charges on their surfaces.

Electrically net-neutral objects are found to attract strongly if a small amount of charge disorder is present on their surface, holding the key to a possible understanding of biological pattern recognition. This article demonstrates that random patches of disordered, frozen electric charges spread throughout surfaces, which are overall neutral, can interact with the long-range twisting force strong enough to be felt across the whole mesoscopic scale and compete with Casimir-van der Waals forces.

The twisting forces acting on a randomly charged sphere mounted on a central axis, which is next to a randomly charged flat substrate, are investigated. Because small amounts of positive and negative charges are spread in a disordered mosaic throughout both surfaces, they induce transient attractive or repulsive twisting forces, regardless of the surfaces’ overall electrical neutrality. These forces’ fluctuations are studied using statistical averaging methods.

It appears that the fluctuations’ root-mean-square value grows in proportion with the total area of the two apposed surfaces. By contrast, it only decreases in inverse proportion to the distance separating the sphere from the substrate. This counter-intuitive result suggests that the long-range twisting force, created by virtue of the disorder of surface charges, is expected to be much stronger and longer-ranged than the elusive Casimir-van der Waals forces.

This could have implications for biological pattern recognition, such as lock and key phenomena based on attraction between biological macromolecules leading to pre-alignment prior to their interaction.

Sample-to-sample torque fluctuations in a system of coaxial randomly charged surfaces
A. Naji, J. Sarabadani, D.S. Dean and R. Podgornik, Eur. Phys. J. E, 35, 24 (2012)
[Abstract]

Superconducting strip: an ultra-low-voltage sensor? (Vol. 43 No. 4)

image Moving vortices inside the weak conducting links carved into a current-carrying superconducting strip.

Minute-scale interactions govern superconductors’ electronic behaviour with potential applications for voltage measurement techniques.

Studying a superconducting strip, an intermittent motion of magnetic flux carrying vortices inside the regularly spaced weak conducting regions has been observed. These vortices resulted in alternating static phases with zero voltage and dynamic phases characterised by non-zero voltage peaks in the superconductor. One knows that superconductors subjected to sufficiently strong magnetic fields feature vortices carrying quantized amounts of magnetic flux. The authors relied on the Ginzburg-Landau theory to study the dynamic of the nanometric to millimetric-scale-width superconducting strip, which was subjected to a magnetic field applied at a right angle and a current applied alongside its length.

It is found that increasing magnetic field also increases the density of mutually-repelling vortices, which in the presence of an external current stimulates vortex motion across the strip. At the same time, the barrier for vortex entry and exit on the strip boundaries is also dependent on the magnetic field. This interplay of magnetic-field-dependent barriers and vortex-vortex interaction results in an on/off vortex motion in increasing magnetic fields.

Eventually, these findings could be applicable in gate devices used to control various modes of on/off states in electrical systems operating in specific windows of temperature, applied magnetic field, current and voltage.

Dynamic and static phases of vortices under an applied drive in a superconducting stripe with an array of weak links
G.R. Berdiyorov, A.R. de C. Romaguera, M.V. Milosevic, M.M. Doria, L. Covaci and F.M. Peeters, Eur. Phys. J. B, 85, 130 (2012)
[Abstract]

Auger electron energy spectrum from N2: new visit (Vol. 43 No. 4)

image The top panel shows the experimental setup. The bottom panels show the angle resolved electron yield for different Auger electron energies (circles) along with the calculated angular distributions (dashed) for different final state symmetries. The bottom right panel shows the molecular distribution.

This article presents a study of electronic relaxation of core-excited molecules. Relaxation occurs through the Auger process where a valence electron fills the core vacancy, created by x-ray ionization, and a second valence electron is released from the molecule. The kinetic energy of this second electron then depends on the final state of the core-relaxed molecule. The Auger electron energy spectrum can therefore be viewed as a kind of fingerprint for molecular composition.

In this study, an infrared laser pulse was used to align an ensemble of nitrogen molecules relative to the laboratory frame. We then photo-ionized the molecules with a ~60 fs x-ray laser pulse. Rotating the polarization of the infrared laser, as shown in the Fig., changed the orientation between the aligned molecules and the laboratory frame electron detector. In this way we measured the angular pattern of the entire Auger electron spectrum in the molecular frame as shown in the Fig.

This experiment demonstrates a new way to measure the molecular-frame angular emission pattern for every Auger electron feature. Adding angular information to the spectral information allows incorporating electronic symmetry in feature identification. These findings suggest reordering some previous Auger feature assignments in the seemingly well-known N2, showcasing the power of this method to measure transient changes in electronic symmetry as a molecule undergoes a chemical reaction.

Molecular frame Auger electron energy spectrum from N2
J. P. Cryan, R. N. Coffee and 32 co-authors, J. Phys. B: At. Mol. Opt. Phys. 45 (2012) 055601
[Abstract]

Make or break for cellular tissues (Vol. 43 No. 4)

image Active, living cells remain governed by the law of physics: image after 1300s.

Models developed to study liquids are used to investigate the mechanics of cellular tissues, which could further our understanding of embryonic development and cancer.

The present study demonstrates that the behaviour of a thin layer of cells in contact with an unfavourable substrate is akin to that of thin fluid or elastic films. Understanding the mechanism by which a thin layer of cells splits into disjointed patches, thus breaking the layer’s structural integrity, bears great significance because the human tissue, or epithelium, covering organs can only fulfil its role if here are no holes or gaps between the cells.

Thanks to the analogy between the cellular layer examined and the well-understood behaviour of thin liquid films, the authors devised a model of the layer’s evolution. They considered it as an active, amorphous material made of a continuum of cells. Because it is subject to a constant competition between neighbouring cell-cell and cell-substrate adhesion, it can either maintain its contiguous structure or break.

The authors investigated the layer’s stability when subjected to chemical and physical disturbances. In particular, they scrutinised how the cellular layer reacted to a non-adhesive substrate with little chemical affinity with the cells. They also subjected the cell to a physical disturbance by laying them in substrates with low stiffness, such as soft gels.

So, the so-called de-wetting phenomenon has been observed, whereby the cellular layer is ruptured leading to islands of cells interspersed with dry patches. The de-wetting phenomenon is therefore due to the cells' distinctive sensitivity to the nature of its substrate, particularly to its decreased stiffness.

De-wetting of cellular monolayers
S. Douezan and F. Brochard-Wyart, Eur. Phys. J. E, 35, 34 (2012)
[Abstract]

A note on Pöschl-Teller black holes (Vol. 43 No. 4)

An interesting feature of black holes is the existence of quasi-normal modes, arising because the system has a peak in the wave potential (scalar, electromagnetic, or gravitational waves). The quasi-normal mode is excited when a disturbance is put in the field near but outside the black hole, (like a wave packet roughly in a circular orbit near the peak).

The excitation then propagates outward and inward and decays. An excitation “mode” has a definite complex frequency: a given oscillation rate in time, and a corresponding decay rate. For gravitational radiation from a spherical (Schwarzschild) black hole, the least damped mode is: ei 0.747t/tH e0.178t/tH with tH the time for light to travel a distance equal to the radius of the black hole [S. Chandrasekhar and S. Detweiler, Proc. Roy. Soc. London, 344 (1975) 441].

To calculate these modes is typically a computational problem, with attendant difficulties in controlling errors and convergence. A partial step to ameliorate these difficulties has been to substitute the black hole potential (long range, polynomial decay to infinity), with more localized potentials decaying exponentially at infinity. Pöschl and Teller [G. Pöschl and E. Teller, Z. Phys. 83 (1933) 143] suggested one such potential in another context: 1/cosh2 α(r- r0).

This is much simpler – and decays more rapidly – than the correct gravitational potential, but to date even this potential has required numerical/computational treatment. Now, however, Zarrinkamar, Hassanabadi and Eskolaki have found an ingenious analytic transformation of the Pöschl-Teller wave equation with immediate solution in terms of Jacobi polynomials. Jacobi polynomials are well studied and characterized classical “special functions”. Thus questions of accuracy and convergence are now under control, and Zarrinkamar et al. have completely solved the quasi-normal mode problem for the Pöschl-Teller black hole.

A note on Pöschl-Teller black holes
S. Zarrinkamar, H. Hassanabadi, A.A. Rajabi and P. Ghalandari Eskolaki, Eur. Phys. J. Plus 127, 56 (2012) [Abstract]

The game of go as a complex network (Vol. 43 No. 4)

image Moduli squared of right eigenvectors of the 7 largest eigenvalues of the Google matrix for the first 100 most frequent moves, showing that each eigenvector is localised on specific moves.

The study of complex networks attracts more and more interest, fuelled in particular by the development of communication and information. It turns out that such networks can also modelize many important aspects of the physical world or of social interactions. However, they have never been used in the study of games.

Games have been played for millennia, and besides their intrinsic interest, they represent a privileged approach to the working of human decision-making. They can be very difficult to modelize or simulate: only recently were computers able to beat chess champions. The old Asian game of go is even less tractable, as no computer program has been able to beat a very good player.

The paper presents the first study of the game of go from a complex network perspective. It constructs a directed network, which reflects the statistics of tactical moves. Study of this network for datasets of professional and amateur games shows that the move distribution follows Zipf's law, an empirical law first observed in word frequencies. Differences between professional and amateur games can be seen, e.g. in the distribution of distances between moves. The constructed network is scale-free, with statistical peculiarities, such as the symmetry between ingoing and outgoing links distributions. The study of eigenvalues and eigenvectors of the matrices used by ranking algorithms singles out certain strategic situations (see figure), and vary between amateur and different professional tournaments. These results should pave the way to a better modelization of board games and other types of human strategic scheming.

The game of go as a complex network
B. Georgeot and O. Giraud, EPL, 97, 68002 (2012)
[Abstract]

Molecular machines with continuous phase space (Vol. 43 No. 4)

image EMP for a tightly coupled motor as a function of the chemical driving force, for different values of the asymmetry. Note the regime in which the EMP exceeds 50%.

Molecular motors exploit the free energy released in the hydrolysis of energetic molecules like ATP to perform work useful for the cell. It is therefore important to know the efficiency of this process, i.e., the ratio between the performed work and the released free energy. The efficiency could reach 100% if the motor worked reversibly, i.e., infinitely slowly, but then its output power would vanish. Thus the relevant quantity is the Efficiency at Maximum Power (EMP). It has been shown that the EMP reaches 50% when the motor operates in the linear regime close to equilibrium. However, it has only recently been investigated further from equilibrium in models describing the motor as a discrete random process.

One can provide a more fundamental model of a molecular motor as a Brownian particle evolving in a two-dimensional continuous space, in which one coordinate represents its spatial position on the substrate and the other coordinate the advancement of the ATP-hydrolysis reaction, subject to a periodic “egg-carton” potential, whose tilt in the direction of the chemical coordinate expresses the free-energy imbalance. We have evaluated the EMP for such a model, with special choices of the potential, and found that it reaches the highest values when the displacements in the spatial and chemical coordinates are tightly bound: in this regime, efficiencies larger that 50% can be reached sufficiently far from equilibrium. When the binding is not tight, the EMP decreases since the motor can perform a chemical hydrolysis cycle without advancing. Our formalism thus allows us to gain a deeper insight into the connection between the mechanics and the thermodynamics of molecular motors.

Efficiency of molecular machines with continuous phase space
N. Golubeva, A. Imparato and L. Peliti, EPL, 97, 60005 (2012)
[Abstract]

The neutron-rich superheavy element 116 confirmed (Vol. 43 No. 4)

image Cross-sections and cross-section limits of the reaction 48Ca + 248Cm → 296116* measured elsewhere and in this work. The data for synthesis of 293116 (3n channel, triangles) and 292116 (4n channel, squares) are shown. The experimental data are compared with results of theoretical calculations.

The synthesis of a superheavy element with the proton number Z=116 has been studied at the velocity filter SHIP of GSI in Darmstadt using a 48Ca beam on radioactive 248Cm targets. At excitation energies of the compound nuclei of 40.9 MeV, four decay chains were measured, which were assigned to the isotope 292116 produced in 4n channel, and one chain, which was assigned to 293116 produced in 3n channel. All chains are terminated by spontaneous fission decays of either 277Hs or 284Cn isotopes on the shoreline of the neutron-rich superheavy island.

Measured cross-sections of 3.4 pb and 0.9 pb, respectively, and decay data of the chains confirm previous data at the Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna. As a new result, one alpha-decay chain was measured, which terminates after four alpha decays by spontaneous fission. The alpha energies of the second to fourth decay are considerably higher than those measured for the alpha decays of 289114, 285Cn, and 281Ds and the spontaneous fission half-life is significantly longer than that of 277Hs measured in previous experiments.

Possible assignments and role of isomeric states are discussed in the frame of excited quasiparticle states of nuclei populated in the decay chain from 293116.

The experience gained in this experiment will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP. For this purpose, related very detailed experimental study of sources of background fission events was also carried out and published in a related article. Here it was shown that such events occur mainly in connection with transfer reactions leading to target-like residues having half-lives similar to the ones of superheavy isotopes from respective fusion reactions.

The reaction 48Ca + 248Cm → 296116* studied at the GSI-SHIP
S. Hofmann et al. (38 co-authors), Eur. Phys. J. A, 48, 62 (2012)
[Abstract]

The source of “background” fission events in experiments on superheavy elements
S. Heinz et al. (9 co-authors), Eur. Phys. J. A, 48, 32 (2012)
[Abstract]

Mitigating disasters by hunting down Dragon Kings (Vol. 43 No. 4)

image Dragons Kings stem from a combination between the supernatural powers of dragons and the anomaly of a king’s wealth relative to that of his subjects. © Anek Suwannaphoom /photos.com.

Scientists aim at forecasting natural or economic disasters by identifying statistical anomalies in complex systems that deviate from behaviour obeying traditional power laws. This article and the special issue it comes from, present the many facets of Dragon Kings in a review alongside nineteen other contributions exploring to which extent this emerging field of statistical analysis could become further established.

Dragon Kings are events akin to catastrophes. They don’t belong to the same power law regime as the more standard events. For example, they can be found in financial market bubbles ending in crashes and neuron-firing cascades leading to epileptic seizures.

This review focuses on elucidating how Dragon Kings are created and can be detected. It also gives an overview of their empirical evidence in abnormal rainfall, hurricanes, and sudden events such as landslides and snow avalanches. The authors also outline the limitations of this sort of statistical analysis. For example, despite being sometimes interpreted as featuring characteristic events of Dragon Kings, great earthquakes may not be formally confirmed as such.

Finally, the authors share their views on the importance of devising prediction models that could become the basis for Dragon Kings simulators. These could be designed to help interpret the warning signs of complex systems evolving out of their safe equilibrium into extreme events such as the subprime crisis, and to steer them into sustainability and ultimately avoid such crisis.

Dragon-kings: mechanisms, statistical methods and empirical evidence
D. Sornette and G. Ouillon, Eur. Phys. J. Special topics 205 1-26 (2012, Discussion and debate issue: from black swans to dragon-kings - Is there life beyond power laws?)
[Abstract]

Astrophysics in lab via collisions of heavy systems (Vol. 43 No. 4)

image High resolution X-ray spectra of Ar17+ -> Ar

Collisions between slow highly charged ions and atoms are one of the most common fundamental processes in space. The consequent emitted light is used to diagnose the relative abundance of constituents in intergalactic clouds and comets. During the collision, the projectile-ions capture, in a highly excited-state, from one to many target-electrons. By a series of atomic cascades the electrons "tumble" from the very high atomic levels onto the ground state through multiple and complex pathways. These cascades lead to photon and/or electron emissions. The accurate analysis of the light (from UV to hard X) emitted during the interaction provides direct insights into the early stages of capture mechanisms.

Until now, for systems involving a large number of electrons, only low-resolution X-ray spectra recorded with solid state detectors were available. In the present work, the contribution of single-electron capture from multiple-capture processes in the X-ray emission have been successfully disentangled for an Ar17+ projectile colliding with N2 or Ar gaseous target at v=0.53 a.u.

Thanks to an accurate calibration of the spectrometers and a complete determination of the ion beam-gas target overlap, absolute X-ray emission cross section has been extracted with a significant improvement in uncertainty. Using a mosaic crystal spectrometer, 2 orders of magnitude in resolving power have been reached. The whole He-like Ar16+ Lyman series from n = 2 to 10 has been resolved as well as the fine structure of 1s2l → 1s2 transitions. The role of single-electron capture, leading to transitions from n = 7 to 10 levels, has been clearly discriminated from multiple capture processes that populate lower lying states. Furthermore, a precise determination of the influence of metastable states emphasizes that transposition of the measurements via ’laboratory ion-atom collisions’ towards interpretation of astrophysical spectra should be made with caution.

Investigation of slow collisions for (quasi) symmetric heavy systems: what can be extracted from high-resolution X-ray spectra
M. Trassinelli et al. (8 co-authors), J. Phys. B 45, 085202 (2012). [Abstract]