Dynamics of ultrathin soft matter systems (Vol. 45 No. 1)

Scheme (not in scale) of the molding approach used to determine the interfacial free volume ζ (white circles in the left lower panel), seen here as the opposite of the sur-face coverage of PS (blue circles).

Thin films of liquids and polymers are interesting systems for those seeking to test glass transition theories and their prediction of a characteristic transition length scale of a few nanometers. The anomalous phenomena observed in some of these nano-confined films has greatly advanced our understanding of theoretical and experimental soft matter physics.

These films are treated as equilibrium systems where surfaces and interfaces introduce monotonous long-range mobility gradients. Considering finite size and interfacial effects provides an intuitive but oversimplified picture that falls short of explaining many phenomena, such as enhancement of segmental mobility near an absorbing surface or long-lasting metastable states in the liquid.

The authors propose a new picture of the dynamics of these confined soft matter systems, which focuses on non-equilibrium states and on the impact of irreversible chain adsorption on the structural relaxation.

They review the experimental approaches that have been used to study the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy. Then propose methods to determine gradients of mobility in thin films and discuss the as-yet untapped potential of analyses based on the time, temperature and thickness dependence of the orientational polarization.

S. Napolitano, S. Capponi, and B. Vanroy, “Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films”, Eur. Phys. J. E, 36, 61 (2013)

A step closer to composite-based electronics (Vol. 45 No. 1)

An illustration of a small portion of a square lattice.

A new study demonstrates that electrical resistivity obeys a staircase-like dependence on the conducting particle concentration in composite materials. These materials are attractive because they have a controllable electrical resistivity combined with their light and flexible properties. This makes them suited for applications in flexible electronics. Now, a theoretical model, confirmed experimentally, elucidates how electrical resistivity varies with the concentration of the particles in these composite materials.

The authors made the theoretical prediction - and proved experimentally using granular metal and carbon-black composites – that the dependence of the electrical resistance on the conducting particle concentration is manifested by a staircase. This was particularly obvious in nanometric scale systems, in which there is a well-defined discrete series of distances between a particle and its neighbours. Each stair exhibits a universal behaviour— independent of the details of the system—predicted by percolation theory. The electrical resistivity associated with subsequent stairs decreases as the concentration of the conducting particles increases.

I. Balberg, D. Azulay, Y. Goldstein, J. Jedrzejewski, G. Ravid and E. Savir, ‘The percolation staircase model and its manifestation in composite materials’, Eur. Phys. J. B, 86, 428 (2013)

No qualms about quantum theory (Vol. 45 No. 1)

‘Schrödinger’s cat state associated with an imagined superposition of a dead and live cat has no reality.’

The alleged shortcomings of quantum theory do not hold up to scrutiny. A colloquium paper peers into the alleged issues associated with quantum theory. The author reviews a selection of the potential problems of the theory. He sets out to demystify a selected set of objections targeted against quantum theory in the literature. He takes the example of Schrödinger’s infamous cat. The term ‘Schrödinger’s cat state’ is routinely applied to superposition of so-called quantum states of a particle. However, this imagined superposition of a dead and live cat has no reality. Indeed, it confuses a physical object with its description. Other myths debunked in this paper include the provision of proof that quantum theory is well defined, has a clear interpretation, is a local theory, is not reversible, and does not feature any instant action at a distance. It also demonstrates that there is no measurement problem, despite the fact that the measure is commonly known to disturb the system under measurement.

B.G. Englert, ‘On Quantum Theory’, Eur. Phys. J. D, 67, 238 (2013)

Revisiting quantum effects in MEMS  (Vol. 45 No. 1)

“Example of MEMS”
Credit: United States Government Work

Micro- and nano-electromechanical devices, referred to as MEMS and NEMS, are ubiquitous. They are found in car airbags and smart phones. The trouble is that, as their size decreases, forces typically experienced at the quantum level start to matter in these nanodevices. The authors have studied the mechanical and electrical stability of MEMS and NEMS, depending on the plate thickness and the nature of the material used. They show that previous works overestimated the operating conditions of the devices by not taking into account this Casimir/van der Waals effect. In addition, they demonstrate that the stability of these devices under the Casimir force changes depending on the nature and thickness of the metal coatings used. It also depends on the variation of concentration of the free charges in the silicon used, which changes with doping levels.

R. Esquivel-Sirvent and R. Perez-Pascual, “Geometry and charge carrier induced stability in Casimir actuated nanodevices”, Eur. Phys. J. B, 86, 467 (2013)

Covariance mapping of molecular Coulomb explosion (Vol. 45 No. 1)

Schematic representation of the partial covariance experiment at FEL.

Free-electron lasers (FELs) provide pulses of XUV or X-ray radiation with unprecedented intensity. The number of photons in these pulses is high enough to allow the reconstruction of the structures of large macromolecules (such as proteins and viruses) from diffraction patterns formed by a single FEL shot. However this process is always accompanied by massive ionization and Coulomb explosion of the molecules. Understanding the dynamics of Coulomb explosion is thus required for the correct assessment of sample degradation. We present a general experimental technique which can address this problem. In the experiment, schematically depicted in the Figure, the molecules (in this case nitrogen and iodine) are ionized by intense XUV pulses at the FLASH FEL. The molecules dissociate via Coulomb explosion to form fragments carrying different charges. These fragments are detected for each single shot by a simple time-of-flight spectrometer. The method of statistical analysis, known as partial covariance mapping, identifies positive correlations of signals induced by ions coming from the same molecule and allows us to extract information about individual dissociation channels from the highly congested experimental spectra.

O. Kornilov, M. Eckstein, M. Rosenblatt, C. P. Schulz, K. Motomura, A. Rouzée, J. Klei, L. Foucar, M. Siano, A. Lübcke, F. Schapper, P. Johnsson, D. M. P. Holland, T. Schlathölter, T. Marchenko, S. Düsterer, K. Ueda, M. J. J. Vrakking and L. J. Frasinski, “Coulomb explosion of diatomic molecules in intense XUV fields mapped by partial covariance”, J. Phys. B: At. Mol. Opt. Phys., 46, 164028 (2013)

Superconductivity and magnetic order in ErPdBi (Vol. 45 No. 1)

Superconducting (SC) and magnetic (AFM) phase diagram of ErPdBi. Inset: Half-Heusler crystal structure.

Half-Heusler compounds attract ample attention because of their flexible electronic structure. A new electronic state in this respect is the topological insulator, where the interior of the material is insulating, while the surface states are conducting. Surprisingly some of the topological half-Heusler compounds become superconducting at low temperatures. Topological superconductors are predicted to have a fully gapped unconventional pairing state in the interior, while the non-trivial topology gives rise to Majorana fermion states at the edge of the sample. The further interplay with magnetic order may lead to exotic superconducting phases.

In the paper the discovery is reported of a new candidate for topological superconductivity: ErPdBi. Magnetic and transport measurements demonstrate superconductivity at Tc = 1.22 K, and, moreover, magnetic order at TN= 1.06 K. Since TN ≈ Tc the interaction of superconductivity and magnetic order is expected to give rise to a complex ground state. Electronic structure calculations reveal a topologically non-trivial band inversion. Accordingly, ErPdBi is advocated as a novel, unique platform to study the interplay of topological states, superconductivity and magnetic order.

Y. Pan, A.M. Nikitin, T.V. Bay, Y.K. Huang, C. Paulsen, B.H. Yan and A. de Visser, "Superconductivity in the noncentrosymmetric half-Heusler compound ErPdBi", EPL, 104, 27001 (2013).

Metallic nanoparticles for plasmonic absorption enhancement (Vol. 45 No. 1)

Expected optical generation rate for a solar cell with integrated random nanoparticles assemblies as visible from the overlapped topography.

Random arrangements of nanoparticles are easy to fabricate and therefore find widespread application. But how do these random assemblies influence the local optical generation rate? The authors investigated Ag nanoparticle assemblies with scanning near-field optical microscopy (SNOM) detecting the optical response to local near-field excitation through an aperture tip while simultaneously recording the topography with atomic force microscopy. 3D simulations in finite-difference time domain confirmed that areas in between irregularly arranged nanoparticles show the strongest response and that no direct correlation of hot spots to particularly sized nanoparticles is possible. An overall highly non-uniform distribution of the electric field is found around the nanoparticles for various wavelengths both in experiment and theory. These variations in local electric field are expected to translate directly to the optical generation rate, which therefore will equally suffer from inhomogeneities (see figure), and will thus crucially determine the effectiveness of plasmonic absorption enhancement. Therefore mapping the local field distributions as possible with SNOM is expected to be highly advisory to optimize nanoparticle geometries.

M. Schmid, J. Grandidier and H. A. Atwater, “Scanning near-field optical microscopy on dense random assemblies of metal nanoparticles“, J. Opt., 15, 125001 (2013)

Advances in positron-molecule annihilation physics (Vol. 45 No. 1)

The interactions of positrons with atoms and molecules are important in diverse settings, from applications in materials science to understanding the origin of the positron-electron annihilation gamma-ray line emanating from the centre of our galaxy. Furthermore, there are important applications in Positron Emission Tomography.

A vital parameter is termed Zeff, the effective number of electrons available to the positron for annihilation. This quantity is almost always greater than the actual number of electrons, Z, due to the dominant electron-positron attraction. Careful measurements of the gas density dependence of the positron annihilation rate were performed for a number of species. By incorporating corrections for non-linearities due to many-body interactions, accurate values of Zeff for N2, O2, CO, N2O and CH4 were derived. It is hoped that this work will serve to benchmark the positron annihilation rates in these molecules and act as a spur to further theoretical development in positron-molecule scattering.

M. Charlton, T. Giles, H. Lewis and D.P. van der Werf, “Positron annihilation in small molecules” J. Phys. B: At. Mol. Opt. Phys. 46, 195001 (2013)

Hydrodynamics role in heavy-ion collisions (Vol. 45 No. 1)

The energy distribution of a random event

In ultra-relativistic nuclear collisions, hydrodynamics plays an important role in governing the system’s evolution. As various empirical evidence strongly points towards local thermalization or, at least, the validity of a certain kind of equation of state in the very early stages of the collisions, it is quite amazing that the resulting hot and dense system evolves more like a perfect fluid, rather than a collection of ‘hard’ binary collisions. Consequently, collectivity builds up inside the system, which can be measured by observables such as collective flow or multi-particle correlations.

In this paper, the authors explicitly decomposed and manipulated the initial conditions and studied the hydrodynamic evolution of individual cumulant components. In particular, they discussed to what extent linearity breaks down in hydrodynamics.

They found that, when expanding in azimuthal angles, each cumulant component possesses non-trivial radius dependence. Though in general linearity is approximately obtained, the authors found that flow harmonics of higher orders are produced, deviating from the linearity between eccentricities and flow coefficients. These results can be seen as a natural consequence of the non-linear nature of hydrodynamics, and they can be understood intuitively in terms of the peripheral-tube model.

W.-L. Qian, Ph. Motta, R. Andrade, F. Gardim, F. Grassi, Y. Hama and T. Kodama, “Decomposition of fluctuating initial conditions and flow harmonics” J. Phys. G: Nucl. Part. Phys. 41, 015103 (2014)

Uniformity: the secret of better fusion ignition (Vol. 45 No. 1)

Non-uniformity as a function of the power imbalance.

One of the ways to achieve thermonuclear fusion is through a controlled reaction between deuterium and tritium. The authors have made theoretical calculations indicating how best to improve the ignition stage of fusion reaction. This approach involves increasing the uniformity of irradiation using high-power laser beams on the external shell of a spherical capsule containing a mix of deuterium and tritium.

Reaching uniformity of irradiation matters for reaching the ignition conditions of thermonuclear fusion. In this study, the authors analyse the possibility of using the UK-based Orion facility’s high-power laser beams of to study uniformity. Specifically, the authors use numerical simulations to analyse the uniformity of the illumination of a spherical target both in the case of circular or elliptical laser intensity profiles. They demonstrate that this approach reduces considerably the non-uniformity of the capsule irradiation—by 50% and 35%, for elliptical and circular intensity profiles respectively.

M. Temporal, B. Canaud, W.J. Garbett, F. Philippe and R. Ramis, “Polar Direct Drive Illumination Uniformity Provided by the Orion Facility”, Eur. Phys. J. D, 67, 205 (2013)

Computer and brain-logic gates (Vol. 45 No. 1)

An illustration for the differences between computer and brain logic-gates.

This year we are celebrating the 70th anniversary of the publication of the seminal work by McCulloch and Pitts "A logical calculus of the ideas immanent in nervous activity". They suggested that the brain is composed of threshold units, neurons, composing reliable logic-gates similar to the logic at the core of today's computers. This suggested computational framework had a tremendous impact on the development of artificial neural networks and machine learning theory, but had limited impact on neuroscience, since neurons exhibit far richer dynamics. Here we propose a new experimentally corroborated paradigm in which the functionality of the brain's logic-gates depends on the history of their activity, the stimulation frequencies of their input neurons, as well as the activity of their interconnections. Our results are based on an experimental procedure where conditioned stimulations were enforced on circuits of neurons embedded within a large-scale network of cortical cells in-vitro. We demonstrate that the underlying biological mechanism is the unavoidable increase of neuronal response latency to ongoing stimulations, which imposes a non-uniform gradual stretching of network delays. This computational paradigm is anticipated to lead to better understanding of the brain's functionalities.

R. Vardi, S. Guberman, A. Goldental and I. Kanter, “An experimental evidence-based computational paradigm for new logic-gates in neuronal activity”, EPL, 103, 66001 (2013).

A topological analysis of plasma flow structures (Vol. 45 No. 1)

Filamentary structure of plasma turbulence.

In toroidally confined plasmas and from a fluid perspective, plasma turbulence is characterized by the existence of multiple vortices located at the magnetic surfaces where the magnetic field lines close on themselves after a finite number of turns around the torus. When we look at transport in such systems, we see that these vortices may cause the trapping of particles, while large scale flows may carry them from vortex to vortex. We develop an analysis approach that has allowed us a complete characterization of the structures of the vortices, determining which ones form close loops, cycles, and which ones have just a finite length, filaments, and make a determination of their length. By comparing these structures at different times we also can determine the life times of the cycles. We have found that both life times of the cycles and lengths of the filaments are well described by lognormal distributions. Having the distribution of the life times of the cycles and lengths of the filaments, we can connect them to the trapping time of particles moving with the turbulence.

B. A. Carreras, I. Llerena Rodríguez and L. García, “A topological analysis of plasma flow structures”, J. Phys. A: Mth. Theor., 46, 375501 (2013)