Vol. 42 No. 6 - Highlights

Positron and electron collisions with formaldehyde (Vol. 42, No. 6)

image Experimental and theoretical total cross sections (TCS) for positron (e+) and electron (e-) scattering from formaldehyde and calculations of the elastic integral cross section (ICS) for positrons. The black arrows labelled "Ps" and "IP" indicate the energy thresholds of the opening of the positronium formation and direct ionisation scattering channels, respectively.

New interest in electron and positron scattering from atoms and molecules has grown in the last few years. Understanding the fundamental forces, like the Coulomb and the dipole interaction, driving the collisional processes between the incident particle and the target, both experimentally and in the development of scattering theory, is a crucial topics in physics.

Formaldehyde (CH2O) is a relatively small and simple fundamental organic species, from which many chemical compounds are derived. In addition, this molecule is characterised by a strong dipolar nature, a property which is expected to play a significant role in affecting the probability of very low-energy scattering. Despite these interesting properties, it had attracted only very little attention so far. The very first absolute cross sections for low energy positron and electron scattering from formaldehyde are reported here, hereby filling in a gap in the available knowledge on this key species.

Experimental total cross sections and calculated elastic integral cross sections for positrons in the energy range ~0.25-50 eV, together with theoretical results of electron total cross sections are presented. As can be seen from the results shown in the figure, the very large slope and magnitude of the low-energy cross sections reflect well the largely polar nature of formaldehyde. As a result of this work, formaldehyde can be used as an excellent candidate species against which further advances in scattering theory might be benchmarked.

An experimental and theoretical investigation into positron and electron scattering from formaldehyde
A. Zecca, E. Trainotti, L. Chiari, G. García, F. Blanco, M. H. F. Bettega, M. T. do N. Varella, M. A. P. Lima and M. J. Brunger, J. Phys. B: At. Mol. Opt. Phys. 44 195202 (2011)

Curvilinear shapes by Virtual Image Correlation (Vol. 42, No. 6)

image Image of a fibre dragged in a fracture by a flow motion and its shape identification (in red)

The problem of the accurate identification of a contour arises in many fields of physics. This contour can be an image of an elongated object, for example a fibre, a filament or a structure of larger dimension such as the cable of a bridge. It can also be a front of a physical event, such as a thermal or chemical front revealed by a convenient marker.

Current methods are optimized in the sense of the capacity of detection of a contour hidden in a noisy image, mainly for medicinal applications. The given information consists in a discrete set of locations (generally a set of pixels). Due to this discontinuous nature, the derivatives (slopes and curvatures) require a filtering and the result depends on its choice. However this knowledge is crucial for many applications. For example fluxes computations depend upon slopes; momentums on beams (in mechanics) depend upon curvatures.

The proposed method, issued from the Digital Image Correlation methods used in solid mechanics, is more focused on the precision of the identification, in the sense of metrology. It consists in the research of the best fit between the image of the physical contour and a virtual image, a curve roughly of the same thickness as the contour. This curve (the virtual beam) is defined analytically thus the obtained shape and its derivatives are smooth, given with confidence and defined at any scale of refinement.

The method can be easily extended to edge detection. It will also be possible to use an analytical expression of the considered problem thus the method will give its best coefficients, leading to a straightforward identification of the phenomenon from an unfiltered image.

Accurate measurement of curvilinear shapes by Virtual Image Correlation
B. Semin, H. Auradou, and M.L.M. François, Eur. Phys. J. Appl. Phys. 56, 10701 (2011)

Effective long-range interactions in confined curved dimensions (Vol. 42, No. 6)

image Phase space picture of the effective long-range interactions indicating many local equilibriums. (Inset: Number of minima as a function of the pitch of the helix).

Fundamental forces of nature such as the gravitational or Coulomb force mediated by the mass and charge of particles, respectively, decay according to universal laws with increasing distance between the particles. Still, these simple laws lead to the enormous beauty and amazing complexity of matter surrounding us.

In ultra-cold atomic physics the three-dimensional motion of neutral atoms or charged particles can be confined to a lower dimensional typically one- or two-dimensional trap by employing electromagnetic forces. If these traps are of curved character, which is possible e.g. in evanescent fields surrounding optical nano-fibres, they confine the motion to a curved low-dimensional manifold.

This opens the perspective of designing novel effective finite-range and possibly also long-range interactions since the dynamics is constrained to the curved geometry but the interaction takes places via the dynamically forbidden dimensions. The corresponding forces can now become oscillating with increasing distance between the particles and are widely tunable via the parameters of the confining curved manifold. Exploring as a prototype example the one-dimensional helix, it can be immediately shown that a plethora of local equilibrium configurations and consequently bound states emerge already for two particles even if the particles were repelling each other in free space. The number and depths of the local minima and wells can be tuned by modifying the pitch or curvature of the helix thereby establishing bound state configurations of different symmetries.

With an increasing number of interacting particles an ever-increasing wealth of symmetry-adapted and symmetry-distorted configurations create a very complex energy landscape exhibiting a dense spectrum of local equilibriums. It can be anticipated that the thermodynamical properties and quantum physics of the many-body interacting helical chain show novel structural properties such as enriched phase diagrams as well as an intriguing dynamical behaviour.

Effective long-range interactions in confined curved dimensions
P. Schmelcher, EPL, 95, 50005 (2011)

Stable p-type conductivity in Bi-doped ZnO (Vol. 42, No. 6)

Zinc Oxide (ZnO) has potential applications in varistors, light emitting diodes and photo detectors. The primary obstacle against its use in optoelectronic devices is the lack of stable p-type material. ZnO is naturally an n-type material, its majority carriers being electrons and it must be doped to become p-type. Most p-type dopants introduce either deep or shallow acceptors and the resulting material is p-type with either very low carrier concentration or unstable conductivity. Many previous efforts to synthesize p-type ZnO used N, As and P as dopants. In the present study bismuth has been chosen.

Thin films of Bi-doped ZnO were grown with a pulsed laser deposition system using a homogeneous target. Sample thickness was about 150 nm. XRD results showed the wurtzite structure of the films and XPS confirmed the presence of Bi. No evidence of secondary phases was found. From Hall measurements made over repeated cycles, in-situ annealed 3 and 5% Bi-doped samples are p-type. However as-grown 3% doped films showed unstable p-type conductivity, which suggests that some form of activation of Bi in ZnO occurs during the post-growth annealing leading to the p-type conduction. In the as grown 5% doped samples, the Bi concentration seems high enough to impose p-type conduction. Bi substitution in ZnO lattice is known to produce acceptors; this is the case here as shown by photoluminescence experiments. Carrier concentrations were 5.4 x 1018 and 4.8 x 1019 cm-3 in annealed 3% and 5% Bi-doped samples, respectively. Temperature-dependent photoluminescence leads to an acceptor energy level at about 0.13 eV above the valence band. The p-type conductivity of these Bi-doped ZnO thin films is stable under oxygen-rich ambient or upon annealing. Thus, this study suggests a possible pathway for developing ZnO based optoelectronic devices.

Study of stable p-type conductivity in bismuth-doped ZnO films grown by pulsed-laser deposition
J. W. Lee, N. G. Subramaniam, J. C. Lee, S. Kumar S and T. W. Kang, EPL, 95, 47002 (2011)

Electron impact dissociation of ND+: formation of D+ (Vol. 42, No. 6)

image Lowest potential energy curves of ND+ leading to the formation of the D+ fragment.

Nitrogen is a common contaminant species in fusion reactors such as the ITER (International Thermonuclear Experimental Reactor). Thus, the collisional properties of nitrogen-containing plasma compounds are widely studied experimentally and theoretically. Here we show the results of absolute cross section measurements for the electron impact dissociative excitation of ND+ yielding D+, especially at low electron energies near the onset of the dissociation. The identification of indirect and resonant processes is a particular challenge in that energy regime. Excitation is likely to be influenced by vibrationally excited levels populated within the X2Π ground state of ND+(v). Two mechanisms can produce these levels: (i) the endothermic reaction D2+(v') + N and (ii) the ion-molecule reaction D2(v") + N+. The latter is the first step in a sequence of molecular activated processes, which confirms that such a reaction chain is important for an understanding of the overall plasma chemistry. The low experimental energy threshold observed in the present studies indicates that the importance of contribution of Rydberg states via the capture of the incoming electron into doubly excited electronic states of (ND)**.

Electron impact dissociation of ND+: formation of D+
J. Lecointre, D.S. Belic, S. Cherkani-Hassani, and P. Defrance, Eur. Phys. J. D, 63, 475 (2011)

Metal contacts in terahertz quantum cascade lasers (Vol. 42, No. 6)

image (a) Threshold current densities of four THz quantum cascade lasers with different metal contacts. (b) Current-voltage curves of the devices at 10 K.

Schottky contacts should be avoided in electrically-pumped semiconductor laser devices because they cause an extra voltage drop at metal-semiconductor interfaces - wasting power, overheating device active region and degrading performance. Metal stacks of Ni/Ge/Au and Ti/Pt/Au are commonly employed to form ohmic contacts with n-type and p-type III-V semiconductors, respectively. The optical loss of these ohmic metal contacts is negligible in the visible-light/near-infrared range (~hundreds of THz) due to their much lower Plasmon frequencies (1-10 THz). The optical properties of these metals are therefore not a concern in conventional semiconductor diode lasers.

This drastically changes if the lasing frequency approaches the terahertz range (1012 Hz), i.e., THz quantum cascade lasers (QCLs) that are based on GaAs/AlGaAs multiple quantum-well structures. The high tangent loss of the commonly-used metals in this frequency range could become a substantial part of the total waveguide loss of the lasers, however metal stacks (such as Ti/Au and Ta/Cu/Au) that exhibit low optical loss in the terahertz frequency range from non-ohmic contacts with III-V semiconductors. Researchers often face a dilemma when picking the metals - ohmic or non-ohmic contacts?

We experimentally investigated the electrical and optical behaviours of THz QCLs with four different Au- and Cu-based metal contacts. The QCL device with non-alloyed Ta/Cu/Au exhibits the lowest threshold current density and the highest lasing temperature in pulsed mode. The better performance is attributed to the lower optical loss of the device waveguide in spite of the formation of a Schottky contact. The findings clarify an important issue that will help researchers design and fabricate THz QCLs operating at higher temperatures and eventually at room temperature.

On metal contacts of terahertz quantum cascade lasers with a metal-metal waveguide
S. Fathololoumi, E. Dupont, S. G. Razavipour, S. R. Laframboise, G. Parent, Z. Wasilewski, H. C. Liu and D. Ban, Semicond. Sci. Technol. 26, 105021 (2011) [Abstract]

Renormalisation group for 3-body interactions in 1D (Vol. 42, No. 6)

image The three-body interaction induced by the renormalization group evolution starting from a Hamiltonian with only two-body forces. The three-body potential is plotted as a function of initial and final relative momentum variables, for three values of the flow parameter. It can be seen to approach a diagonal form as the flow progresses.

One important message emerging from developments of effective field theories and effective Hamiltonians for nuclear physics is that many-body forces are inevitable whenever degrees of freedom are eliminated. At the same time, first-principles calculations have shown that two-body forces alone are not able to give an accurate account of the energies of light nuclei and the saturation of nuclear matter. Three- (possibly more-) body forces are thus essential in low-energy nuclear physics. The construction of effective interactions through elimination of degrees of freedom can be done either by imposing a cut-off on the Hilbert space or by applying a transformation putting the Hamiltonian into a simpler form, such as a diagonal matrix.

The Similarity Renormalization Group follows the latter route by means of a continuous set of transformations. It has proved to be a powerful tool in low-energy nuclear physics, when applied mainly in the context of expansions using harmonic-oscillator basis states. The present paper provides the first application of this method to three-body interactions in a momentum-space basis. Although the models studied are simple ones (bosons in 1D), the structure of the evolution equations has the full complexity of any set of three-body equations. The results show the expected decoupling of high- from low-momentum states for both two- and three-body interactions, which means that only low-momentum matrix elements of the evolved potentials are needed to describe low-energy states. This work paves the way for applications to few-nucleon scattering processes and nuclear matter, starting from realistic nuclear forces in three dimensions.

The Similarity Renormalization Group for Three-Body Interactions in One Dimension
O. Åkerlund , E.J. Lindgren, J. Bergsten, B. Grevholm, P. Lerner, R. Linscott, C. Forssén, L. Platter, EPJ A, 47, 1 (2011)

How do protein binding sites stay dry in water? (Vol. 42, No. 6)

image Simulations are used to study the ability of model tunnel structure to stay dry

What is the the condition for model cavity and tunnel structures resembling the binding sites of proteins to stay dry without losing their ability to react, a prerequisite for proteins to establish stable interactions with other proteins in water? To answer this, models of nanometric-scale hydrophobic cavities and tunnels are used to understand the influence of geometry on the ability of those structures to stay dry in solution. The authors study the filling tendency of cavities and tunnels carved in a system referred to as an alkane-like monolayer, chosen for its hydrophobic properties, to ensure that no factors other than geometrical constraints determine their ability to stay dry.

They show that the minimum size of hydrophobic cavities and tunnels that can be filled with water is in the nanometer range. Below that, the structure stays dry because it provides a geometric shield; if a water molecule was to penetrate the cavity it would pay an excessive energy cost to release its hydrogen bonds. By comparison, water fills carbon nanotubes that are twice smaller (but slightly less hydrophobic) than the alkane monolayer, making them less prone to stay dry.

It is also shown that the filling of nanometric cavities and tunnels with water is a dynamic process that goes from dry to wet over time. Water molecules inside the cavities or tunnels may arrange in a network of strong cooperative hydrogen bonds. Their disruption through thermal fluctuations induces the temporary drying of the holes until new bonds are re-established. Among many potential applications, one in biophysics would be to study water-exclusion sites of proteins, and understand the physical phenomenon linked to the geometry of those sites, underpinning the widespread biological process of protein-protein associations.

Behavior of water in contact with model hydrophobic cavities and tunnels and carbon nanotubes
E.P. Schulz, L.M. Alarcón and G.A. Appignanesi, Eur. Phys. J. E 34, 114 (2011) [Abstract]

New complex may offers safer alternative for gene therapy (Vol. 42, No. 6)

image Self-organized DNA fragments in a non-cationic Lα lipid phase

The authors have created a complex system designed to hold DNA fragments in solution between the hydrophilic layers of a matrix of fatty substances (also known as lipids) combined with a surfactant (used to soften the layers' rigidity). One possible application that has yet to be tested is gene therapy.

Although gene therapy was initially delivered using viral vectors, recent attempts at devising alternative vectors have exploited positively charged lipids to form complex structures holding DNA fragments with electrostatic forces. However the positively charged ions (cations) used in this type of vector have proven toxic for human cells.

Until now, only positively charged fatty substance were thought capable of holding DNA in a complex vector. The authors of this study have proved otherwise by creating an electrically neutral matrix, structured like a multi-layered cake, which holds the DNA fragments at a high concentration in solution between the layers.

It appears that DNA fragments within the complex self-organise over time. These fragments spontaneously align parallel to one another and form rectangular and hexagonal structures across the layers. This ordering in a special matrix holds the key to creating non-toxic gene therapy delivery vectors. The change of atomic-level interactions within the layers and the appearance of interactions at the interface between the layers and the DNA molecules may explain the emergence of ordered structures at high DNA concentrations.

The next step of this research involves elucidating the precise physical forces that hold the complex together. Applications of such technology go beyond gene therapy vector design, as the same principle can be applied for the delivery of other particles such as chemical drugs.

Supramolecular polymorphism of DNA in non-cationic Lα lipid phases
E.R. Teixeira da Silva, E. Andreoli de Oliveira, A. Février, F. Nallet and L. Navailles, Eur. Phys. J. E, 34, 83 (2011)

Observation of metastable hcp solid helium (Vol. 42, No. 6)

image Partial phase diagram of helium and minimum pressures achieved at different temperatures between 1.1K and 1.4K. The metastable domain is below the melting line.

Crystalline solids can be brought into metastable state with respect to fusion only if surface melting is avoided. Overheated metals have indeed been observed by embedding small samples in carefully chosen matrices. Because of its constant melting pressure at low temperatures, hcp solid helium offers a unique possibility to achieve a metastable solid via pressure variations. Intense positive and negative pressure swings far from any interface can be achieved using focused sound waves. In hcp solid helium, the sound velocity is anisotropic and a dedicated non-spherical sound emitter has to be used. The wave amplitude is small enough not to melt the crystal at its interface with the emitter. As it propagates, its amplitude increases and pressures below the static melting line are obtained in the solid bulk. The pressure is measured via the refraction index changes of the medium using an interferometric imaging technique. The main result of this work is shown on the figure: hcp solid helium between -2 and -4 bars below the melting line has been produced and observed. A side result is that the crystal seems to become unstable beyond this value. We feel that the stretched quantum solid is an interesting new system to be understood in details.

Observation of metastable hcp solid helium
F. Souris, J. Grucker, J. Dupont-Roc and Ph. Jacquier, EPL, 95, 66001 (2011)