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Ordered Si oxide nanodots at atmospheric pressure (Vol. 43 No. 1)

image SEM image of the surface morphology of SiOxHyCz nano-islands localized at the center of nano-indents

It is now possible to simultaneously create highly reproductive three-dimensional silicon oxide nanodots on micrometric scale silicon films in only a few seconds. The present study shows that one can create a square array of such nanodots, using regularly spaced nanoindents on the deposition layer, that could ultimately find applications as biosensors for genomics or bio-diagnostics.

A process called atmospheric pressure plasma-enhanced chemical vapour deposition is used. This approach is a much faster alternative to methods such as nanoscale lithography, which only permits the deposition of one nanodot at a time. It also allows the growth of a well- ordered array of nanodots, which is not the case of many growth processes. In addition, it can be carried out at atmospheric pressure, which decreases its costs compared to low-pressure deposition processes.

One goal was to understand the self-organization mechanisms leading to a preferential deposition of the nanodots in the indents. By varying the indents' interspacing, they made it comparable to the average distance travelled by the silicon oxide particles of the deposited material. Thus, by adapting both the indents' spacing and the silicon substrate temperature, they observed optimum self-ordering inside the indents using atomic force microscopy.

The next step will be to investigate how such nanoarrays could be used as nanosensors. It is planed to develop similar square arrays on metallic substrates in order to better control the driving forces producing the highly ordered self-organisation of nanodots. Further research will be needed to give sensing ability to individual nanodots by associating them with probe molecules designed to recognise target molecules to be detected.

Ordering of SiOxHyCz islands deposited by atmospheric pressure microwave plasma torch on Si(100) substrates patterned by nanoindentation
X. Landreau, B. Lanfant, T. Merle, E. Laborde, C. Dublanche-Tixier and P. Tristant, Eur. Phys. J. D, 65/3, 421 (2011)
[Abstract]

Organic nanoparticles, more lethal to tumours (Vol. 46 No. 4)

Man receiving radiation therapy for cancer treatment.
© Mediteraneo / Fotolia

Carbon-based nanoparticles could be used to sensitize cancerous tumours to proton radiotherapy and induce more focused destruction of cancer cells, a new study shows.

Radiotherapy used in cancer treatment is a promising treatment method, albeit rather indiscriminate. Indeed, it affects neighbouring healthy tissues and tumours alike. Researchers have thus been exploring the possibilities of using various radio-sensitizers; these nanoscale entities focus the destructive effects of radiotherapy more specifically on tumour cells. In a study published recently, the authors have now shown that the production of low-energy electrons by radio-sensitizers made of carbon nanostructures hinges on a key physical mechanism referred to as plasmons—collective excitations of so-called valence electrons; a phenomenon already documented in rare metal sensitizers. This reseach may lead to the development of novel types of sensitizers composed of metallic and carbon-based parts.

A. Verkhovtsev, S. McKinnon, P. de Vera, E. Surdutovich, S. Guatelli, A. V. Korol, A. Rosenfeld and V. Solov’yov,, Comparative analysis of the secondary electron yield from carbon nanoparticles and pure water medium, Eur. Phys. J. D 69, 116 (2015)
[Abstract]

P2 – The weak charge of the proton (Vol. 50, No. 1)

The experimental setup of the P2-experiment to measure the weak mixing angle at the new electron accelerator MESA in Mainz

The P2-experiment at the new electron accelerator MESA in Mainz aims at a high-precision determination of the weak mixing angle at the permille level at low Q2. This accuracy is comparable to existing measurements at the Z-pole but allows for sensitive tests of the Standard Model up to a mass scale of 50 TeV. The weak mixing angle will be extracted from a measurement of the parity violating asymmetry in elastic electron-proton scattering. The asymmetry measured at P2 is smaller than any asymmetry measured so far in electron scattering, with an unprecedented accuracy. This review describes the underlying physics and the innovative experimental techniques, such as the Cherenkov detector, beam control, polarimetry, and the construction of a novel liquid hydrogen high-power target. The physics program of the MESA facility comprises indirect, high-precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclei, transverse single-spin asymmetries, and a possible future extension to the measurement of hadronic parity violation.

D. Becker and 45 co-authors, The P2-Experiment - A future high-precision measurement of the weak mixing angle at low momentum transfer, Eur. Phys. J. A 54, 208 (2018)
[Abstract]

Partial synchronization as a model for unihemispheric sleep (Vol. 50, No. 5-6)

Partial synchronization as a model for unihemispheric sleep
Brain connectivity.

Human brains exhibit a slight structural asymmetry of their two hemispheres (see Figure). We have investigated the dynamical asymmetry arising from this natural structural difference in healthy human subjects, using a minimum model which elucidates the modalities of unihemispheric sleep in human brain, where one hemisphere sleeps while the other remains awake. In fact, this state is common among migratory birds and mammals like aquatic species.

By choosing appropriate coupling parameters in a network of FitzHugh-Nagumo oscillators with empirical structural connectivity, we have observed that our brain model exhibits spontaneous symmetry breaking and bistability, where each hemisphere may engage into either of two dynamical states, characterized by a relatively high and low degree of synchronization. However, a high degree of synchronization in one of the hemispheres always coincides with a low degree of synchronization in the other. This dynamical asymmetry can be even enhanced by tuning the inter-hemispheric coupling strength. These results are in accordance with the assumption that unihemispheric sleep requires a certain degree of inter-hemispheric separation.

The structural asymmetry in the brain allows for partial synchronization dynamics, which may be used to model unihemispheric sleep or explain the mechanism of the first-night effect in human sleep.

L. Ramlow, J. Sawicki, A. Zakharova, J. Hlinka, J. Ch. Claussen and E. Schöll, Partial synchronization in empirical brain networks as a model for unihemispheric sleep, EPL 126, 50007 (2019)
[Article]

Particle accelerators for the study of Cultural Heritage (Vol. 42, No. 3)

image Details of the painting by Mantegna (above, part of the veil of the Virgin; below, the eye of the Virgin), and corresponding X ray distribution maps obtained by PIXE at the external scanning micro-beam set-up at INFN-LABEC.

This paper is a first critical discussion of the contemporary role that small particle accelerators play in the field of applications related to Cultural Heritage for non-destructive materials analysis and dating, such as ion beam analysis (IBA) and accelerator mass spectroscopy (AMS). This analysis is put in perspective by comparing accelerator-based methods to other techniques, pointing out the corresponding pros and cons. It is stressed that IBA can address questions that cannot be answered by conventional techniques like X-ray fluorescence (the latter having however the advantage of portability). It is shown in particular that IBA can still yield unrivalled results thanks to novel uses of its potential, such as providing elementalmaps and resolving layer structures. This was strikingly demonstrated in a recent PIXE (Particle Induced X-ray Emission) analysis at LABEC of Mantegna’s famous painting Madonna con Bambino (see figure), which used the scanning external proton microbeam set-up to produce a high-resolution elementalmapping.The use of different, even precious, pigments to paint tiny details is thus pointed out, which can be of great interest to art historians and restorers. In addition, by utilizing the different absorption rates of different X-ray energies, even the paint layer structure can sometimes be reconstructed: for example, in the case shown in the upper part of the figure, it is shown that the Virgin’s veil was made with a gold paint over a background of ultramarine blue, lightened with lead-white. No other technique can provide such information in a totally non-destructive way and with no pick-up of samples

The present role of small particle accelerators for the study of Cultural Heritage
P.A. Mandò et al., Eur. Phys. J. Plus 126, 41 (2011)
[Abstract]

Particles near absolute zero do not break the laws of physics (Vol. 45 No.4)

A free particle strongly coupled to a heat bath. Credit: Adamietz et al.

A change of models demystifies anomalous particle behaviour at very low temperatures, confirming that the third law of thermodynamics cannot be violated

In this work, the authors have demonstrated that a theoretical model of the environment’s influence on a particle does not violate the third law of thermodynamics, despite appearances to the contrary. These findings are relevant for systems at the micro or nanometer scale that are difficult to decouple from the heat or the quantum effects exerted by their environment.

Previous theoretical predictions suggested that, under certain circumstances, the specific heat—the amount of energy is needed to raise the temperature of a particle coupled to a heat bath by a certain amount—can decrease below zero at strictly zero temperature (−273.15 °C). This prediction appears to breach the third law of thermodynamics, indicating that the specific heat must drop to zero value at strictly zero temperature. Yet, these findings show that previous studies need to be modified in order to account for a spatial confinement of the particle.

R. Adamietz, G.-L. Ingold and U. Weiss, “Thermodynamic anomalies in the presence of general linear dissipation: from the free particle to the harmonic oscillator”, Eur. Phys. J. B, 87, 90 (2014)
[Abstract]

Passivated Tunneling Contacts for c-Si Solar Cells by Al2O3 and ZnO Nanolayers (Vol. 44 No. 5)

Band diagram of the proposed novel front contact

Al2O3 nanolayers are well-known for their ability to reduce recombination losses at crystalline silicon surfaces, making Al2O3 an attractive material for passivation of the next-generation high efficiency solar cells. In this work, we try to take the application of Al2O3 one step further: when Al2O3 is deposited on n-type silicon, a high concentration of holes accumulates at the surface due to the high density of negative charges in the Al2O3. Consequently a pn-junction is formed which can replace the traditional front side p-doped region made by high temperature diffusion.

The idea presented in the article is to deposit a stack consisting of an Al2O3 and ZnO layer on the silicon. The holes separated from the electrons at the junction can tunnel through the Al2O3 into the transparent conductive ZnO where they are collected with minimal energy loss when the Al2O3 charge density and ZnO doping density are properly tailored. Using atomic layer deposition, it was demonstrated that Al2O3 and Al-doped ZnO films deposited with sub-nanometer precision can be used for this purpose with sufficiently high tunneling currents when the Al2O3 is about 1-2 nm thick.

D. Garcia-Alonso, S. Smit, S. Bordihn and W. M. M. Kessels, ‘Silicon passivation and tunneling contact formation by atomic layer deposited Al2O3/ZnO stacks’, Semicond. Sci. Technol. 28, 082002 (2013)
[Abstract]

Pattern Formation Induced by Fixed Boundary Condition (Vol. 48 No. 2)

Time development of the reaction diffusion dynamics. Fixed boundary condition at x=0 transforms uniform temporal oscillation to stationary periodic pattern

Pattern formation in nonequilibrium systems has been extensively investigated in physical and chemical systems as well as for biological morphogenesis, since the seminal study by Alan Turing: How perturbations to uniform, stationary states are amplified to form a spatially periodic pattern is thoroughly understood with extensive experimental demonstrations. In contrast to this spontaneous pattern formation, however, little is understood how given boundary condition leads to global pattern formation. Here, we demonstrate that the fixed boundary can transform a temporally-periodic, spatially-uniform state to a spatially-periodic, stationary pattern– a novel class of pattern formation mechanism. This pattern formation is not understood by the Fourier-mode linear stability analysis – the standard tool for Turing instability. Rather, by introducing a one-dimensional ‘spatial’ map, the emergent pattern is reproduced well as its periodic attractor, by replacing the time with space. Accordingly, linear dispersion relationship between the period and wavelength is obtained. This provides a general tool to analyze the pattern formation in reaction-diffusion systems, while the boundary-induced pattern formation mechanism will explain several biological morphogenesis, including recent experimental observations.

T. Kohsokabe and K. Kaneko, Boundary-induced pattern formation from temporal oscillation: Spatial map analysis, EPL 116, 48005 (2016).
[Abstract]

Photocurrent simulation in TH photoconductive detectors (Vol. 44 No. 1)

The simulated signals are shown for two different gate fluences as well as the input THz field. The schematics on the right depict the reason for the difference between signals, which is caused by the saturation of the traps in the high fluence regime resulting in long lived mobile carriers which do not appear in the low fluence case.

Nowadays the most widely used spectroscopic technique in the terahertz band (0.1 to 10 THz) is called terahertz time-domain spectroscopy, which generates and detects pulses of terahertz light by triggering photoconductive antennae using infrared pulses from an ultrafast laser.

The influence of geometrical structure and semiconductor properties on the performance of photoconductive antennae has been studied extensively from the experimental point of view. However, theoretical studies on the semiconductor carrier dynamics of these devices have only emerged recently and have mostly focused on simulating the performance of emitters.

The present work develops a semi-classical Monte-Carlo model that can simulate ultrafast carrier dynamics in photoconductive detectors. The simulation tracks the motion of millions of charges under the electric field of a terahertz pulse at various times after their photo-generation taking into account the quantum mechanical scattering of each particle. By utilising a sequence of simulations the transient photocurrent was modelled precisely. In photoconductive detectors the rate at which electrons become trapped is an important parameter that determines how the measured current transient differs from the actual terahertz pulse's shape. By examining the role of carrier trapping at various illumination levels the authors demonstrated that high powers can distort the measured photocurrent. This model will set the path for further development of detectors of pulsed terahertz radiation by providing insights into semiconductor material design for that application.

E. Castro-Camus, M.B. Johnston and J. Lloyd-Hughes, ‘Simulation of fluence-dependent photocurrent in terahertz photoconductive receivers’, Semicond. Sci. Technol. 27, 115011 (2012)
[Abstract]

Photoproduction of n'-mesons off the deuteron (Vol. 42, No. 2)

image From left to right of panels in Fig. 1, we have the inclusive reaction data first, then the world free and, this experiment, quasi-free proton data and last quasi-free neutron data from coincidence and an extraction of proton from deuteron inclusive data. Solid lines: solution (I) NH model, dashed lines: η'-MAID for neutron and dotted lines: CLAS proton data.

A key to our understanding of Quantum Chromodynamics (QCD) in the strong regime is our ability to reproduce the hadronic excitation spectrum. Up to now, and due to their limited predictive power, quark models forecast of this spectrum at high excitation energies is unsatisfactory and is dubbed "the missing resonances problem". To explore the high excitation energies in the hadron spectrum production or scattering of heavier mesons from a nucleon target is essential.

In a recent tour-de-force experiment, I. Jaegle et al. report on an impressive first measurement of η' photoproduction off a deuteron target at beam energies between 1.47 - 2.45 GeV at the tagged photon beam of the ELSA electron accelerator. Differential cross sections with a wide angular coverage were derived for quasi-free production both on protons and neutrons validating the quasi-free picture. And the first estimate of the coherent γd -> dη' contribution is found consistent with an impulse approximation, pointing to a viable isospin composition of model amplitudes and weak final state interactions.

Legendre polynomials coefficients from angular distributions fits of this experiment and world data are reported in the Fig. where proton and neutron cross sections for photon energies above 2 GeV, in a region where contributions from t-channel exchange are important, display a similar behaviour. At lower photon energies from where the proton cross-section peaks, the behaviour points to different resonance contributions and would require polarization observables for future investigation.

Photoproduction of η'-mesons off the deuteron
I. Jaegle et al. (the CBELSA/TAPS Collaboration), Eur. Phys. J. A, 47, (2011)
[Abstract] | [PDF]

Physical parameters matter in terms of cancer cells’ metastatic ability (Vol. 47 No. 2)

Plots of single-cell trajectories stimulated by different levels of epidermal growth factor

Scientists develop potential visual test for diagnosing invasive states of breast cancer cells

The micro-environment surrounding cancer cells is just as important as genes in regulating tumour progression. Scientists have therefore examined the biophysical and biochemical cues occurring in the vicinity of cancer cells. This represents a departure from the traditional measurement of secreted molecules, called biomarkers. The latest research in this field, recently published, found that the presence of a substance called Epidermal Growth Factor (EGF) promotes the motility of elongated mesenchymal tumour cells, which migrate depending on their adhesive properties by climbing along collagen fibres, in contrast to rounded tumour cells, which migrate in an adhesion-independent manner. These findings stem from the work of the authors. The study found that micro-environmental cues linked to the presence of EGF contribute to modulating the mobility of tumour cells—which by their nature can easily change and vary in form. These findings suggests that the cell aspect ratio could constitute a potential visual cue for diagnosing invasive states of breast cancer cells, and ultimately other cancer cells.

D. T. Geum, B. J. Kim, A. E. Chang, M. S. Hall, and M. Wu, Epidermal growth factor promotes a mesenchymal over an amoeboid motility of MDA-MB-231 cells embedded within a 3D collagen matrix, Eur. Phys. J. Plus 131, 8 (2016)
[Abstract]

Physical properties of solids elucidated by zooming in and out of high resolution (Vol. 49 No.5-6)

A new study shows how to couple highly accurate and simplified models of the same system to extract thermodynamics information using simulations

Computer simulations are used to understand the properties of soft matter—such as liquids, polymers and biomolecules like DNA –which are too complicated to be described by equations. They are often too expensive to simulate in full, given the intensive computational power required. Instead, a helpful strategy is to couple an accurate model—applied in the areas of the system that require greater attention—with a simpler, idealised model. In a paper published recently, the authors make the accurate model in high-resolution coincide seamlessly with an exactly solvable representation at lower resolution.

M. Heidari, R. Cortes-Huerto, K. Kremer, and R. Potestio , Concurrent coupling of realistic and ideal models of liquids and solids in Hamiltonian adaptive resolution simulations, Eur. Phys. J. E 41, 64 (2018)
[Abstract]

Physics of Squeezed Helices (Vol. 44 No. 1)

When confined to a surface helical filaments form "squeelices". Two categories of squeelices exist, depending on the sign of the twist-kink self-energy (positive and negative for g >1 and g <1 respectively). (a,b) Typical filament-helices in 3-D before confinement. (a',b') When confined, (a) transforms into untwisted circularized shapes (g > 1) while (b) assumes twisted and wavy shapes (g<1).

Helically coiled filaments are everywhere in living nature. In experimental situations, filaments are often squeezed flat (or nearly flat) onto two-dimensional surfaces. Under such 2D confinement filament-helices form what we call "squeelices" - peculiar squeezed conformations often resembling looped waves, spirals or circles. Many such shapes have been observed and reported for a variety of biological and man-made filaments.

With filament-helices being such a ubiquitous structure, we asked the question: what happens when filament helices become confined? We found that the confinement produces some dramatic changes in filaments shape, giving rise to several notable and surprising effects. In particular “squeelices” can display an enhanced cyclisation probability, unusually strong end-to-end fluctuations and a conformational multistability. The conformational dynamics of confined helices is most naturally described in terms of discrete particle-like entities – which we call the "twist-kinks". These "twist-kinks" turn out to be analogous and are physically related to crystal dislocations in solids and Sine-Gordon-kinks from soliton physics. Twist-kinks move thermally along the confined helix and interact much like quasi-particles. Confined helices can further thermally switch between discrete twist-quantized states comprising different numbers of twist-kinks.

Doing simple things (confining) to simple objects (helical filaments) can give rise to complex physics.

G-M. Nam, N-K. Lee, H. Mohrbach, A. Johner and I. M. Kulíc, ‘Helices at interfaces’, EPL, 100, 28001 (2012)
[Abstract]

Picosecond-range control over information processing (Vol. 46 No. 2)

Energy levels of the parabolic QD versus the strength of the Rashba SOC Credit: J. A. Budagosky et al.

Optical manipulation is key to reaching the necessary speed to control the furtive underlying physical mechanism used in quantum information processing.

Quantum computing will, one day, bring quicker information processing. One of the keys to such speed is being able to control the short-lived physical phenomenon holding quantum information, also known as quantum bits (qubits). A new study presents a novel optical manipulation technique to control one possible kind of qubit—represented, in this case, by polarised electron spins—exposed to an ultra-short pulsed laser in the picosecond-range. The authors have tested this novel optics approach using a quantum dot—nanoscopic artificial structures with a small number of electrons—in this study.

They used optical manipulation relying on very high-frequency—terahertz—laser pulses to induce a 180⁰ rotation of the polarisation of the spin of a single electron confined in a semiconductor quantum dot. They then used a set of mathematical tools to define the most effective manipulation technique.

J. A. Budagosky Marcilla and A. Castro, , “Ultrafast single electron spin manipulation in 2D semiconductor quantum dots with optimally controlled time-dependent electric fields through spin-orbit coupling”, Eur. Phys. J. B 88, 15 (2015)
[Abstract]

Pionic Deuterium (Vol. 42, No. 5)

image De-excitation cascade in pionic deuterium

A new precise measurement of the pD(3p-1s) X-ray transition in the pionic deuterium atom has been performed at the PSI accelerator in Switzerland. The pionic deuterium is a short lifetime atom, where the negative pion (p-) replaces the electron, resulting in an atomic size scaled down by the ratio of the pion mass over electron mass, a factor of about 270.

The experiment makes use of a high intensity decelerated beam of - stopping in a cooled deuterium gas target where the p- is captured. Following the capture an atomic de-excitation quantum cascade of 0.1ns duration takes place and the atom ends up in the 1s ground state as shown in the Figure. A Bragg spectrometer equipped with a bent Silicon crystal and pixel semiconductor detectors provides the precise X-ray detection in the appropriate keV region.

The measurement of the energy of the X-ray emitted in the pD(3p-1s) transition leads to a new value of 3075.583 ± 0.030 eV. A new and updated calculation of this transition energy assuming a pure electromagnetic system (pure QED - no strong interaction) leads to a value of 3077.939 ± 0.008 eV. The difference between these two quantities gives exactly the hadronic shift e1s = -2.336 ± 0.031 eV. The line-shape has been analysed, providing a new and precise value of the hadronic broadening G1s = 1.171+0.023 -0.049 eV.

The accuracy of 1.3% achieved for the shift e1s leads to a more precise determination of the isoscalar scattering length a+ (pD being an isoscaler object). The new precise value obtained for the hadronic broadening G1s leads to a new determination of the threshold parameter a, the transition strenght for a S-wave pion, with unprecedented accuracy.

Pionic Deuterium
Th. Strauch, F.D. Amaro, D.F. Anagnostopoulos, P. BÅNuhler, D.S. Covita, H. Gorke, D. Gotta, A. Gruber, A. Hirtl, P. Indelicato, E.-O. Le Bigot, M. Nekipelov, J.M.F. dos Santos, Ph. Schmid, S. Schlesser, L.M. Simons, M. Trassinelli, J.F.C.A. Veloso and J. Zmeskal, Eur. Phys. J. A, 47, 88 (2011)
[Abstract]

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