Europhysics News

Highlights

Pionic-Hydrogen Atom and Quantum Chromodynamics (Vol. 46 No. 3)

Analogous to the vast amount of knowledge acquired on the electronic hydrogen atom over the last century as a probe of Quantum Electrodynamics, hadronic physics is using “pionic hydrogen” - a hydrogen atom where the electron is replaced by a negatively charged pion - as a laboratory for investigating Quantum Chromodynamics (QCD). The small Bohr radius of pionic hydrogen offers a large sensitivity to the strong pion-proton interaction, leading to an energy shift compared to the ground state energy if only the electromagnetic interaction is considered. The precise determination of this shift provides a benchmark of our understanding of the strong interaction from basic principles in QCD. To this end an exquisite experiment was devised and performed at the high intensity, low energy pion beam at the Paul Scherrer Institut using the cyclotron trap and an ultimate resolution Bragg spectrometer leading to an impressive four fold improvement compared to the previous best measurement as shown in the figure.

M. Hennebach et al. (+14 co-authors), Hadronic shift in pionic hydrogen, Eur. Phys. J. A 50, 190 (2014)
[Abstract]

Plasma and Nano put novel biomaterials into life (Vol. 48 No. 1)

Example of the sophisticated plasma+nano process: plasma and nano work together to produce biocompatible system of silver nanowires in nanoporous membrane.

Low-temperature plasma, i.e. ionized gas produced by electric discharges in gas or liquid, is a powerful tool for fabricating novel biocompatible nanomaterials.

Typically, complex nanomaterials are produced using chemistry-based techniques. They are cheap and efficient, and could be further utilized to fabricate nanomaterials vitally needed for novel devices. However, emerging applications require a new generation of nanomaterials to boost their characteristics and occupy devoted application niches. The low-temperature plasma could play a pivotal role in the nanosynthesis of immense complexity. This review paper reveals advantages of approaches based on the plasma environment to fabricate nanoscaled biomaterials exhibiting very high biological activity, biological inertness, and other features of the biomaterials capable of making them highly attractive. Plasma-assisted fabrication of gold and silicon nanoparticles for bio-applications; carbon nanoparticles for cancer therapy; carbon nanotube-based platforms for enzyme production and bacteria growth control; and other applications of low-temperature plasmas in the production of biologically-active materials were discussed. The effect of plasmas have led to better results, as compared with the conventional neutral-gas based methods.

I. Levchenko, M. Keidar, U. Cvelbar, D. Mariotti, A. Mai-Prochnow, J. Fang and K. Ostrikov, Novel biomaterials: plasma-enabled nanostructures and functions. Topical Review, J. Phys. D: Appl. Phys. 49, 273001 (2016).
[Abstract]

Plasma density evolution in a microwave pulse compressor (Vol. 46 No. 2)

Plasma density vs. time after the laser triggering for different microwave output.

Microwave plasma discharges have been widely investigated for many years; there is, however, a subject that is insufficiently studied. It is the plasma formation at the initial – nanosecond time-scale – stage of the high-pressure discharge in a resonant cavity and its interrelation with the process of microwave energy release from the cavity that goes out of resonance during the plasma generation. This subject directly concerns the operation of microwave compressors using commercial magnetrons and klystrons for short-pulse high-power microwave generation. In this work, for the first time, spectroscopic measurements were performed to investigate nanosecond dynamics of the plasma density in the S-band compressor with laser triggering. For pressurized helium filling the compressor cavity and switch, the plasma density was evaluated from the shapes of the 3888.65 Å and 4471.5 Å He I spectral lines. The measured evolution of the density was found to correlate with the peak power of the compressor output pulse and efficiency of the stored microwave energy extraction. With increasing microwave output, the plasma appears earlier in time after the laser beam enters the system, the plasma density rises more steeply, and it reaches higher values.

L. Beilin, A. Shlapakovski, M. Donskoy, T. Queller and Ya. E. Krasik, “Plasma density temporal evolution in a high-power microwave pulse compressor switch”, EPL 109, 25001 (2015).
[Abstract]

Plasma killing of Leukemia cells (Vol. 43 No. 6)

Cell suspensions in multi-well plate being exposed to the low temperature plasma plume emitted by the Plasma Pencil.

Plasmas are ionized gases that contain a mixture of electrons, ions, and neutrals. Plasmas are generated by adding some form of energy to a neutral gas. The most common method to generate plasma is to subject a gas to high level of electrical stress, which initiates an electronic avalanche and thus generating electrons, ions, and molecular fragments such as radicals and other reactive species. Low temperature plasmas in particular produce chemical species including reactive oxygen species (ROS) and reactive nitrogen species (RNS). ROS and RNS exhibit strong oxidative properties and can potentially trigger signaling pathways in biological cells. For example oxidation of the lipids and proteins that constitute the membrane of biological cells leads to the loss of their functions. In such an environment, bacterial cells were found to die within minutes or even seconds. In eukaryotes, very low doses of low temperature plasmas were found to help the proliferation of some skin cells and at slightly higher doses, plasmas can induce apoptosis, or programmed cell death, opening the possibility to use plasma technology against cancerous cells.

In this study, we investigate the effect of low temperature atmospheric pressure plasma towards the progression of cancerous human T-cell leukemia cells. The source of low temperature plasma is the plasma pencil, which utilizes short duration high voltage pulses. Our data shows that cell morphology and cell viability was affected in a dose-dependent manner after treatment with low temperature plasma. The outcome of this study revealed that the effect of plasma exposure was not immediate, but had a delayed effect and increasing the time of plasma exposure resulted in increased leukemia cell death.

N Barekzi and M Laroussi, ‘Dose dependent killing of Leukemia cells by low temperature plasma’, J. Phys. D: Appl. Phys. 45 422002 (2012)
[Abstract]

Plasma screens enhanced as disorder strikes (Vol. 43 No. 6)

Disorder within the inner structure of plasma material gives a higher quality to plasma screens (Credit: FreeDigitalPhotos.net).

A new study improves our understanding of plasma sources, a state of matter similar to gas in which a certain portion of the particles are ionised and which are used for example in plasma display panels. Under certain circumstances, plasma tends to form structures such as filaments of electric discharge akin to mini-lightning. The authors investigated the transition from a highly ordered filament pattern, which is arranged hexagonally, to a disordered system due to the reduction of the externally applied voltage.

To analyse the transition in the order of the discharge, Wild and colleagues used two approaches. First, they used a method commonly employed to analyse spatial patterns, called 2D Fourier transformation. Then, for the first time, they applied an analysis tool typically used to evaluate dusty plasma discharges, known as triple correlation function.

The authors observed a pivot point in the voltage at which the decaying order started occurring. This information can ultimately be used to guarantee the quality of applications such as plasma screens. That is because the dissolution of self-organised electric discharge filaments in plasma matter enhances the homogeneity of the matter.

R. Wild and L. Stollenwerk, ‘Breakdown of order in a self-organised barrier discharge’, EPJ D, 66, 214 (2012)
[Abstract]

Plasma tool for destroying cancer cells (Vol. 45 No.3)

The plasma discharged used to study DNA at ambient air conditions. Credit: Han et al.

Inducing biological tissue damage with an atmospheric pressure plasma source could open the door to many applications in medicine The authors conducted a quantitative and qualitative study of the different types of DNA damage induced by atmospheric pressure plasma exposure. This approach, they hope, could ultimately lead to devising alternative tools for cancer therapy as well as applications in hospital hygiene, dental care, skin diseases, antifungal care, chronic wounds and cosmetics treatments.

To investigate the DNA damage from so-called non-thermal Atmospheric Pressure Plasma Jet (APPJ), the team adopted a common technique used in biochemistry, called agarose gel electrophoresis. They studied the nature and level of DNA damage under two different conditions of the helium plasma source with different parameters of electric pulses.

The next step would involve investigating plasma made from helium mixtures with different molecular ratios of other gases to increase the level of radical species, such as reactive oxygen species and reactive nitrogen species, known to produce severe DNA damage. These could, ultimately, help to destroy cancerous tumour cells.

X. Han, W. A. Cantrell, E. E. Escobar and S. Ptasinska, “Plasmid DNA damage induced by Helium Atmospheric Pressure Plasma Jet”, Eur. Phys. J. D, 68, 46 (2014)
[Abstract]

Plasma: Casimir and Yukawa mesons (Vol. 46 No. 1)

Energy correction factor for two gold plates in the absence of any intervening plasma.
Credit: Ninham et al.

The Casimir electromagnetic fluctuation forces across plasmas are analogous to so-called weak nuclear interaction forces, as new findings show.

A new theoretical work establishes a long-sought-after connection between nuclear particles and electromagnetic theories. Its findings suggests that there is an equivalence between generalised Casimir forces and what are referred to as weak nuclear interactions between protons and neutrons. The Casimir forces are due to the quantisation of electromagnetic fluctuations in vacuum, while the weak nuclear interactions are mediated by subatomic scale particles, originally called mesons by Yukawa. These results have been found by the authors.

The authors extended the formulae for the Casimir force between these ideal metal plates to include interactions across a plasma and temperature, explicitly. The new formulae that emerge show that long-range electromagnetic fluctuations are qualitatively different from those across a vacuum. They also shed some new light on measurements of Casimir forces between metal plates, an issue that has long puzzled physicists.

B. W. Ninham, M. Boström, C. Persson, I. Brevik, S. Y. Buhmann and B. E. Sernelius, “Casimir Forces in a Plasma: Possible Connections to Yukawa Potentials”, Eur. Phys. J. D 68, 328 (2014)
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Plasmas as gaseous electrodes for aqueous electrochemistry (Vol. 44 No. 1)

Atmospheric-pressure microplasma stably formed in a flow of argon gas at the surface of acidic water

Plasmas are ionized gases made-up of electrons and gaseous ions. In theory, plasmas are a source of charge that can be coupled with liquids to initiate electrochemical reactions in solution. This has been known for some time, at least since 1887 when Gubkin first reported the use of plasmas for electroplating silver from silver nitrate. Unfortunately, plasmas are inherently difficult to stabilize at atmospheric pressure, and vacuum operation has limited prior experiments to solvents with low vapour pressures such as ionic liquids.

The present work shows how to get stable atmospheric-pressure plasmas allowing aqueous electrochemical reactions. The approach is based on pd scaling (gas pressure p and dimension d of the plasma) which imposes to reduce the dimension of the plasma to go to higher pressure. Atmospheric operation is achieved by forming a µm-scale plasma, or microplasma.

Upon operating the microplasma as the cathode at the surface of an aqueous solution, the electrons from the plasma reduce the protons to produce hydrogen gas. At the platinum counter-electrode, oxidation reactions lead to the formation of oxygen gas. The electrolysis of water is well known in electrochemistry, but this is the first time it has been demonstrated with a plasma electrode. As plasmas are increasingly in use with wet electrodes for medical and materials applications, it is crucial to understand the high complexity of plasma-liquid interactions. The role of electrons has thus far been largely overlooked, and this work brings a crucial piece of the puzzle.

M. Witzke, P. Rumbach, D. B. Go and R. M. Sankaran, ’Evidence for the electrolysis of water by atmospheric-pressure plasmas formed at the surface of aqueous solutions’, J. Phys. D 45, 442001 (2012)
[Abstract]

PLED polymers evolve characteristically during operation (Vol. 51, No. 5)

Molecular dynamics simulations have shown that the mysteriously high efficiency of polymer LEDs arises from interactions between triplet excitons in their polymer chains, and unpaired electrons in their molecular impurities.

Polymer LEDs (PLEDs) are devices containing single layers of luminescent polymers, sandwiched between two metal electrodes. They produce light as the metal layers inject electrons and holes into the polymer, creating distortions which can combine to form two different types of electron-hole pair: either light-emitting ‘singlets’, or a non-emitting ‘triplets’. Previous theories have suggested that the ratio between these two types should be around 1:3, which would produce a light emission efficiency of 25%. However, subsequent experiments showed that the real value can be as high as 83%. We found that this higher-than-expected efficiency can be reached through interactions between triplet excitons, and impurities embedded in the polymer.

Y D Wang, J J Liu, Y X Liu, X R Wang, Y Meng, Dynamic Recombination of Triplet Exciton with Trapped Counterion in Conjugated Polymers, Eur. Phys. J. B 93, 173 (2020)
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Polarization of the surface emission from quantum dashes (Vol. 43 No. 6)

Degree of the linear polarization for standard (a) and columnar quantum dashes (b) versus their height (symbols); solid lines show a theoretical dependence. The respective cross-sections are schematically shown for the reference.

We propose a novel approach to control the polarization of surface emission from quantum-dot-like objects - an issue of great interest for optoelectronic applications where both fully polarized and polarization-insensitive gain is highly desired. The experimental study of strongly asymmetric In(Ga)As/InP nanostructures in a wide range of geometries verifies our theoretical modelling.

A simple analytical formula describing the dependence of the degree of polarization on the nanostructure height is established, which is of practical importance when aiming at fabricating structures with predictable polarization properties without sophisticated atomistic modelling. Furthermore, it shows that the observed changes in polarization are a consequence of the valence band states mixing. It appears that the surface emission is enhanced for the polarization along the in-plane larger dimension and sensitive to the lateral shape anisotropy. Its saturation for highly asymmetric geometries makes fully polarized emission unachievable solely by the lateral symmetry control.

Our study reveals very high sensitivity of the surface emission polarization to the object height and demonstrates its continuous height-driven enhancement up to 0.9 for so called columnar quantum dashes – a record value for any epitaxial system. It is possible only through the combination of a strong in-plane anisotropy and an enhanced nanostructure height – the condition shown to be essential to fully tailor the polarization properties of the surface emission. Moreover, it opens up a possibility to combine unpolarized edge emission and strongly polarized surface emission in one device via its active region engineering.

Anna Musiał + 11 co-authors, ‘Height-driven linear polarization of the surface emission from quantum dashes’, Semicond. Sci. Technol. 27, 105022 (2012)
[Abstract]

Polychromatic cylindrically polarized beams (Vol. 47 No. 5-6)

Various polarization patterns (arrows) and intensity distributions (underlying doughnut) of a co-rotating radially polarized X-wave

Cylindrically polarized beams represent a class of solutions, where the polarization can be radially or azimuthally distributed across the intensity profile. These beams have very intriguing properties, both from a fundamental and an applied perspective. Despite their great success, they have been almost exclusively studied and realized within the monochromatic regime.

An open question is if non-monochromatic cylindrically polarized solutions of Maxwell equations exist. New research answers to this question by employing X waves with orbital angular momentum (the polychromatic counterpart of Bessel beams) as building blocks to generate optical pulses with radial and azimuthal polarization. This approach is different from the monochromatic case where Hermite-Gaussian beams are typically used. Solutions are investigated in the paraxial and the nonparaxial regime and the role of the pulse’s spectrum in the polarization properties of the pulse itself is pointed out. Analysis shows that the generalization of the concept of non-uniform polarization to the domain of optical pulses leads to new intriguing applications, such as spatially resolved Raman spectroscopy. Cylindrically polarized X-waves with orbital angular momentum could also open new intriguing scenarios for fundamental research and quantum optics.

M. Ornigotti, C. Conti and A. Szameit, Cylindrically polarized nondiffracting optical pulses, J. Opt. 18, 075605 (2016)
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Polycrystalline diamond: thermal conductivity versus optical defects (Vol. 49, No. 1)

Thermal conductivity of polycrystalline diamond versus optical transmission (%) at 633 nm

With development of technologies, diamond can be produced in big amount and various qualities. With the continuing advances in production of synthetic diamonds, diamond is feasible for more applications such as heat sink for electronics. In this work, the authors have worked together with one of the largest synthetic diamond company (IIa Technologies Pte. Ltd. Singapore). The authors have systematically studied the influence of various optical active defects on thermal conductivities for synthetic polycrystalline diamonds. It is found that the top surface, which shows higher thermal conductivity compared to the bottom surface (where growth nucleation started), also shows lower densities of defects than the bottom surface. The influence from non-diamond carbon phase and C-H stretch for top surface is not significant because of the low concentration of these defects on the growth surface. However the heat transport is still limited by the presence of Ns⁰ defect, which is the main contributor lowering the thermal conductivities on the top surfaces. For the bottom surface, non-diamond carbon phase, Si vacancy, C-H stretch and Ns0 defects all lead to an obvious reduction in the thermal conductivity. Furthermore, a well fitted equation was given to quickly estimate the thermal conductivity by optical transmission, and the equation was demonstrated to be valid at any wavelength in visible region. This enables a fast and reasonable estimation of thermal conductivity by visual inspection.

Q. Kong, A. Tarun, C. M. Yap, S. Xiao, K. Liang, B. K. Tay, D. S. Misra, Influence of optically active defects on thermal conductivity of Polycrystalline diamond, Eur. Phys. J. Appl. Phys. 80, 20102 (2017)
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Positron and electron collisions with formaldehyde (Vol. 42, No. 6)

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)
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Positronium formation in the noble gases (Vol. 44 No. 4)

Positronium formation cross-section for argon: solid curve, present results; dashed curves, previous calculations. Experimental measurements: hollow squares, UCL; solid squares, UCSD; triangles, ANU.

Positronium (Ps) is a neutral atom composed of an electron and its anti-particle, the positron. Since its reduced mass is essentially half that of atomic hydrogen, its binding energy is 6.8 eV. Being anti-particles, this electron-positron pair will annihilate producing gamma rays in about a tenth of a microsecond, long enough for experiments to be carried out. Ps can be formed in positron scattering from atoms and its cross section measured.

For most atoms, Ps formation has the lowest inelastic threshold. Theoretically, this is a two-centre problem (atomic nucleus and the center-of-mass of Ps) and thus difficult to calculate directly. We have simplified this problem by treating it as direct ionization with a threshold 6.8 eV below the true ionization threshold. Our method also ensures the rapid decrease in the Ps formation cross section at high energies. Even with this simple model our results for the noble gases are in much better agreement with experimental measurements than calculations based on more elaborate theories.

R.P. McEachran and A.D. Stauffer,, ‘Positronium formation in the noble gases’, J. Phys. B: At. Mol. Opt. Phys. 46, 075203 (2013)
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Practical limits for detection of ferromagnetism (Vol. 42, No. 4)

Ferromagnetic saturation moment of a ZnO substrate measured in five consecutive stages, exemplifying two of the most common sources of ferromagnetic contamination and showing a type of reversibility upon annealing under different atmospheres, which is often observed in some of the recently discovered nanomagnets mentioned in the text (the detection of ferromagnetism below 5 10^-7 emu is hindered by setup-related artefacts).

Over the last ten years, signatures of room-temperature ferromagnetism have been found in thin films and nanoparticles of various materials that are non-ferromagnetic in bulk. The implications of such high temperature ferromagnetism are in some cases so extraordinary, e.g. dilute magnetic semiconductors (DMS) with carrier-mediated ferromagnetism well above room temperature would revolutionize semiconductor-based spintronics, that they triggered an enormous volume of materials research and development. However, the magnetics community soon started realizing the dangers of measuring the very small magnetic moments of these nanomagnets (nanometer sized materials with nano-emu magnetic moments). Pushing state-of-the-art magnetometers to their sensitivity limits, where extrinsic ferromagnetic signals originating from magnetic contamination and measurement artefacts are non-negligible, these new nanomagnets raise a number of challenges to magnetometry techniques and, most of all, to its users' methods and procedures. While new nanomagnets continue being "discovered" based on magnetometry measurements, the general opinion is moving towards the notion that finding a signature of ferromagnetism by means of magnetometry, i.e. a magnetic hysteresis, is only necessary but not sufficient to claim its existence.

Through an extensive analysis of various materials subject to different experimental conditions, the authors aim at re-establishing the reliability limits for detection of ferromagnetism using high sensitivity magnetometry. The paper provides a roadmap describing how extrinsic ferromagnetism can be avoided or otherwise removed, its magnitude when such optimum conditions cannot be guaranteed, and to what extent its characteristics may or may not be used as criteria to distinguish it from intrinsic ferromagnetism.

Practical limits for detection of ferromagnetism using highly sensitive magnetometry techniques
L.M.C. Pereira, J.P. Araújo, M.J. Van Bael, K. Temst and A. Vantomme, J. Phys. D: Appl. Phys. 44, 215001 (2011)
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