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Valence band of graphite oxide (Vol. 42, No. 1)

image The valence band of HOPG (Highly oriented pyrolytic graphite), PG (precursor graphite), GO (graphite oxide) with a photon energy of 130eV.

Graphite oxide (GO) has been the subject of intense study for its use in producing cheap and vase amounts of graphene by reduction through chemical or physical methods. Reduced graphite oxides have been used in graphene-based applications such as transparent conducting film (TCF), flexible displays, field effect transistors (FET), supercapacitors, and batteries. Therefore, it is essential to study the fundamental electronic and structural properties of graphite oxide to exploit its possible applications.

This paper investigated the valence band structure of graphite oxide by photoelectron spectroscopy for the first time. The typical sp2 hybridization states (π and σ) found in graphite were also observed in graphite oxide. However, the π state near the Fermi level disappeared because of bonding between the π and oxygen-related states originating from graphite oxide, indicating electron transfer from graphite to oxygen and resulting in a downward shift of the highest occupied molecular orbital (HOMO) state to the higher binding energies. The band gap opening increased to about 1.8 eV, and additional oxygen-related peaks (O2p and O2s) were observed at 8.5 and 27 eV in graphite oxide. The work function of graphite oxide was also first measured using the kinetic energy cut-off of the secondary electrons to be 5.9 ± 0.1 eV. This research results can improve fundamental understanding of graphite oxide for its possible applications.

Valence band of graphite oxide
Hae Kyung Jeong, Cheolsoo Yang, Bong Soo Kim and Ki-jeong Kim, EPL 92, 37005 (2010)
[Abstract]

Validity of the Silver-Blaze property for QCD at finite chemical potential (Vol. 51, No. 1)

Sketch of the QCD phase diagram in the temperature and baryon chemical potential plane.

The properties of the theory of strong interactions, QCD, at finite chemical potential are of great interest for at least two reasons: (i) model studies suggest a potentially rich landscape of different phases with highly interesting analogies to those found in solid state physics; (ii) the resulting thermodynamic properties have far reaching consequences for the physics of neutron stars and neutron star mergers. Investigating the properties of light scalar and pseudo-scalar quark-antiquark bound states at finite chemical potential by solving coupled sets of Dyson-Schwinger equations , the meson masses, wave functions, and decay constants are computed, as well as changes in the quark dressing functions for chemical potentials below the first-order chiral phase transition while tracing charge-conjugation parity breaking. Eventually, we confirm the validity of the Silver-Blaze property: in observables all dependencies of colored quantities (propagators, wave-functions, etc.) on chemical potential cancel out and we observe constant masses and decay constants up to and into the coexistence region of the first-order chiral phase transition.

Pascal J. Gunkela, Christian S. Fischerb, and Philipp Isserstedtc, Quarks and light (pseudo-)scalar mesons at finite chemical potential, Eur. Phys. J. A 55, 169 (2019)
[Abstract]

Vector Correlators in Lattice QCD (Vol. 43 No. 1)

image Comparison of the vacuum polarisation calculated from e+e- annilhilation cross section with recent lattice simulations.

Vacuum polarisation, the modification of the photon propagator due to virtual electron-positron pairs, is one of the first quantum loop corrections encountered in field theory. In both QED and QCD it causes the running of the appropriate fine structure constant as the physical scale is varied, and also corrects the magnetic moments of electrons and muons from the value 2 predicted by the Dirac equation. For scales below a few GeV the QCD vacuum polarisation cannot be calculated perturbatively, but can be accessed via the optical theorem from the annihilation cross section of e+e- into hadrons, which is simply related to the spectral density ρ(s) in the vector isoscalar channel.. This paper opens a new direction by first assessing the current state-of-the-art in calculating the vacuum polarisation in lattice QCD, the most systematic non-perturbative approach, and then by setting out two different routes to improving on this, and identifying applications to strong interaction phenomenology.

Comparison with experimental data reveals that current results are badly finite-volume affected. The paper provides technical details enabling these distortions to be understood and ultimately extrapolated to the large volume limit. It also uses the same data to estimate the current-current correlator as a function of Euclidean time exposing the possibility that different ranges are amenable to different theoretical approaches; the dominant hadronic correction to (g-2) for the muon, about to be measured with unprecedented precision at Fermilab, comes from the range 0.5fm<ct<1.5fm. This reasoning is also suggests a new QCD reference scale, to help callibrate the lattice spacing using high-precision numerical estimates of the vector correlator.

Vector Correlators in Lattice QCD: Methods and Applications
D. Bernecker and H. Meyer, Eur. Phys. J. A, 47 11, 1 (2011)
[Abstract]

Versatile method yields synthetic biology building blocks (Vol. 47 No. 5-6)

Fluorescence microscopy image of polymersomes, taken 3 days after production

New high-throughput method to produce both liposomes and polymersomes on the same microfluidic chip Synthetic biology involves creating artificial replica that mimic the building blocks of living systems. It aims at recreating biological phenomena in the laboratory following a bottom-up approach. Today scientists routinely create micro-compartments, so called vesicles, such as liposomes and polymersomes. Their membranes can host biochemical processes and are made of self-assembled lipids or a particular type of polymers, called block copolymers, respectively. In a new study, researchers have developed a high-throughput method--based on an approach known as microfluidics--for creating stable vesicles of controlled size. The method is novel in that it works for both liposomes and polymersomes, without having to change the design of the microfluidic device or the combination of liquids. The authors recently published these findings. Typical applications in synthetic biology include the encapsulation of biological agents and creation of artificial cell membranes with a specific biochemical function. They anticipate that their method might also be applicable for the controlled fabrication of hybrid vesicles used in bio-targeting and drug-delivery.

J. Petit, I. Polenz, J.- C. Baret, S. Herminghaus and O. Bäumchen, Vesicles-on-a-chip: A universal microfluidic platform for the assembly of liposomes and polymersomes, Eur. Phys. J. E 39, 59 (2016)
[Abstract]

Vibrationally assisted quantum engines – a new scheme for directed coherent transport (Vol. 46 No. 5-6)

Schematic of the vibrationally rocked excitonic quantum ratchet

Energy transport at the nano/quantum scale has a long history of research, with significant interest being paid in the debate over whether quantum coherence plays a role in the efficiency of exciton transport in photosynthetic complexes. Much attention has also turned to improving energy transport for man-made energy harvesting systems and nanodevices, such as in solar cells and quantum dot arrays.

Achieving directed quantum transport permits far superior collection of the deposited energy. The study of quantum ratchets shows how directed energy transport is achievable in quantum dot arrays. Recent experimental work on light harvesting molecules have implicated the role of discrete mechanical modes in enhancing the energy transport through dipole arrays, but say less about directed transport. Here the authors bring together these two apparently unrelated models to present a scheme for a new type of quantum engine. Utilising both excitonic and vibrational motions it is shown that the resulting coherent mechanical dynamics causes directed enhanced energy transport towards one end of the exciton chain. The quantum engine is autonomous, requiring no external pumping or modulation but works off the initial charge on the exciton chain which excites the vibrational motion.

C. R. Myers, G. J. Milburn and J. Twamley, Vibrationally assisted quantum energy pumps, New J. Phys., 17, 093030 (2015)
[Abstract]

Water window imaging opportunity (Vol. 45 No.5-6)

Time-frequency analysis of dipole acceleration extracted from the numerical simulations performed in argon, for three different regimes of laser intensity.
Credit: Pérez-Hernández et al.

A new theoretical study elucidates mechanisms that could help in producing coherent radiations, and could ultimately help to achieve high-contrast images of biological samples.

Ever heard of the water window? It consists of radiations in the 3.3 to 4.4 nanometre range, which are not absorbed by the water in biological tissues. New theoretical findings show that it is possible to develop coherent radiations within the water window. These could be the basis of an optimal technique to obtain a high-contrast image of the biological samples or to be used in high-precision spectroscopy. Now, a new theoretical study identifies the physical mechanism needed to efficiently generate the harmonic radiations at high laser intensities that occur beyond the saturation threshold of atoms and molecules. These findings, aimed at improving conventional methods of coherent radiation production to reach the water window, were recently found by the authors. In previous similar work, studies focused on hydrogen as the atomic target. In the present work, the authors extend the study to argon atoms.

J. A. Pérez-Hernández, M. F. Ciappina, M. Lewenstein, A. Zaïr and L. Roso (2014), “High-order harmonic generation at high laser intensities beyond the tunnel regime”, Eur. Phys. J. D 68, 195 (2014)
[Abstract]

Wavy energy potential patterns from scattering nuclei reveal hidden information (Vol. 48, No. 4)

A Feynman diagram of proton-neutron scattering mediated by a neutral pion

New approach to analysing anomalies in collisions between atomic nuclei promises a new perspective on how they interact

Anomalies always catch the eye. They stand out from an otherwise well-understood order. Anomalies also occur at sub-atomic scale, as nuclei collide and scatter off into each other—an approach used to explore the properties of atomic nuclei. The most basic kind of scattering is called ‘elastic scattering,’ in which interacting particles emerge in the same state after they collide. Although we have the most precise experimental data about this type of scattering, the author contends in a paper published recently that a new approach to analysing such data harbours potential new interpretations of fundamental information about atomic nuclei.

R.S. Mackintosh, Elastic scattering phenomenology, Eur. Phys. J. A 53, 66 (2017)
[Abstract]

Weak localization and Berry phases in HgTe quantum wells (Vol. 44 No. 2)

Berry phase Γ and corresponding weak localization (WL) and weak antilocalization (WAL) regions for transport in HgTe quantum wells.

Topological insulators represent a novel class of materials featuring conducting surface states at the boundaries of an insulating bulk crystal. These surface states are induced by an inverted band-ordering, which arises if the states of the valence and the conduction band interchange their electron-hole character. In a Hall bar geometry fabricated from a two-dimensional topological insulator, the boundaries carry two quasi one-dimensional edge states with opposite spins and opposite propagation directions. Since both states carry a universal conductance quantum and backscattering between them is absent, the total conductance is quantized, which is known as the hallmark of the quantum spin Hall effect. For the first time, this quantized conductance was measured in heterostructures based on different HgTe and CdTe compounds, which exhibit a transition from a normal to an inverted band ordering for increasing HgTe layer widths.

Reflectionless transport, and thereby the quantum Spin Hall effect, can also be explained by Berry phases inducing destructive interference of backscattered waves. However, this transport phenomenon can only be demonstrated if the Fermi energy can be tuned into the bulk insulating state.

The present work shows that, besides the measurement of the quantum spin Hall effect, the different band orderings and the associated Berry phases of HgTe heterostructures can also be detected in the bulk conducting state by an investigation of the energy dependent magnetoconductance profile. Depending on the strength of the spin-orbit interaction and the chemical potential, the charge transport features characteristic transitions between weak localization and weak antilocalization, which are distinctly different for both band orderings and consequently introduce another way to verify the topological insulator properties.

V. Krueckl and K. Richter, ‘Probing the band topology of mercury telluride through weak localization and antilocalization’, Semicond. Sci. Technol. 27, 124006 (2012)
[Abstract]

Wetting routes of droplet upon patterned hydrophilic surface (Vol. 50, No. 1)

Different wetting routes of droplet upon patterned hydrophilic surface

The wetting transition of the droplet on the patterned hydrophilic surface can occur spontaneously and may further lead to superwetting that has the potential to develop novel technologies in the field of anti-fogging, printing and heat transfer. However, it is still unknown how the wetting transition occurs on such a patterned surface. In contrast to the conventional view that wetting occurs immediately in the vertical direction upon the contact of the droplet with the solid surface due to the capillary force, we find that the droplet spreads first in the horizontal direction if the patterned surface has a large enough roughness. Then, the wetting transition occurs at the periphery rather than at the middle part of droplet, which is termed as “one-dimensional wetting”. We ascribe such an interesting phenomenon to the competition between the horizontal force arising from the non-equilibrium surface tension and the vertical capillary force as well as to the different pressure under the droplet, which lead to three different wetting routes (one-dimension wetting (One), two dimension wetting (Two), Between one and two dimension wetting (BOT)).

T. Li, X. Liu, H. Zhao, B. Zhang and L. Wang, Counterintuitive wetting route of droplet on patterned hydrophilic surface, EPL 123, 36003 (2018)
[Abstract]

Weyl states mean magnetic protectorates (Vol. 49, No. 1)

Pictorial view of a Weyl quasi-particle state and its magnetic field loops

Electrons in conductors are basically free particles subject to residual collisions, a picture first proposed by Paul Drude in 1900 and later developed by Arnold Sommerfeld in 1927 with quantum concepts. The Drude-Sommerfeld assumption that in between any two collisions the electrons move freely is only approximate since electrons transport charge. While in motion they give rise to an electric current that creates a magnetic field able to influence each other’s trajectories. In the Drude-Sommerfeld scenario this small magnetic field interaction among electrons is simply discarded. However this magnetic field can lead to topologically protected states in case the electrons move in a layer no matter its strength assuming residual collisions. The magnetic field streamlines, created by the electronic motion, form loops that pierce the layer twice. These magnetic field loops are a consequence that electrons occupy Weyl states and yet live in the parabolic band of the Drude-Sommerfeld scenario. Indeed this residual magnetic interaction brings these Weyl states to a higher energy since they acquire magnetic energy. Nevertheless they fall in magnetic protectorates forbid to decay into a lower energy state.

M. M. Doria and A. Perali, Weyl states and Fermi arcs in parabolic bands, EPL 119, 21001 (2017)
[Abstract]

What protects minority languages from extinction? (Vol. 51, No. 4)

Greetings in some of the world’s 6,000+ languages.
(CC BY-SA 2.0, by Flickr user quinn.anya).

Mathematical modelling of competing languages in a geographical area suggests two scenarios in which one or more minority languages will be more likely to survive.

Over 6,000 languages are currently spoken worldwide, but a substantial minority - well over 5% - are in danger of dying out. It is perhaps surprising that this fraction is no higher, as most models have so far predicted that a minority language will be doomed to extinction once contacts with speakers of the majority language reach a certain level. This work describes, using mathematical modelling, two mechanisms through which this doomsday scenario does not occur, i.e. several languages come to coexist in the same area.

J-M Luck and A Mehta, On the coexistence of competing languages, Eur. Phys. J. B 93, 73 (2020)
[Abstract]

When diffusion depends on chronology (Vol. 44 No. 5)

Motorways are examples of nodes connected by edges studied as complex networks. (Photo Credit: Highways Agency)

The present work shows that the order of events taking place in complex networks may dramatically alter the way diffusion occurs in them. The Internet, motorways and other transport systems as well as many social and biological systems are composed of nodes connected by edges, and can be represented as networks.

Scientists studying diffusion over such networks over time have now identified the temporal characteristics that affect their diffusion pathways. In this paper, the authors show that one key factor that can dramatically change a diffusion process is the order in which events take place in complex networks.

They developed an analytical model to better understand the properties of time-dependent networks that either accelerate or slow down diffusion. Their study focused on different classes of popular models for diffusion, namely random walks and epidemic spread models, and found the way in which the temporal ordering of events matters. They expect these results to help in building more appropriate metrics to understand real-world complex network data.

R. Lambiotte, L. Tabourier and J.C. Delvenne, ‘Burstiness and spreading on temporal networks’, Eur. Phys. J. B (2013)
[Abstract]

Winner and losers of the EU funding challenge (Vol. 46 No. 1)

A representative Minimum Spanning Tree of the network of countries involved in the FP7 accepted proposals, which captures the backbone of interactions between the countries.
Credit: M.Tsouchnika et al.

Successfully attracting EU funding could depend on the nature of the research consortium.

The European Union has a well-oiled funding mechanism in the form of grants given to research consortia. Understanding which type of consortium work receives funding could help future applicants. And it could also bring further transparency on how public funds are spent. Now, the authors have brought valuable insights into the structure of research consortia that are most likely to attract EU funding.

They found that a proposal from a partnership made up of small-scale institutes is more likely to be rejected. The authors also found that large-scale institutes favour collaborations with small-scale ones, in both successful and unsuccessful research consortia. This means they are different from other social networks of similar interactions. Finally, the team also revealed that in both network types the same five countries are the most influential ones: France, Germany, the UK, Spain and Italy.

M. Tsouchnika and P. Argyrakis, “Network of participants in European Research: accepted vs. rejected proposals”, Eur. Phys. J. B, 87, 292 (2014)
[Abstract]

WONDER-2018: a workshop on nuclear data (Vol. 50, No. 5-6)

WONDER-2018: a workshop on nuclear data
By combining experimental data (left, example of experimental setup) and theoretical calculations (middle, example of theoretical calculations), it becomes possible to perform an evaluation of nuclear data (right, example of evaluated cross-sections). Those evaluated nuclear data are collected in a regularly updated international library such as JEFF (Joint Evaluated Fusion and Fission).

To describe the path of neutrons in the material but also the chain reactions that take place in a reactor and the changes in the composition of matter due to nuclear reactions, neutronics uses computer codes. These codes have also acquired such a level of performance since the last two decades that the main source of uncertainty in neutronic calculations comes today from nuclear data. In this context, the 5th edition of the International Workshop On Nuclear Data Evaluation for Reactor Applications (WONDER-2018), organized by the French Alternative Energies and Atomic Energy Commission (CEA) in collaboration with the NEA (Nuclear Energy Agency of the OECD) was held in Aix-en-Provence, France, on October 2018. The main objective was to identify future trends in the measurement, modeling and evaluation of nuclear data needed for current reactors and innovative reactor concepts. Proceedings were published in EPJ Web-of-Conferences: EPJ Web of Conferences Volume 211 (2019).

WONDER-2018 – 5th International Workshop On Nuclear Data Evaluation for Reactor applications, EPJ Web of Conferences 211 (2019)
[Article]

X mode Doppler Reflectometry k-spectral measurements in ASDEX Upgrade: experiments and simulations (Vol. 48, No. 4)

Power spectra over perpendicular wavenumber. "GENE" shows density fluctuations, the others are scattered microwave power (a.u.)

Doppler reflectometry is a microwave backscattering diagnostic for measuring flows and density fluctuation spectra in fusion plasmas. One longstanding problem is the discrepancy between the Doppler spectrum and the density fluctuation spectrum from turbulence simulations: The red "GENE" curve has its knee at a different wavenumber compared to the experimental Doppler measurements. The knee position is intrinsic to the turbulent drive mechanism and should be the same in both.

We coupled the sophisticated plasma turbulence code GENE to the fullwave code IPF-FD3D to model the scattering and the power response of the reflectometer in the presence of realistic turbulence. Dashed lines in the figure are the result of fullwave simulations. The blue curve (A=1) reproduces the knee position, but not the power law of the experiment. Reduction of the density fluctuation strength yields a better fit (A=0.5). The apparent shift of the knee is therefore a characteristic of the diagnostic.

This breakthrough reconciles turbulence simulations and experiment and shows that extra-ordinary mode scattering is taking place in the non-linear regime.

C. Lechte, G. D. Conway, T. Görler, C. Tröster-Schmid and the ASDEX Upgrade Team, X mode Doppler reflectometry k-spectral measurements in ASDEX Upgrade: experiments and simulations, Plasma Phys. Control. Fusion 59, 075006 (2017)
[Abstract]

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