Europhysics News

Highlights

Fragmentation of random trees (Vol. 46 No. 4)

Networks are ubiquitous, appearing in the study of subjects as diverse as gene-protein interactions, power grids, and algorithms.

The function of a network is closely linked to its structure. For instance, in biochemical reaction networks, removal of a species or reaction can dramatically change the output of the system. Evolving networks often undergo degradation, making it important to understand how the structure breaks apart when components are randomly removed, also revealing how resilient a network is to attacks.

We studied the fragmentation of a random tree, a network formed by repeatedly attaching new nodes to an existing node chosen uniformly randomly.

We present exact equations governing the evolution of fragment sizes after a fraction of the nodes are removed at random, along with asymptotic solutions. For very large trees, fragment size distribution decays as a power law, with an exponent of 1+1/m, m being the fraction of remaining nodes. This implies that a few very large fragments coexist with many small ones (see figure).

Our findings reveal unusual fragmentation kinetics, where the fragment size distribution is characterized by a time-dependent exponent, and can provide insight into other fragmentation processes where dynamic parameters are observed.

Z. Kalay and E. Ben-Naim, Fragmentation of random trees, J. Phys. A, Math. Theor., 48, 045001 (2015)
[Abstract]

Fragmenting ions and radiation sensitizers (Vol. 50, No. 5-6)

A new study using mass spectrometry is helping piece together what happens when DNA that has been sensitized by the oncology drug 5-fluorouracil is subjected to the ionising radiation used in radiotherapy.

The anti-cancer drug 5-fluorouracil (5FU) acts as a radiosensitizer: it is rapidly taken up into the DNA of cancer cells, making the cells more sensitive to radiotherapy. However, little is known about the precise mechanism through which radiation damages cells. A team of scientists have now used mass spectrometry to shed some light on this process; their work was recently published in EPJD. A full understanding of this process could ultimately lead to new ways of protecting normal tissues from the radiation damage caused by essential cancer treatments.

P.J.M. van der Burgt, M.A. Brown, J. Bockova, A. Rebelo, M. Ryszka, J-C. Poully and S. Eden, Fragmentation processes of ionized 5-fluorouracil in the gas phase and within clusters, Eur. Phys. J. D 73, 184 (2019)
[Article]

From experiment to evaluation, the case of n+238U (Vol. 49, No. 1)

Experimental set-up for capture measurements at the Joint Research Centre in Belgium.

Evaluated nuclear data represent the bridge between experimental and theoretical achievements and final user applications. The complex evolution from experimental data towards final data libraries forms the cornerstone of any evaluation process. Since more than 90% of the fuel in most nuclear power reactors consists of ²³⁸U, the respective neutron induced cross sections are of primary importance towards accurate neutron transport calculations. Despite this significance, the relevant experimental data for the ²³⁸U(n,γ) capture reaction have only recently provided for a consistent description of the resonance region. In this work, the ²³⁸U(n,γ) average cross sections were evaluated in the energy region 5-150 keV, based on recommendations by the IAEA Neutron Standards projects and experimental data not included in previous evaluations.

A least squares analysis was applied using exclusively microscopic data. This resulted in average cross sections with uncertainties of less than 1%, fulfilling the requirements on the High Priority Request List maintained by the OECD-NEA. The parameterisation in terms of average resonance parameters maintained consistency with results of optical model and statistical calculations. The final deliverable is an evaluated data file for ²³⁸U, which was validated by independent experimental data.

I. Sirakov, R. Capote, O. Gritzay, H.I. Kim, S. Kopecky, B. Kos, C. Paradela, V.G. Pronyaev, P. Schillebeeckx, and A. Trkov, Evaluation of cross sections for neutron interactions with 238U in the energy region between 5 keV and 150 keV, Eur. Phys. J. A 53, 199 (2017)
[Abstract]

Frozen-planet states in exotic helium atoms (Vol. 51, No. 3)

Low energy anti-proton accelerator. © Wikipedia, Tom Purves from Toronto, Canada

Mapping the energy levels and estimated the stability of a ‘frozen planet’ configuration of anti-protonic helium.

Exotic subatomic particles that are like ‘normal’ particles apart from one, opposite, property - such as the positron, which is like an electron but positively rather than negatively charged - are collectively known as antimatter. Direct studies of collisions between particles of matter and those of antimatter using giant facilities such as those at CERN can advance our understanding of the nature of matter. In this work, the energy levels of an exotic form of helium produced in this way are mapped. The work has been described by one commentator as ‘... a new jewel in the treasure of scientific achievements in atomic physics theory”.

T.P. Grozdanov, E.A. Solov'ev , Hidden-crossing explanation of frozen-planet resonances in antiprotonic helium; their positions and widths, Eur. Phys. J. D 74, 50 (2020)
[Abstract]

Functional Multiplex PageRank: the centrality is a function (Vol. 48 No. 2)

Correlations between the Functional Multiplex PageRank of Heathrow, Gatwick, Frankfurt and Düsseldorf airports in the multiplex network formed by Lufthansa and British Airways flights

Multiplex networks are formed by a set of nodes connected by different types of interactions. The centrality of a node in a multiplex network depends on the influence one attributes to different types of connections. For example consider a multiplex of two layers formed by Lufthansa and British Airways flights. If we attribute maximum influence to Lufthansa flights Frankfurt airport is more central than Heathrow airport while the opposite is true if we attribute maximum influence to British Airways flights. The Functional Multiplex PageRank combines this information through a paradigmatic shift: the centrality of a node is not a single number but an entire function associated to the relevance given to the different types of connections. For each node it allows to characterize which types of connections contribute the most to its centrality. Interestingly the correlations between the Functional Multiplex PageRank of different nodes reveal the similarity in the role of the nodes.

J. Iacovacci, C. Rahmede, A. Arenas and G. Bianconi, , Functional Multiplex PageRank, EPL 116, 28004 (2016)
[Abstract]

Gap function of hexagonal pnictide superconductor SrPtAs (Vol. 45 No.4)

Above: Quasiparticle equal energy (ω) surfaces for A_1g(a) and E_g(b) gap functions at ω/Δ₀=0.5 (Δ₀ is the gap amplitude). Below: QPI spectrum (c,d) for corresponding gap functions. The scattering vectors q_i are characteristic for the nodal gap structure.

The pnictide superconductor SrPtAs has a hexagonal layered structure that breaks in-plane inversion symmetry while overall the crystal is still centrosymmetric. This has peculiar consequences for the electronic structure as well as the Cooper pairing. It leads to a strong Rashba spin orbit coupling and splitting of quasi-2D 5d Pt bands that dominate the Fermi surface. Although the superconducting gap functions are even or odd under inversion the in-plane pairing is nevertheless a mixture of singlet and triplet pairs due to the large Rashba coupling. Microscopic theories have obtained possible s+f and p+d wave candidates with unconventional nodal structure. We propose to apply Bogoliubov quasiparticle interference (QPI) technique to SrPtAs which records the spectral density fluctuations at the surface due to impurities. We show that their analysis can give important clues on the nodal structure of the unknown SrPtAs gap function.

A. Akbari and P. Thalmeier, “Gap function of hexagonal pnictide superconductor SrPtAs from quasiparticle interference spectrum”, EPL, 106, 27006 (2014)
[Abstract]

Gauge theory of topological phases of matter (Vol. 44 No. 3)

Topologically protected states of matter are the focus of recent intensive research efforts. Such states may play an important role in future concrete implementations of devices for topological quantum computing. Prominent examples are incompressible 2D electron gases exhibiting the Quantum Hall effect or the spin Hall effect, 3D topological insulators and superconductors, etc. From a conceptual point of view it is important to note that the low-energy effective theories describing all these states can be derived, using only very general principles, from a unified theoretical framework which we have called “gauge theory of states of matter”.

A key idea underlying our framework is to promote fundamental or emergent global symmetries of idealized systems to local gauge symmetries of realistic systems, and to then study the response of such systems under variations of the corresponding gauge fields. For systems with a bulk energy gap, our theory predicts the general form of the response laws, transport equations, and the structure of gapless surface modes. It also elucidates how the structure of the ionic background, electromagnetic fields, velocity fields and curvature influence the properties of such systems.

J. Fröhlich and P. Werner, ‘Gauge theory of topological phases of matter’, EPL, 101, 47007 (2013)
[Abstract]

Geometry of quantum evolution in a nonequilibrium environment (Vol. 50, No. 3)

The geometric effect of quantum dynamical evolution has potential applications in studying quantum phase transition and realizing geometric quantum computation. Due to the fact that a quantum system unavoidably interacts with its environments and undergoes decoherence, much extensive attention has been paid to theoretical investigations on the geometric dynamical evolution in open quantum systems under nonunitary dynamics. The investigation on the geometry in the dynamical evolution of an open quantum system is crucial for further understanding the origins of decoherence, quantum-classical transition and so on.

There are many significant situations where the nonequilibrium feature of the environment becomes dominant. In these situations, the statistical properties of the environmental noise are nonstationary, corresponding physically to impulsively excited phonons of the environment in certain nonequilibrium states initially. We show that the renormalization of the intrinsic energy of the system, namely, the frequency shift induced by the nonequilibrium feature of the environment has a significant impact on the geometry of quantum dynamical evolution.

X. Cai, R. Men, Y. Zhang and L. Wang, Geometry of quantum evolution in a nonequilibrium environment, EPL 125, 30007 (2019)
[Abstract]

Germanium detectors get position sensitive (Vol. 47 No. 5-6)

Interaction positions determined by the pulse shape analysis of AGATA and the AGATA spectrometer at GANIL (picture by P. Lecomte)

High purity germanium detectors have grown into very popular devices within the field of gamma ray spectroscopy. The sensitive part of these detectors consists of the largest, purest and monocrystalline semi-conductors used on earth. Ge detectors are famous for their outstanding energy resolution for electromagnetic radiation, especially in the field of nuclear physics and astrophysics. Recently technical advances and the segmentation of the Ge crystals opened up new opportunities. In this way, the Ge detector becomes a position sensitive device and allows for the novel gamma-ray tracking technique.

New gamma ray spectrometers are currently under construction and implement the new method. The article describes all the theoretical concepts, which are needed for a precise understanding of all detector properties. Moreover, an elaborate computer code, named ADL, was developed and yielded a huge set of hundred thousands of detector pulses. These pulses are compared to measured pulses from individual gamma rays in order to extract the position where the radiation interacted with the detector material and created charges. ADL utilizes all relevant aspects of signal creation and formation with the Ge detector and the subsequent electronics. Meanwhile the code is successfully used for position sensitive spectroscopy within the AGATA project.

B. Bruyneel, B. Birkenbach and P. Reiter, Pulse shape analysis and position determination in segmented HPGe detectors: The AGATA detector library, Eur. Phys. J. A 52, 70 (2016)
[Abstract]

Geyser oscillations in the vacuum expansion of solid He (Vol. 41, No. 5)

Vacuum expansion of solid ⁴He exhibits identical oscillations in flow (a) in the source chamber (SC) pressure (b), thus providing new insight into the geyser effect.

Following Galli and Reatto's scenarios for the existence of the Andreev-Lifshitz supersolid phase of 4He as an effect of excess vacancies [J. Low Temp. Phys. 124, 197 (2001)], the vacuum expansion of solid He through a micrometric orifice was suggested to inject excess vacancies into the bulk [R. Grisenti et al, J. Electr. Spectr. 129, 201 (2003)]. Unexpectedly these vacuum expansion experiments exhibited, superimposed on the uniform He flow out of the orifice, spectacular periodic intensity bursts (geyser effect), with their period increasing for decreasing solid He temperatures or increasing pressures [G. Benedek et al, Phys. Rev. Lett. 95, 095301 (2005)]. Anomalies were observed at temperatures below the lambda point, in a restricted pressure domain of the solid just above the melting pressure, suggesting frictionless flow. These early measurements, almost contemporary to Kim and Chan torsional oscillator experiments [Nature 427, 225 (2004)], raised new questions about the effective role of vacancies in explaining the regular collapses of the solid leading to the geyser bursts.

The present study, while confirming the geyser effect over a wider temperature and pressure domains, provides an answer to those questions by monitoring the pressure inside the source and comparing data with the gas inlet valve (Figure) open and closed. The new results indicate that the geyser collapse does not occur near the orifice, as previously suggested, but at a plug in the feed line upstream of the source chamber. Each collapse is triggered by the increasing vacancy concentration which makes the solid behave much as a liquid. On this basis it is argued that vacuum expansion provides a novel approach for investigating exotic non-equilibrium phases of quantum solids such as helium.

The Geyser effect in the expansion of solid helium into vacuum
G. Benedek, P. Nieto, and J. P. Toennies, Eur. Phys. J. B 76, 237 (2010)
[Abstract] | [PDF]

Giant negative group time delay by microwave mode adaptors (Vol. 43 No. 6)

Input (black) of a pulse traversing 20 m adaptor cavity and its output (red). The adaptor cavity delay is 2200ns and leads to a negative time shift.

In 1960 Brillouin wrote a footnote in his famous book on ‘Wave Propagation And Group Velocity’ (p.79): “The negative parts of the (theoretical) group velocity have no physical meaning. A negative velocity shows the maximum of the group at the output before it has entered the input of a special medium”. However, since 1985 several physicists measured a negative delay and thus a negative group velocity at a sharp molecular resonance. Recently a giant negative group time delay was observed in a medium of two microwave mode adaptors separated by a 20 m long waveguide when they are not parallel aligned. Instead of +60 ns vacuum time -2.2 µs were measured for the same distance. Such adaptors are used in communication technology to transform rectangular waveguide modes into circular waveguide modes: For instance, in the case of TV reception via satellite. The strange behaviour is based on a 90° shift of the linear polarization of the superimposed right and left circular wave modes in the case of a perpendicular adaptor orientation. Remarkable, the polarization shift of 90° takes place at each reflection and in this way makes the shifted adaptors to reflectors, whenever the distance between the adaptors equals a multiple of half the wavelength.

A. Carôt, H. Aichmann and G. Nimtz, ‘Giant negative group time delay by microwave adaptors’, EPL, 98, 64002 (2012)
[Abstract]

Giraffes are living proof that cells’ pressure matters (Vol. 43 No. 6)

If homeostatic pressure within biological tissues was proportional to fluid pressure, giraffes could not exist (Credit: © bourbon numérik - Fotolia.com).

This article presents a model that describes dividing cells within human tissues from the perspective of physicists could help further the understanding of cancer growth. It explores the relative impact of the mechanical pressure induced by dividing cells in biological tissues. and could have significant implications for the understanding of cancer growth.

The authors create a two-component mathematical model accounting for both the cells and the fluid caught in between. On the one hand, cells are modelled as behaving like a dividing fluid subject to expansion. On the other hand, the interstitial fluid is akin to an ideal fluid that cannot be compressed. This model is designed to elucidate the nature of mechanical pressure exerted upon dividing cells by their surrounding tissues, referred to as homeostatic pressure.

It replaces a previous single-component model they developed last year. Its assumption: the homeostatic pressure is proportional to the fluid pressure within the tissue. If that were the case, very tall organisms such as giraffes could not exist, because the cells in their lower body would die under pressure.

Thanks to the two-component model, the authors find that it is the cells’ pressure and not the interstitial fluid’s pressure that influences the level of cell division. Going one step further, they pinpoint the range of fluid pressure required to drive cell flow within the body.

Such models could help gain a greater understanding of the importance of the disruption of homeostatic pressure in biological tissues caused by cancer cells that are characterised by abnormal levels of cell proliferation.

J. Ranft, J. Prost, F. Jülicher and F. J. Joanny, ‘Tissue dynamics with permeation’, Eur. Phys. J. E (2012) 35, 46
[Abstract]

Gold-diamond nanodevice for hyperlocalised cancer therapy (Vol. 46 No. 5-6)

Colocalisation studies with confocal fluorescence microscopy and acidotropic probes show particles trapped in the lysosomes of the living HeLa cells

Gold nanorods can be used as remote controlled nanoheaters delivering the right amount of thermal treatment to cancer cells, thanks to diamond nanocrystals used as temperature sensors.

Precise targeting biological molecules, such as cancer cells, for treatment is a challenge, due to their sheer size. Now, the authors have proposed an advanced solution, based on a novel combination of previously used techniques, which can potentially be applied to thermal cancer therapy. The authors presented in this work an improved sensing technique for nanometre-scale heating and temperature sensing. Using a chemical method to attach gold nanorods to the surface of a diamond nanocrystal, they have invented a new biocompatible nanodevice. It is capable of delivering extremely localised heating from a near-infrared laser aimed at the gold nanorods, while accurately sensing temperature with the nanocrystals.The novelty of this study is that it shows that it is possible to use diamond nanocrystals as hypersensitive temperature sensors with a high spatial resolution—ranging from 10 to 100 nanometres—to monitor the amount of heat delivered to cancer cells.

.-Ch. Tsai, O. Y. Chen, Y.-K. Tzeng, Y. Y. Hui, J. Y. Guo, Ch.-Ch. Wu, M.-Sh. Chang and H.-Ch. Chang, Gold/diamond nanohybrids for quantum sensing applications, EPJ Quantum Technology, 2, 19 (2015)
[Abstract]

Granular material conductivity increases in mysterious ways under pressure (Vol. 48, No. 4)

Scientists reveal how electrical resistance in metallic granular media decreases as the pressure on the micro-contact interface between the grains increases.

What happens when you put pressure on bunch of metallic microbeads? According to physicists, the conductivity of this granular material increases in unusual ways. So what drives these changes? The large variations in the contact surface between two grains or the rearranging electrical paths within the granular structure? In a recent study published recently, the authors made systematic measurements of the electrical resistance—which is inversely related to conductivity—of metallic, oxidised granular materials in a single 1D layer and in 3D under compression. They showed that the granular medium conducts electricity in a way that is dictated by the non-homogenous contacts between the grains. These findings have implications for industrial applications based on metallic granular material.

M. Creyssels, C. Laroche, E. Falcon and B. Castaing, Pressure dependence of the electrical transport in granular materials, Eur. Phys. J. E 40, 56 (2017)
[Abstract]

Graphene Mini-Lab (Vol. 44 No. 1)

Dirac electron diffusion (represented by a red semi classical trajectory) on a ratchet asymmetric potential represented by blue triangles can be rectified by ac drives.

A mini-laboratory is proposed to study fast moving electrons in the carbon-based material called graphene as a model for massless particles moving in a noisy environment with velocities close to the speed of light, in analogy to relativistic Brownian particles such as cosmic rays.

In graphene (one atom-thick carbon layer forming a honeycomb lattice) the interaction of electrons with atoms changes the effective mass of the electrons. As a result, the energy of these electrons becomes similar to the photon energy. Therefore, electrons in graphene can be regarded as ultra-relativistic particles even though their actual velocity is 100 times lower than the speed of light.

The authors used the classical Brownian motion formalism to study the dynamics of electrons within the confines of the graphene mini-laboratory. They considered different chip geometries AND subjected them to changing conditions affecting the way these electrons diffuse through the material such as temperature and electric field strength.

Going one step further, the authors were able to rectify electron fluctuations and to control the electron motion itself from an unusual chaotic to a periodic motion by varying the electric field. Future work would experimentally demonstrate how variation of the temperature could be used positively to enhance the performance of graphene chips by gaining a greater control over electron transport. Such graphene mini-labs could also ultimately help understand the dynamics of matter and anti-matter in cosmic rays.

A. Pototsky, F. Marchesoni, F. V. Kusmartsev, P. Hanggi and S. E. Savel'ev, ‘Relativistic Brownian motion on a graphene chip’, Eur. Phys. J. B (2012) 85: 356
[Abstract]

Subcategories

Page 14 of 39