Vol. 47 No.2 - Highlights

Bright sparks shed new light on the dark matter riddle (Vol. 47 No. 2)

Data gathered by the detector module Lise depicted in the light yield energy plane

Highest sensitivity detector ever used for very light dark matter elementary particles

The origin of matter in the universe has puzzled physicists for generations. Today, we know that matter only accounts for 5% of our universe; another 25% is constituted of dark matter. And the remaining 70% is made up of dark energy. Dark matter itself represents an unsolved riddle. Physicists believe that such dark matter is composed of (as yet undefined) elementary particles that stick together thanks to gravitational force. In a study recently published, the authors use the so-called phonon-light technique to detect dark matter. They are the first to use a detection probe that operates with such a low trigger threshold, which yields suitable sensitivity levels to uncover the as-yet elusive particles responsible for dark matter. The asymmetric dark matter particle models are one of the candidates for a new elementary particle to explain dark matter. The experimental detection is no different from the scattering of two billiard balls, as the particle scatters on an atomic nucleus. The challenge: the lighter the dark matter particle is, the smaller the energy deposited in the crystal used for detection is. Currently, no other direct dark matter search method has a threshold for nuclear recoils as low as 0.3 keV. As such, the CRESST-II team are the first to ever probe dark matter particle masses at such low mass scale.

G. Angloher +39 co-authors, Results on light dark matter particles with a low-threshold CRESST-II Detector, Eur. Phys. J. C 76, 25 (2016)

Helping turn waste heat into electricity (Vol. 47 No. 2)

The resonant structure of electron scattering on the bismuth lattice

How the collective motion of electrons interacting with bismuth crystal atoms can be fine-tuned to harvest excess heat

At the atomic level, bismuth displays a number of quirky physical phenomena. A new study reveals a novel mechanism for controlling the energy transfer between electrons and the bismuth crystal lattice. Mastering this effect could, ultimately, help convert waste heat back into electricity, for example to improve the overall efficiency of solar cells. These findings have been published now. The author investigates the collective motion of electrons in bismuth, which behaves in a fluid manner with waves propagating in it, a phenomenon referred to as a low energy plasmon. This study demonstrates that the low energy plasmons, when tuned to the same wavelength as the lattice vibrations of the bismuth crystal, or phonons, can very efficiently slow lattice motion. In essence, this plasmon-phonon coupling mechanism, once intensified under specific conditions, could be a new way of transferring energy between electrons and the underlying crystal lattice.

One implication is that the plasmon-phonon coupling can help to explain a long-since observed, significant effect in bismuth: the so-called Nernst effect. This occurs when a sample is warmed on one side and subjected to a magnetic field, causing it to produce a significant electrical voltage in the perpendicular direction. Hence it turns heat into useful electricity.

P. Chudzinski, Resonant plasmon-phonon coupling and its role in magneto-thermoelectricity in bismuth, Eur. Phys. J. B 88, 344 (2015)

Unfolding quantum jumps (Vol. 47 No. 2)

Example of a quantum trajectory displayed in real time (top) and in effective time (bottom)

A novel way of defining time parameterisation in continuous measurements sheds a new light on quantum jumps and helps unravel hidden phenomena.

The evolution of a continuously and weakly monitored quantum system is given by a quantum trajectory. The stronger the measurement gets, the less regular the trajectory becomes with the progressive emergence of seemingly instantaneous jumps and sharp spikes which make the analysis of this interesting regime difficult in practice. The authors have proposed to locally redefine time as a function of the measurement back-action in order to blow up the details which are lost in physical time. This new parameterisation unfolds quantum jumps which become continuous and provides a well defined strong measurement limit. This method yields a finer description than the standard von Neumann projective approach to strong measurements: anomalous observables that show non-zero fluctuations in the former would appear trivial in the latter. In addition to its theoretical interest, the technique presented has the advantage of being readily applicable to existing experimental datasets.

M. Bauer, D. Bernard and A. Tilloy, Zooming in on quantum trajectories, J. Phys. A: Math. Theor. 49, 10LT01 (2016)

Anti-hydrogen origin revealed by collision simulation (Vol. 47 No. 2)

Scientists studying the formation of antihydrogen ultimately hope to explain why there is more matter than antimatter in the universe. © vpardi / Fotolia

Numerical model takes us one step closer to understanding anti-hydrogen formation, to explain the prevalence of matter and antimatter in the universe

Anti-hydrogen is a particular kind of atom, made up of the antiparticle of an electron—a positron—and the antiparticle of a proton—an antiproton. Scientists hope that studying the formation of anti-hydrogen will ultimately help explain why there is more matter than antimatter in the universe. In a new study published recently, the authors demonstrate that the two different numerical calculation approaches they developed specifically to study collisions are in accordance. As such, their numerical approach could therefore be used to explain antihydrogen formation. The authors employed two very different calculations —using a method dubbed coherent close-coupling — for both one- and two-centre collisions respectively in positron scattering on hydrogen and helium. Interestingly, they obtained independently convergent results for both approaches. Such convergence matters, as it is a way to ascertain the accuracy of their calculations for anti-hydrogen formation.

I. Bray, J. J. Bailey, D. V. Fursa, A. S. Kadyrov and R. Utamuratov, Internal consistency in the close-coupling approach to positron collisions with atoms, Eur. Phys. J. D 70, 6 (2016)

Adjustable adhesion power: what fakirs can learn from geckos (Vol. 47 No. 2)

The model of adhesion between two patterned, yet elastic, surfaces.

New study models adhesion force as key to contact between two rough, yet elastic, surfaces

The authors have now developed a model to study the importance of adhesion in establishing contact between two patterned, yet elastic, surfaces. Nature is full of examples of amazing adjustable adhesion power, like the feet of geckos, covered in multiple hairs of decreasing size. Until now, most experimental and theoretical studies have only focused on the elastic deformation of surfaces, neglecting the adhesion forces between such surfaces. This new approach just published, matters when the scale of adhesive forces, is comparable to elastic forces on materials such as tyres. What the authors focused on was the transition between what they describe as the “happy fakir” scenario, where the sphere hardly presses against the pillars, and the “impaled fakir” scenario, where there is a strong adhesion between the two surfaces. By comparing experimental data on the size of the contact area--which gives rise to the so-called van der Waals cohesive forces between the molecules--with the findings of their new theoretical model, they revealed the importance of adhesion between the two different surfaces in establishing contact.

L. Dies, F. Restagno, R. Weil, L. Léger and C. Poulard, Role of adhesion between asperities in the formation of elastic solid/solid contacts, Eur. Phys. J. E 38, 130 (2015)

Scrutinising the tip of molecular probes (Vol. 47 No. 2)

The solid lines indicate the temperature range used to estimate the amount of molecules loaded onto the probe

Nature of interaction of probe molecules on the surface of oxide particles elucidated

Studies of molecules confined to nano- or micropores are of considerable interest to physicists. That’s because they can manipulate or stabilise molecules in unstable states or obtain new materials with special properties. In a new study published recently the authors have discovered the properties of the surface layer in probe molecules on the surface of oxide particles. These properties depend on the interaction at the interface. In this particular study, probes are formed by adsorption of rod-like cyanophenyl derivates on the surface of oxide particles. The authors found that their surface layers behave like glass-forming liquids.

They used data from infrared spectroscopy and thermogravimetry to identify the strength of the interaction between the probe and the oxide surface, which also helped them determine the type of bonding to the surface. The study shows that the value of the surface density can be used to divide the composites into several groups. This helps to determine that the probe molecules applied to the surface of a given group can display similar interactions, as observed in surfaces of the same family.

S. Frunza, L. Frunza, C. P. Ganea, I. Zgura, A. R. Brás and A. Schönhals, Rod-like cyanophenyl probe molecules nanoconfined to oxide particles: Density of adsorbed surface species, Eur. Phys. J. Plus 131, 27 (2016)

Lifshitz transitions and correlation effects in unconventional superconductors (Vol. 47 No. 2)

Calculation of the effective mass as a function of thermal energy and the shift of the top of a hole pocket (Et) relative to the Fermi energy

Unconventional superconductivity is observed in heavy fermion systems, cuprates, molecular crystals, and iron-based superconductors close to a point in the phase diagram where as a function of a control parameter (pressure or doping), the antiferromagnetic order is suppressed. A widespread view is that at this point, which is called a quantum critical point (QCP), strong antiferromagnetic fluctuations are a candidate for the glue mediating superconductivity and also account for the normal state non-Fermi-liquid behaviour. Recent ARPES results on ferropnictides have shown that in these compounds the non-Fermi-liquid like scattering rate does not diverge at the QCP, as expected in the quantum critical scenario. Rather, near the QCP it is constant over a large range of the control parameter. In this study, a new scenario is proposed using minimum model calculations: a co-action of hole vanishing Lifshitz transitions and correlation effects is able to explain the ARPES results as well as the strange normal state transport and thermal properties.

J. Fink, Influence of Lifshitz transitions and correlation effects on the scattering rates of the charge carriers in iron-based superconductors, EPL 113, 27002 (2016)

Single atom manipulations at the LT-UHV-4-STM (Vol. 47 No. 2)

The LT-UHV-4-STM head and a 5.12 x 5.12 nm2 STM image of a letter C constructed atom by atom with 6 Au ad-atoms on Au(111) using here scanner PS3. I = 50 pA, V = 500 mV with ΔZ = 0.12 nm. Single atom manipulations tunnel resistance: 333 K.Ω

The new ScientaOmicron LT-UHV scanning tunneling microscope is installed at Pico-Lab CEMES-CNRS (Toulouse) with its 4 STM scanners performing on the same surface. At 4.3 K, we report state-of-art STM experiments on Au(111) usually performed on the most stable single tip LT-UHV STMs. Operating the 4 scanners independently or in parallel with an inter tip apex distance < 100 nm, the ΔZ stability is better than 2 pm per STM. Single Au atom manipulations were performed on Au(111) recording the pulling, sliding or pushing signal. When contacting one Au ad-atom, a jump to contact leads to a perfect linear low voltage I-V characteristics with no averaging. Two tips surface conductance measurements were also performed with one lock-in and in a floating sample mode to capture the Au(111) surface states via two STM tips dI/dV characteristics. This new instrument is exactly 4 times a very precise single tip LT-UHV-STM.

J. Yang, D. Sordes, M. Kolmer, D. Martrou and C. Joachim, Imaging, single atom contact and single atom manipulations at low temperature using the new ScientaOmicron LT-UHV-4 STM, Eur. Phys. J. Appl. Phys. 73, 10702 (2016)

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)

Exact formula now available for measuring scientific success (Vol. 47 No. 2)

Scientists develop formula to describe the growth of scientists’ h-index.
https://eu.fotolia.com/id/84276789 © treenabeena

Physicists team has developed equations governing the growth of authors’ h-index using an agent-based model.

Scientometrics research is the science of evaluating scientific performance. Physics methods designed to predict growth based on a scale-free network have rarely been applied to this field. Now, the authors have developed an analytical method using a previously developed agent-based model to predict the h-index, probably the most popular citation-based scientific measurement, using bibliometric data. They are the very first to succeed in developing an exact formula to calculate the number of external citations and self-citations for each paper written by an author. These findings have just been published by the authors. It opens the door to applying this growth analysis to social network users or citations from different scientific fields.

B. Żogała-Siudem, G. Siudem, A. Cena and M. Gagolewski, Agent-based model for the h-index – exact solution, Eur. Phys. J. B 8, 21 (2016)

Liquid foam: plastic, elastic and fluid (Vol. 47 No. 2)

Snapshot of a foam in a convergent channel

New study elucidates the plastic flows behind the motion of liquid foams, whose ability to absorb all kinds of waves makes them well-suited as acoustic insulators, or as explosion wave absorbers.

What differentiates complex fluids from mere fluids? What makes them unique is that they are neither solid nor liquid. Among such complex fluids are foams. They are used as a model to understand the mechanisms underlying complex fluids flow. Now, the authors have gained new insights into predicting how complex fluids react under stretching conditions due to the interplay between elasticity, plasticity and flow. These findings were recently published by the authors. Ultimately, potential applications include the design of new, optimised acoustic insulators based on liquid forms, or the mitigation of blast waves caused by explosions.

B. Dollet and C. Bocher, Flow of foam through a convergent channel, Eur. Phys. J. E 38, 123 (2015)

Superconductivity found in BaPd2As2 single crystal (Vol. 47 No. 2)

Superconducting Meissner effect in ThCr2Si2-type BaPd2As2 crystal

In this work the single crystal of ThCr2Si2-type BaPd2As2 was successfully prepared by a self-flux growth method. The crystal structure was characterized by powder X-ray diffraction method, with the space group I4/mmm and lattice parameters a = 4.489(2) Å, c = 10.322(3) Å. From the characterizations of low temperature electrical resistivity, magnetic susceptibility and specific heat measurements, bulk superconductivity was clearly revealed in this compound, although it was not found in other structural types of BaPd2As2. The superconducting onset Tc (critical temperature) is 3.85 K and the zero resistivity happens at 3.80 K. Surprisingly, this Tc is much higher than those of all other isostructural Pd-based superconductors, such as CaPd2As2 (Tc = 1.27 K) and SrPd2As2 (Tc = 0.92 K). The reason that leads to a higher Tc in this compound deserves more detailed studies to understand the underlying mechanism.

Q. Guo, J. Yu1, B.-B. Ruan, D.-Y. Chen, X.-C. Wang, Q.-G. Mu, B.-J. Pan, G.-F. Chen and Z.-A. Ren, Superconductivity at 3.85 K in BaPd2As2 with the ThCr2Si2-type structure, EPL 113, 17002 (2016)