Nuclear and Quark Matter at High Temperature (Vol. 48, No. 4)

In high-temperature field theory applied to nuclear physics, in particular to relativistic heavy-ion collisions, it is a longstanding question how hadrons precisely transform into a quark-gluon matter and back. The change in the effective number of degrees of freedom is rather gradual than sudden, despite the identification of a single deconfinement temperature. In order to gain an insight into this issue while considering the structure of the QGP we review the spectral function approach and its main consequences for the medium properties, including the shear viscosity. The figure plots a sample spectral density on the left and the effective number of degrees of freedom (energy density relative to the free Boltzmann gas) to the right. Two thin spectral lines result in a doubled Stefan-Boltzmann limit (SB), while any finite width reduces the result down to a single SB. When spectral lines become wide, their individual contributions to energy density and pressure drops. Continuum parts have negligible contribution. This causes the melting of hadrons like butter melts in the Sun, with no latent heat in this process.

T.S.Biró, A.Jakovác and Z.Schram, : Nuclear and quark matter at high temperature, Eur. Phys. J. A 53, 52 (2017)
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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)
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Proving Einstein right using the most sensitive Earth rotation sensors ever made (Vol. 48, No. 4)

A new study use the most precise inertial sensor available to date to measure whether Earth partially drags inertial frames along with its rotation.

Einstein’s theory of gravity, also referred to as General Relativity, predicts that a rotating body such as the Earth partially drags inertial frames along with its rotation. In a study recently published, a group of scientists based in Italy suggests a novel approach to measuring what is referred to as frame dragging. The authors propose using the most sensitive type of inertial sensors, which incorporate ring lasers as gyroscopes, to measure the absolute rotation rate of the Earth. The experiment aims to measure the absolute rotation with respect to the local inertial frame, which is what is referred to as frame dragging. In principle, the ring laser should show one rotation around the Earth's axis every 24 hours. However, should observation by reference to fixed stars in the sky show a slightly different rate of rotation, the difference can be attributed to frame dragging. The authors’ proposed experiment, called GINGER, requires two ring lasers to provide a reference measurement. Their proposed solution can accurately test the frame dragging effect at 1%, a vast improvement compared to previous experiments, which has 19% and 5% error in their measurement.

A. D. V. Di Virgilio, J. Belfi, W.-T. Ni, N. Beverini, G. Carelli, E. Maccioni and A. Porzio, GINGER: a feasibility study, Eur. Phys. J. Plus 132, 157 (2017)
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X mode Doppler Reflectometry k-spectral measurements in ASDEX Upgrade: experiments and simulations (Vol. 48, No. 4)

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)
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Accurate determination of Curie temperature in helimagnet FeGe (Vol. 48, No. 4)

Cubic helimagnet FeGe, the prototype of skyrmion materials near room temperature, has emerged and may impact future information technology. The magnetic entropy change (MEC) of helimagnet FeGe and the close relationship between the MEC and critical exponents of a second-order phase transition were studied. A relatively small MEC under external high magnetic field indicates the coexistence and competition between exchange anisotropy and magneto-crystalline anisotropy, and a stable balance is formed in the precursor region when the applied magnetic field cannot completely transform FeGe into a single magnetic structure phase. Based on the obtained magnetic entropy change and critical exponents, an accurate Curie temperature of helimagnet FeGe under zero magnetic field is confirmed to be 279.1 K, lower than 282 K deduced directly from the derivative magnetic susceptibility and higher than 278.2 K previously reported. So，the accurate determination of Curie temperature is conducive to reconsider the inhomogeneous chiral-spin state and reconstruct the magnetic phase diagram in the precursor region of helimagnet FeG.

L. Xu, H. Han, J. Fan, D. Shi, D. Hu, H. Du, L. Zhang, Y. Zhang and H. Yang, Magnetic entropy change and accurate determination of Curie temperature in single-crystalline helimagnet FeGe, EPL 117, 47004 (2017)
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Imaging helps to spot fake ancient daggers (Vol. 48, No. 4)

Combining neutron and X-ray imaging gives clues to how ancient weapons were manufactured

Since the 19^th century, collectors have become increasingly interested in weapons from ancient Asia and the Middle East. In an attempt to fight forged copies, physicists are now adding their imaging power to better authenticate these weapons; the fakes can't resist the investigative power of X-rays combined with neutron imaging. In a study published recently, the authors have demonstrated the usefulness of such a combined imaging approach to help museum curators in their quest to ensure authenticity. They can now reliably tell first-class modern copies of early daggers and swords from authentic ones. In this study, the authors focus on a kris—the distinctive weapon of Malaysia and Indonesia—and a kanjar— a double-edged dagger with a slightly curved blade and a pistol-grip made of metal, ivory, jade or some other hard-stone found e.g. in Persia and India. The authors found the internal structure of the traditional kris examined in this study was inconsistent with descriptions of traditional forging methods to be found in the extant literature, thus suggesting the artefact was a fake. By contrast, the kanjar analysed in the study is most likely to be authentic, as the material distribution in the volume of the blade conforms to traditional metallurgical processes.

F. Salvemini, F. Grazzi, N. Kardjilov, F. Wieder, I. Manke, D. Edge, A. Williams and M. Zoppi, Combined application of imaging techniques for the characterization and authentication of ancient weapons, Eur. Phys. J. Plus 132, 228 (2017)
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Unidirectional control of optically induced spin waves (Vol. 48, No. 4)

For future information technologies, the field of magnonics is rapidly emerging. Spin waves ̶collective modes of spin precessions ̶ are promising information carriers in magnonics, as Joule heating is negligible and propagation damping is low. Spatial control of the spin wave is indispensable for future application such as spin-wave switching, spin-wave-assisted recording, and sensing of small magnetic fields. In this article, unidirectional control of optically induced spin waves in a rare-earth iron garnet crystal is demonstrated. We observed the interference of two spin-wave packets with different initial phases generated by circularly polarized light pulses. This interference results in unidirectional propagation if the spin-wave sources are spaced apart at 1/4 of the wavelength of the spin waves and the initial phase difference is set to π/2. The propagating direction of the spin wave is switched by the polarization helicity of the light pulses. Moreover, in a numerical simulation, applying more than two spin-wave sources with a suitable polarization and spot shape, arbitrary manipulation of the spin wave by the phased array method was replicated. This achievement opens up a field of magnetic materials science and explores an alternative sensing technique using magnetic fields.

I. Yoshimine, Y. Y. Tanaka, T. Shimura and T. Satoh, Unidirectional control of optically induced spin waves, EPL 117, 67001 (2017)
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Wavy energy potential patterns from scattering nuclei reveal hidden information (Vol. 48, No. 4)

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)
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