## EPJ D Highlight - Stresses and flows in ultra-cold superfluids

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- Published on 08 May 2020

Mathematical modelling of superfluids, which exhibit quantum mechanical properties at a macroscopic scale, shows that they become deformed when flowing around impurities.

Superfluids, which form only at temperatures close to absolute zero, have unique and in some ways bizarre mechanical properties, Yvan Buggy of the Institute of Photonics and Quantum Sciences at Heriot-Watt University in Edinburgh, Scotland, and his co-workers have developed a new quantum mechanical model of some of these properties, which illustrates how these fluids will deform as they flow around impurities. This work is published in the journal *EPJ D*.

## EPJ D Topical review - Recent total cross section measurements in electron scattering from molecules

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- Published on 05 May 2020

Accurate new experimental data on electron interactions with matter are necessary for the understanding of a wide variety of natural and technological processes occurring in complex environments. Knowledge of the efficiency of electron interactions with biomolecules is crucial for the description and modeling of ionizing radiation damage to living cells and biomolecules radiolysis. Accurate experimental data concerning electron interactions are also important for the description of many phenomena occurring in plasma physics and gaseous electronics, including modeling of processes in cometary and planetary atmospheres.

## EPJ D Highlight - Questionable stability of dissipative topological models for classical and quantum systems

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- Published on 15 April 2020

Physicists Rebekka Koch and Jan Carl Budich make important contributions to understanding dissipative topological systems by studying the spectral instabilities that occur in the mathematical description and their effect on experimental setups in a new paper in EPJ D.

Energy conservation lies at the core of every physical theory. Effective mathematical models however can feature energy gain and/or loss and thus break the energy conservation law by only capturing the physics of a subsystem. As a result, the Hamiltonian, the function that describes the system's energy, loses an important mathematical property: it is no longer Hermitian. Such non-Hermitian Hamiltonians have successfully described experimental setups for both classical problems – in e.g. some optical systems and electrical circuits - and quantum ones, in modelling the motion of electrons in crystalline solids. In a new paper in *EPJ D*, physicists Rebekka Koch from the University of Amsterdam in the Netherlands and Jan Carl Budich from Technische Universität Dresden, in Germany, describe how these functions provide new insights into behaviour at the edges of topological materials.

## EPJ D Highlight - Looking for dark matter

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- Published on 08 April 2020

A new paper in EPJ D, ‘Constraining domain wall dark matter with a network of superconducting gravimeters and LIGO’, suggests two novel methods of searching for dark matter by measuring tiny perturbations in fundamental constants.

Dark matter, which cannot be physically observed with ordinary instruments, is thought to account for well over half the matter in the Universe, but its properties are still mysterious. One commonly held theory states that it exists as ‘clumps’ of extremely light particles. When the earth passes through such a clump, the fundamental properties of matter are altered in ways that can be detected if instruments are sensitive enough. Physicists Rees McNally and Tanya Zelevinsky from Columbia University, New York, USA, have now published a paper in *EPJ D* proposing two new methods of looking for such perturbations and, thus, dark matter. This paper is part of the EPJD Topical Issue on Quantum Technologies for Gravitational Physics which is still open to submissions.

## EPJ D Topical review - Electron collisions with molecules and molecular clusters

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- Published on 25 March 2020

Over the last ten years, advances in the computational investigation of electron collision processes have seen an overhaul of many of the software packages employed by researchers, in parallel with the development of new tools. In particular, the increased interest in biological molecules as targets has stimulated the development of software which makes use of current computational abilities. These developments have enabled scientists to study small targets with increasing levels of detail, larger targets than ever before, and the effect of the environment by means of the investigation of small molecular clusters.

## EPJ D Topical review - Electron-scattering on molecular hydrogen: convergent close-coupling approach

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- Published on 24 March 2020

Molecular hydrogen is the simplest neutral molecule, the most abundant molecule in the universe and an important constituent of plasmas with applications in astrophysics, fusion, atmospheric physics, and various industries. Elemental collision processes play an important role in modelling these plasmas, and collisions with electrons have attracted significant interest from both experiment and theory. A number of compilations of cross sections for electron collisions with molecular hydrogen have been produced. In all cases these cross section data sets have been produced from an analysis of experimental data, even though there were significant discrepancies between different experiments for many transitions. Theoretical calculations have been largely excluded from critical evaluations of the data due to large uncertainties. This changed with the application of the convergent close-coupling (CCC) method to electron collisions with molecules.

## EPJ D Highlight - Frozen-planet states in exotic helium atoms

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- Published on 18 March 2020

In an elegant study published in EPJ D, physicists from Serbia and Russia have mapped 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. A new study by Tasko Grozdanov from the University of Belgrade in Serbia and Evgeni Solov’ev from the Institute of Nuclear Research near Moscow in Russia has mapped the energy levels of an exotic form of helium produced in this way. This work, which is published in *EPJ D*, has been described by one commentator as ”... a new jewel in the treasure of scientific achievements in atomic physics theory”.

## EPJ D Highlight - A better starting point for exploring entanglement

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- Published on 25 February 2020

Updated mathematical techniques which can distinguish between two types of ‘non-Gaussian curve’ could make it easier for researchers to study the nature of quantum entanglement.

Quantum entanglement is perhaps one of the most intriguing phenomena known to physics. It describes how the fates of multiple particles can become entwined, even when separated by vast distances. Importantly, the probability distributions needed to define the quantum states of these particles deviate from the bell-shaped, or ‘Gaussian’ curves which underly many natural processes. Non-Gaussian curves don’t apply to quantum systems alone, however. They can also be composed of mixtures of regular Gaussian curves, producing difficulties for physicists studying quantum entanglement. In new research published in *EPJ D*, Shao-Hua Xiang and colleagues at Huaihua University in China propose a solution to this problem. They suggest an updated set of equations which allows physicists to easily check whether or not a non-Gaussian state is genuinely quantum.

## EPJ D Highlight - Deconstructing Schrödinger’s Cat

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- Published on 14 February 2020

The French theoretical physicist Franck Laloë presents a modification of Schrödinger’s famous equation that ensures that all measured states are unique, helping to solve the problem that is neatly encompassed in the Schördinger’s cat paradox.

The paradox of Schrödinger’s cat – the feline that is, famously, both alive and dead until its box is opened – is the most widely known example of a recurrent problem in quantum mechanics: its dynamics seems to predict that macroscopic objects (like cats) can, sometimes, exist simultaneously in more than one completely distinct state. Many physicists have tried to solve this paradox over the years, but no approach has been universally accepted. Now, however, theoretical physicist Franck Laloë from Laboratoire Kastler Brossel (ENS-Université PSL) in Paris has proposed a new interpretation that could explain many features of the paradox. He sets out a model of this possible theory in a new paper in *EPJ D*.

## EPJ D Highlight - Breaking up amino acids with radiation

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- Published on 04 February 2020

A new study describes how the amino acid, glutamine, is broken up when bombarded with different doses of electrons. This has implications for cancer radiotherapy and understanding the origin of life.

Small organic molecules, including the amino acids that form the ‘building blocks’ of proteins in living cells, fragment to form ions under the impact of high-energy radiation such as electron beams. A new study published in *EPJ D* has now shown what happens when electrons collide with one amino acid, glutamine. The extent of the damage and the nature of the ions formed are both affected by the energy of the colliding electrons. This work arises from a collaboration between experimental physicists led by Alexander Snegursky at the Institute of Electron Physics, Uzhgorod, Ukraine and theoreticians led by Jelena Tamuliene at Vilnius University, Vilnius, Lithuania.