Superconductivity is a rare physical state in which matter is able to conduct electricity--maintain a flow of electrons--without any resistance. It can only be found in certain materials, and even then it can only be achieved under controlled conditions of low temperatures and high pressures. New research from a team including Carnegie's Elissaios Stavrou, Xiao-Jia Chen, and Alexander Goncharov hones in on the structural changes underlying superconductivity in iron arsenide compounds--those containing iron and arsenic. It is published by . 
This is the tetragonal crystal structure of NaFe2As2. Sodium (Na) is represented by the black balls, iron (Fe) by the red balls, and arsenic (As) by the yellow balls. (Image: Alexander Goncharov) Although superconductivity has many practical applications for electronics (including scientific research instruments), medical engineering (MRI and NMR machines), and potential future applications including high-performance power transmission and storage, and very fast train travel, the difficulty of creating superconducting materials prevents it from being used to its full potential. As such, any newly discovered superconducting ability is of great interest to scientists and engineers. Iron arsenides are relatively recently discovered superconductors. The nature of superconductivity in these particular materials remains a challenge for modern solid state physics. If the complex links between superconductivity, structure, and magnetism in these materials are unlocked, then iron arsenides could potentially be used to reveal superconductivity at much higher temperatures than previously seen, which would vastly increase the ease of practical applications for superconductivity. When iron arsenide is combined with a metal--such as in the sodium-containing NaFe2As2 compound studied here--it was known that the ensuing compound is crystallized in a tetrahedral structure. But until now, a detailed structure of the atomic positions involved and how they change under pressure had not been determined. The layering of arsenic and iron (As-Fe-As) in this structure is believed to be key to the compound's superconductivity. However, under pressure, this structure is thought to be partially misshapen into a so-called collapsed tetragonal lattice, which is no longer capable of superconducting, or has diminished superconducting ability. The team used experimental evidence and modeling under pressure to actually demonstrate these previously theorized structural changes--tetragonal to collapsed tetragonal--on the atomic level. This is just the first step toward definitively determining the link between structure and superconductivity, which could potentially make higher-temperature superconductivity a real possibility. They showed that at about 40,000 times normal atmospheric pressure (4 gigapascals), NaFe2As2 takes on the collapsed tetragonal structure. This changes the angles in the arsenic-iron-arsenic layers and is coincident with the loss in superconductivity. Moreover, they found that this transition is accompanied by a major change in bonding coordination in the formation of the interlayer arsenic-arsenic bonds. A direct consequence of this new coordination is that the system loses its two-dimensionality, and with it, superconductivity. "Our findings are an important step in identifying the hypothesized connection between structure and superconductivity in iron-containing compounds," Goncharov said. "Understanding the loss of superconductivity on an atomic level could enhance our ease of manufacturing such compounds for practical applications, as well as improving our understanding of condensed matter physics."
read more "Linking superconductivity and structure"
This is the tetragonal crystal structure of NaFe2As2. Sodium (Na) is represented by the black balls, iron (Fe) by the red balls, and arsenic (As) by the yellow balls. (Image: Alexander Goncharov) Although superconductivity has many practical applications for electronics (including scientific research instruments), medical engineering (MRI and NMR machines), and potential future applications including high-performance power transmission and storage, and very fast train travel, the difficulty of creating superconducting materials prevents it from being used to its full potential. As such, any newly discovered superconducting ability is of great interest to scientists and engineers. Iron arsenides are relatively recently discovered superconductors. The nature of superconductivity in these particular materials remains a challenge for modern solid state physics. If the complex links between superconductivity, structure, and magnetism in these materials are unlocked, then iron arsenides could potentially be used to reveal superconductivity at much higher temperatures than previously seen, which would vastly increase the ease of practical applications for superconductivity. When iron arsenide is combined with a metal--such as in the sodium-containing NaFe2As2 compound studied here--it was known that the ensuing compound is crystallized in a tetrahedral structure. But until now, a detailed structure of the atomic positions involved and how they change under pressure had not been determined. The layering of arsenic and iron (As-Fe-As) in this structure is believed to be key to the compound's superconductivity. However, under pressure, this structure is thought to be partially misshapen into a so-called collapsed tetragonal lattice, which is no longer capable of superconducting, or has diminished superconducting ability. The team used experimental evidence and modeling under pressure to actually demonstrate these previously theorized structural changes--tetragonal to collapsed tetragonal--on the atomic level. This is just the first step toward definitively determining the link between structure and superconductivity, which could potentially make higher-temperature superconductivity a real possibility. They showed that at about 40,000 times normal atmospheric pressure (4 gigapascals), NaFe2As2 takes on the collapsed tetragonal structure. This changes the angles in the arsenic-iron-arsenic layers and is coincident with the loss in superconductivity. Moreover, they found that this transition is accompanied by a major change in bonding coordination in the formation of the interlayer arsenic-arsenic bonds. A direct consequence of this new coordination is that the system loses its two-dimensionality, and with it, superconductivity. "Our findings are an important step in identifying the hypothesized connection between structure and superconductivity in iron-containing compounds," Goncharov said. "Understanding the loss of superconductivity on an atomic level could enhance our ease of manufacturing such compounds for practical applications, as well as improving our understanding of condensed matter physics."
Alexander Szameit and his team of the University Jena developed a photonic set-up that can simulate non-physical processes in a laboratory. March 1938: The Italian elementary particle physicist Ettore Majorana boarded a post ship in Naples, heading for Palermo. But he either never arrives there - or he leaves the city straight away – ever since that day there has been no trace of the exceptional scientist and until today his mysterious disappearance remains unresolved. Since then, Majorana, a pupil of the Nobel Prize winner Enrico Fermi, has more or less been forgotten. What the scientific world does remember though is a theory about nuclear forces, which he developed, and a very particular elementary particle. “This particle named after Majorana, the so-called Majoranon, has some amazing characteristics“, the physicist Professor Dr. Alexander Szameit of the Friedrich Schiller University Jena says. “Characteristics which are not supposed to be existent in our real world.“ Majorana particles are, for instance, their own antiparticles: Internally they combine completely opposing characteristics – like opposing charges and spins. If they were to exist, they would extinguish themselves immediately. “Therefore, Majoranons are of an entirely theoretical nature and cannot be measured in experiments.“ Together with colleagues from Austria, India, and Singapore, Alexander Szameit and his team succeeded in realizing the impossible. In the new edition of the science magazine they explain their approach: Szameit and his team developed a photonic set-up that consists of complex waveguide circuits engraved in a glass chip, which enables them to simulate charged Majorana particles and, thus, allows to conduct physical experiments. “At the same time we send two rays of light through parallel running waveguide lattices, which show the opposing characteristics separately,“ explains Dr. Robert Keil, the first author of the study. After evolution through the lattices, the two waves interfere and form an optical Majoranon, which can be measured as a light distribution. Thus, the scientists create an image that catches this effect like a photograph – in this case the state of a Majoranon at a defined moment in time. “With the help of many of such single images the particles can be observed like in a film and their behaviour can be analyzed,“ says Keil. This model allows the Jena scientists to enter completely unknown scientific territory, as Alexander Szameit stresses. “Now, it is possible for us to gain access to phenomena that so far only have been described in exotic theories.“ With the help of this system, one can conduct experiments in which conservation of charge – one of the pillars of modern physics – can easily be suspended. “Our results show that one can simulate non-physical processes in a laboratory and, thus, can make practical use of exotic characteristics of particles that are impossible to observe in nature.“ Szameit foresees one particular promising application of simulated Majoranons in a new generation of quantum computers. “With this approach, much higher computing capacities than are possible at the moment can be achieved.“