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Key compound for high-temperature superconductivity found

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A research group in Japan found a new compound H5S2 that shows a new superconductivity phase on computer simulation. Further theoretical and experimental research based on H5S2 predicted by this group will lead to the clarification of the mechanism behind high-temperature superconductivity, which takes place in hydrogen sulfide .

Superconductivity is the total disappearance of electrical resistance when an object is cooled below a definite temperature. If superconductor is used for electric wire, it becomes possible to carry electricity without loss. That's why superconductivity has been drawing attention as an important physical phenomenon for solving environmental and energy problems.

However, the superconducting critical temperature, the temperature at which superconductivity takes place, is so low that its practical realization is difficult. Last year, a striking news came out that H2S broke the record for superconducting critical temperature under high-pressure. However, the chemical composition ratio of sulfur and hydrogen and the crystal structure during the process in which superconductivity takes place have not been well understood.

A research group led by Takahiro Ishikawa, Specially Appointed Assistant Professor, and Katsuya Shimizu, Professor, at Center for Science and Technology under Extreme Conditions, Graduate School of Engineering Science, Osaka University, Tatsuki Oda, Professor at School of Mathematics and Physics, Kanazawa University, and Naoshi Suzuki, Professor at Faculty of Engineering Science, Kansai University predicted a new superconductivity phase of hydrogen sulfide (H5S2), which was presented at a pressure of 1.1 million bar on computer simulation. The superconducting critical temperature obtained from H5S2, whose calculated value was the same as the experimental value. This result may lead to the clarification of the mechanism behind high-temperature superconductivity, which takes place in hydrogen sulfide by further theoretical and experimental research based on H5S2.

Furthermore, by applying methods used and knowledge obtained by this group to other light element hydrides, it will become possible to establish guidelines for enhancing superconducting critical temperature to near room temperature.

This research was featured in the electronic version of Scientific Reports on Thursday, March 17, 2016.


New magnetism research brings high-temp superconductivity applications closer


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A research team led by the U.S. Department of Energy's (DOE's) Argonne National Laboratory has discovered that only half the atoms in some iron-based superconductors are magnetic, providing a conclusive demonstration of the wave-like properties of metallic magnetism in these materials.

The discovery allows for a clearer understanding of the magnetism in some compounds of iron, the iron arsenides, and how it helps induce superconductivity, the resistance-free flow of electrical current through a solid-state material, which occurs at temperatures up to 138 degrees Kelvin, or minus -135 degrees Celsius.

"In order to be able to design novel superconducting materials, one must understand what causes superconductivity," said Argonne senior physicist Raymond Osborn, one of the project's lead researchers. "Understanding the origin of magnetism is a first vital step toward obtaining an understanding of what makes these materials superconducting. Given the similarity to other materials, such as the copper-based superconductors, our goal was to improve our understanding of high-temperature superconductivity."

From an applied perspective, such an understanding would allow for the development of magnetic energy-storage systems, fast-charging batteries for electric cars and a highly efficient electrical grid, said Argonne senior physicist Stephan Rosenkranz, the project's other lead researcher.

Superconductors reduce power loss. The use of high-temperature superconducting materials in the electrical grid, for example, would significantly reduce the large amount of electricity that is lost as it travels though the grid, enabling the grid to operate more efficiently.

The researchers were able to show that the magnetism in these materials was produced by mobile electrons that are not bound to a particular iron atom, producing waves of magnetization throughout the sample. They discovered that, in some iron arsenides, two waves interfere to cancel out, producing zero magnetization in some atoms. This quantum interference, which has never been seen before, was revealed by Mössbauer spectroscopy, which is extremely sensitive to the magnetism on each iron site.

Researchers also used high-resolution X-ray diffraction at the Advanced Photon Source (APS) and neutron diffraction at Oak Ridge National Laboratory's Spallation Neutron Source (SNS) to determine the chemical and magnetic structures and to map the electronic phase diagram of the samples used. The APS and SNS are DOE Office of Science User Facilities.

"By combining neutron diffraction and Mössbauer spectroscopy, we were able to establish unambiguously that this novel magnetic ground state has a non-uniform magnetization that can only be produced by itinerant electrons. These same electrons are responsible for the superconductivity," Rosenkranz said.

The research is available in the January 25 online edition of Nature Physics.

Next, Rosenkranz and Osborn plan to characterize the magnetic excitations, or fluctuations of iron-based superconductors, to determine how they to relate to and possibly cause superconductivity.

 

Superconductivity seen in a new light

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Superconducting materials have the characteristic of letting an electric current flow without resistance. The study of superconductors with a high critical temperature discovered in the 1980s remains a very attractive research subject for physicists. Indeed, many experimental observations still lack an adequate theoretical description. Researchers from the University of Geneva (UNIGE) in Switzerland and the Technical University Munich in Germany have lifted the veil on the electronic characteristics of high-temperature superconductors. Their research, published in Nature Communications, shows that the electronic densities measured in these superconductors are a combination of two separate effects. As a result, they propose a new model that suggests the existence of two coexisting states rather than competing ones postulated for the past thirty years, a small revolution in the world of superconductivity.

Below a certain temperature, a superconducting material loses all electrical resistance (equal to zero). When immersed in a magnetic field, high-temperature superconductors (high-Tc) allow this field to penetrate in the form of filamentary regions, called vortices, a condition in which the material is no longer superconducting. Each vortex is a whirl of electronic currents generating their own magnetic fields and in which the electronic structure is different from the rest of the material.

Coexistence rather than competition

Some theoretical models describe high-Tc superconductors as a competition between two fundamental states, each developing its own spectral signature. The first is characterized by an ordered spatial arrangement of electrons. The second, corresponding to the superconducting phase, is characterized by electrons assembled in pairs.

"However, by measuring the density of electronic states with local tunneling spectroscopy, we discovered that the spectra that were attributed solely to the core of a vortex, where the material is not in the superconducting state, are also present elsewhere—that is to say, in areas where the superconducting state exists. This implies that these spectroscopic signatures do not originate in the vortex cores and cannot be in competition with the superconducting state," explains Christoph Renner, professor in the Department of Quantum Matter Physics of the Faculty of Science at UNIGE. "This study therefore questions the view that these two states are in competition, as largely assumed until now. Instead, they turn out to be two coexisting states that together contribute to the measured spectra," professor Renner says. Indeed, physicists from UNIGE using theoretical simulation tools have shown that the experimental spectra can be reproduced perfectly by considering the superposition of the spectroscopic signature of a superconductor and this other electronic signature, brought to light through this new research.

This discovery is a breakthrough toward understanding the nature of the high-temperature superconducting state. It challenges some theoretical models based on the competition of the two states mentioned above. It also sheds new light on the electronic nature of the vortex cores, which potentially has an impact on their dynamics. Mastery of these dynamics, and particularly of the anchoring of vortices that depend on their electronic nature, is critical for many applications such as high-field electromagnets.

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