Scientists in China and US chart latest discoveries of iron-based superconductors

January 28, 2015, Science China Press

Superconductivity is a remarkable macroscopic quantum phenomenon, discovered just over a century ago. As temperature decreases to below a critical value, the electric resistance of a superconductor vanishes and the magnetic field is repelled. Superconductors have many applications, and can be used to transport electricity without loss of energy. Conventional superconductivity is explained by the Bardeen-Cooper-Schrieffer theory, posited more than five decades ago. In a superconducting state, two electrons with opposite momenta attract each other to form a bound pair. The pairing mechanism in a conventional superconductor is due to couplings between electrons and phonons, which are a quantum version of lattice vibrations.

The (Tc) is very low - usually well below 40 K. The low transition has greatly limited practical applications of . In a paper published in the Beijing-based journal National Science Review, a team of scientists based in the United States and China review recent discoveries of featuring the highest superconducting transition temperature next to copper oxides.

In an article titled, "Iron-based high transition ," co-authors Xianhui Chen, Pengcheng Dai, Donglai Feng, Tao Xiang and Fu-Chun Zhang present an overview of material aspects and physical properties of iron-based superconductors. They outline the transition temperature's dependence on the crystal structure, the interplay between antiferromagnetism and superconductivity, and the electronic properties of compounds obtained by angle-resolved photoemission spectroscopy.

"It has been a dream to realize high-Tc or , which may revolutionarily change power transmission in the world," they explain in sketching out the latest advances in this search for more practical superconductors. One impetus to accelerate this search was triggered by the discovery nearly two decades ago of a high-Tc superconducting cuprate. A worldwide search since then has uncovered the highest transition temperature at ambient pressure of 135 K in an Hg-based cuprate. The second class of high-Tc materials covers iron-based superconductors, initially discovered in 2008. The highest Tc in bulk iron-based superconductors discovered to date is 55 K in SmO1?x FxFeAs.

So far many families of iron-based superconductors have been discovered. "Study of iron-based superconductors and their physical properties has been one of the major activities in condensed matter physics in the past several years," state the authors of the study.

Several powerful new techniques, including angle-resolved photoemission spectroscope and scanning tunneling microscopy, have been developed while studying high-Tc superconductors. These techniques, together with neutron scattering, nuclear magnetic resonance, and optical conducting measurements, have been applied to examine the properties of the new compounds.

Iron-based superconductivity shares many common features with the high-Tc cuprates. Both are in the sense that phonons are unlikely to play any dominant role in their superconductivity. Both are quasi-2D, and their superconductivity is in the proximity of antiferromagnetism. In the cuprates, the low-energy physics is described by a single band, while in the iron-based compounds, there are multi-orbitals involved. Yet some of the physics in the cuprates remain controversial. Deeper investigation of iron-based superconductors might broaden understanding of their unconventional superconductivity and provide a new route for finding .

In mapping out recent breakthroughs, the co-authors outline superconducting crystal structures, the interplay between magnetism and superconductivity, and the electronic structure of iron-based as revealed through angle resolved photoemission spectroscopy and scanning tunneling spectroscopy experiments. They also review current theories regarding superconductivity. Over the past decade, they state in the study, "Tremendous progress has been achieved in the synthesis of materials, growth of single crystals, characterization of crystal structures, measurements of thermodynamics, and transport and various spectroscopic quantities for iron-based superconductors." "This has given us a comprehensive understanding on the chemical and crystal structures, band structures, spin and orbital orderings, pairing symmetry, and other physical properties of iron-based superconductors," they explain. Iron-based superconductors are proximate to antiferromagnetism, which suggests that AF fluctuations are responsible for the observed superconductivity. Investigating the superconductivity mechanism should focus in part on the cause of the pairing of electrons. Meanwhile, a theory should be developed to explain existing experimental data and to predict new experimental effects.

Like cuprate superconductivity, iron-based superconductivity is generally believed to originate predominantly from the electron-electron repulsive interaction, which induces antiferromagnetic fluctuations. A solid theoretical description of high-Tc superconductivity in both cuprate and iron-based materials remains a great challenge. Iron-based superconductors are multi-band materials. All five 3d orbitals of Fe hybridize strongly with As or Se 4p orbitals. They also couple strongly with each other and have contribution to both itinerant conducting electrons and localized magnetic moments.

Scientists in the field are still trying to develop a clear physical picture with reliable theoretical tools to treat an electronic system with strong coupling between itinerant and localized electrons. It is likewise important to design experimental measurements that could solve a number of key problems, which in turn could test theories on iron-based superconductivity. "While we are still far from the stage to predict high-Tc materials, there is good progress along this development," state the paper's coauthors. "It is possible in the future that theory may guide the search or synthesis of the high-Tc superconductors."

Co-authors Tao Xiang, based at the Institute of Physics, Chinese Academy of Sciences in Beijing, and Fu-Chun Zhang, a professor at Zhejiang University in the eastern Chinese city of Hangzhou, said: "Progress achieved in studies of the mechanism of iron-based could have a strong impact on the study of theory of strongly correlated quantum systems."

Co-author Xianhui Chen, a professor at the University of Science and Technology of China, said that the recent discovery of superconductivity at 190 K in H2S under pressure up to 200 GPa, reported in December of 2014 by three scientists based at Germany's Max Planck Institute for Chemistry, "suggests that could be achieved."

Explore further: Electron spin could be the key to high-temperature superconductivity

More information: Xianhui Chen, Pengcheng Dai, Donglai Feng, Tao Xiang, Fu-Chun Zhang. "Iron-based high transition temperature superconductors". National Science Review, nsr.oxfordjournals.org/content … 014/07/03/nsr.nwu007

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Mark_Goldes
1 / 5 (2) Jan 29, 2015
Ultraconductors are polymer equivalents of room temperature superconductors. See that heading at aesopinstitute.org

Four Small Business Innovation Research contracts were successfully completed. Final Reports are available for all four as pdf files upon request.

Almost 1,000 samples were independently reproduced on a separate US Air Force Contract.

After 20 years of R&D work stopped a decade ago following the lack of funding resulting from the dot.com crash. It is in the process of resuming. Commercial products are likely within a year or two.

AESOP and I are attacked by an obsessed Troll using pseudonyms - including a fake Board of physicists. The SBIR Final Reports contradict almost every statement he makes about these materials, which conduct 100,000 times better than copper, gold or silver.
Losik
Jan 29, 2015
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Mark_Goldes
1 / 5 (2) Jan 29, 2015
Actually, they were discovered in 1981 in Moscow, Russia. After the Cold War ended, we supported three floors of labs in Moscow for 6 years and rotated a number of Russian Ph.D. scientists through our labs in Sebastopol, CA. The work continues at the world famous Ioffe lab in St. Petersburg, Russia where a team measured zero resistance. A paper reporting that work was published in 1989. Work has also been done in Israel and Mexico. Applications are close and the work will not die out. Nor is work in cold fusion. See lenr-canr.org for more about that growing arena.
Mark_Goldes
1 / 5 (2) Jan 29, 2015
They were discovered in Moscow, Russia in 1981. We supported three floors of labs in Moscow for 6 years. Several Russian Ph.D.s were rotated through our labs in Sebastopol, CA. Three were awarded Green cards as Distinguished Scientists. The work was paralleled at the world famous Ioffe Institute in St. Petersburg. That team published a paper in 1989 reporting zero resistance. Work has also been done with these materials in Israel and Mexico. It is close to commercialization in the USA. Cold fusion is far from dying out. See lenr-canr.org to learn why.
(Added when it seemed the earlier comment had been deleted).
Losik
Jan 29, 2015
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Mark_Goldes
1 / 5 (2) Jan 29, 2015
As I recall, he told us he was first handed a piece of polypropylene in 1981 with the comment: "Something is wrong with this dielectric material. It is conducting electricity. Fix it!" He was astonished at the conductivity and instead put a team to work, It may have been 1988 when they had done enough research for him to conclude it was equivalent to a room temperature superconductor.

Feel free to contact me for a list of publications regarding these materials.
Losik
Jan 29, 2015
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Mark_Goldes
1 / 5 (2) Jan 30, 2015
I've sent you the list and all four Final SBIR Reports. You are very welcome.

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