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Iron-based Superconducting Body: Experimental Results Decisively Suggest a New Mechanism (Press Release)

Release Date
11 Jul, 2012
  • BL02B2 (Powder Diffraction)

Tokyo Institute of Technology
Japan Synchrotron Radiation Research Institute (JASRI)

• Unexpected discovery in the starting material of iron-based superconductor.
• In addition to the known plateau with high Tc values, another massive plateau with still higher Tc values was found.
• The second, newly found plateau defies explanation by the conventional theory.
• The use of negative hydrogen ions as a dopant led to the discovery.

A research group at the Tokyo Institute of Technology - Prof. Hideo Hosono (Frontier Research Center), Assistant Professor Satoru Matsuishi and graduate student Soushi Iimura (Material & Structure Laboratory), and others - successfully injected more than three times as many electrons than previously possible using negative hydrogen ions*1, and discovered a new region where superconductivity can manifest itself. Further examination revealed that the newly discovered region has higher Tc values than those already known, and provides a wider window for the expression of superconductivity. These results defy precise explanation by the spin mechanism now gathering ground, and these findings from the experiments can be considered to represent decisive support for the mechanism where an orbital effect plays a central role. They also seem to provide a potent guiding principle for the search of new materials with higher Tc values.

The research was conducted as part of the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST), and some of the experiments were implemented in collaboration with SPring-8 (a large-scale synchronous radiation facility).

"Two-dome structure in electron-doped iron arsenide superconductor"
Soshi Iimura, Satoru Matuishi, Hikaru Sato, Taku Hanna, Yoshinori Muraba, Sung Wng Kim, Jung Eun Kim, Masaki Takata & Hideo Hosono
Nature Communications, 3 943(2012), published online 10 July 2012


Fig.1. Superconducting transition temperature (<em>T</em><sub>c</sub>) vs. Doped electron concentration
Fig.1. Superconducting transition temperature (Tc) vs. Doped electron concentration

(Left): Report published in Feb. 2008 (This research group, Journal of the American Chemical Society)
(Right): This report. The use of negative hydrogen ions in place of fluorine enabled twice as high a concentration of doped electrons, leading to the discovery of the new plateau in region II.

*1 Negative hydrogen ion

Hydrogen predominantly forms a positive H+ ion (proton) by releasing an electron. But, it is not uncommon for a hydrogen atom to form a negatively charge H- ion (hydride ion) by taking in an extra electron; typically this occurs when a hydrogen atom is combined with an element that is prone to become a positive ion. As the H- ion has an extra electron, it has a large radius relative to its weight. For example, the H+ included in NaH has an ionic radius of 146pm, roughly equivalent to that of the oxide ion (O2-).

For more information, please contact:
  Prof. Hideo Hosono (Tokyo Institute of Technology, Frontier Research Center)
   E-mail :

   Dr. Jungeun Kim (JASRI)
    E-mail :