SPring-8, the large synchrotron radiation facility

Calcium is an element familiar to us. When it is placed under high pressure, it becomes superconductive. It was found that the crystal structure of calcium changes to become superconductive at -244^oC at ultrahigh pressures of above 200 GPa.

Information on material selection and crystal structure is important when synthesizing and designing superconductors that can be used at high temperatures because the crystal structure and superconductivity properties of a material are closely related. The achievements of this study will provide valuable guidelines for designing superconducting materials in the future.

Masafumi Sakata of the Center for Quantum Science and Technology under Extreme Conditions, Osaka University, in collaboration with scientists of JASRI, has discovered that elemental calcium has a high-pressure phase at ultrahigh pressures of above 200 GPa and that calcium in this phase exhibits the highest ever superconducting transition temperature recorded for an element.

Even elements that do not exhibit superconductivity under atmospheric pressure can exhibit superconductivity when they are placed under high pressure. So far, superconductivity has been observed in 23 elements under high pressure. Among these elements, calcium was found to have the highest superconducting transition temperature of 25 K (-248^oC) at 160 GPa; however, its behavior under further higher pressures remained unclear.

The research group generated an ultrahigh-pressure environment of above 200 GPa and observed the superconductivity of calcium under ultrahigh pressure and its crystal structure by electrical resistance measurement and powder X-ray diffraction measurement at SPring-8. The results indicate that calcium undergoes a phase transition to a high-pressure phase at around 200 GPa and that calcium in this phase exhibits superconductivity at 29 K (-244^oC) at 220 GPa, which is the highest superconducting transition temperature ever reported for an element. Information on material selection and crystal structure is important when synthesizing and designing superconductors that can be used at high temperatures; thus, the achievements of this study will provide valuable guidelines for designing superconducting materials in the future.

The achievements were obtained by a group led by Masafumi Sakata (Specially Appointed Researcher), Yuki Nakamoto (Technician), and Katsuya Shimizu (Professor) from the Center for Quantum Science and Technology under Extreme Conditions, Osaka University, jointly with a group led by Takahiro Matsuoka (contract researcher; currently Specially Appointed Assistant Professor at the Center for Quantum Science and Technology under Extreme Conditions, Osaka University) and Yasuo Ohishi (Senior Scientist) of JASRI as a SPring-8 research proposal project with support from the Funding Program for Next Generation World-Leading Researchers.

The results of this study were published in Physical Review B Rapid Communications, a journal published by the American Physical Society, on 22 June 2011.

Publication:
"Superconducting state of Ca-VII below a critical temperature of 29 K at a pressure of 216 GPa"
Masafumi Sakata, Yuki Nakamoto, Katsuya Shimizu, Takahiro Matsuoka, and Yasuo Ohishi
Physical Review B, Rapid Communications, 83, 220512(R) (2011), published online 22 June 2011