Ab initio study on the high superconducting transition temperature in calcium under high pressure

Abstract

For calcium in the phases IV and V, we estimated the superconducting transition temperature T _c by the use of the Allen–Dynes formula. Setting the effective screened Coulomb repulsion constant μ* at 0.1 in the formula, we obtained T _c =23.42 K at 100 GPa for Ca-IV and T _c =15.87 K at 120 GPa for Ca-V. In order to clarify the origin of such high values of T _c , first, we investigated the band character of electrons and found that the high T _c is not necessarily related to the so called s–d transfer. Then we analyzed the electron–phonon coupling at each phonon mode in Ca-V where the highest T _c in elements has been experimentally observed. As a result, we discovered that an optical mode at the Γ point has the strongest electron–phonon coupling. Such phonon mode can exist only in the complex crystal structure of Ca-V, and the result shows that the high T _c seems to be closely linked with the complex crystal structures like Ca-IV and Ca-V.

^†This paper was presented at the XLVI^th European High Pressure Research Group (EHPRG 46) Meeting, Valencia (Spain), 7–12 September, 2008.

Keywords:

• first-principles calculation
• superconductivity
• phonon
• calcium

Acknowledgements

This work was partially supported by the Grant-in-Aid for Scientific Research in Priority Areas (No. 17064013 and No. 19051016) of the Ministry of Education, Culture, Sports, Science, and Technology, Japan. It was also supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowships for Young Scientists (No. 19·1723), Grant-in-Aid for Scientific Research (No. 15GS0213), and the 21st COE Program of JSPS.

Notes

^†This paper was presented at the XLVI^th European High Pressure Research Group (EHPRG 46) Meeting, Valencia (Spain), 7–12 September, 2008.