Researchers from the High Magnetic Field Laboratory of Chinese Academy of Sciences (CHMFL) made significant progress in the research of single crystal topological superconductors. High quality Sr_xBi₂Se₃ single crystals have been successfully grown, which exhibit superconducting volume fraction as high as 91.5% and are quite stable in the air. Using the stable high magnetic field facilities, researchers find that the materials exhibit periodic quantum oscillation signals between 10 and 35 Tesla. They provide transport evidence of the existence of topologically-protected surface state in the Sr_xBi₂Se₃ samples.

Topological superconductivity is a new quantum state of matters. Different from traditional superconductors, there exists the topologically-protected gapless metallic surface state at the surface of a topological superconductor. In the inner part, it exhibits superconductivity. This unique topological property makes the topological superconductor the ideal material for the fabrication of quantum computer.

In recent years, scientists have put tremendous interest in the searching of possible topological superconductors. In 2010, Prof. R. J. Cava’s group in Princeton University reported the observation of superconductivity in Cu_xBi₂Se₃ system. This report has attracted much attention in the investigating of possible topological superconductivity. Unfortunately, the Cu_xBi₂Se₃ superconductor has relatively low superconducting volume fraction (the maximum superconducting volume fraction can be only about 50%). Moreover, the Cu_xBi₂Se₃ samples are unstable in air. These factors greatly limited the further investigation of topological superconductivity in this compound.

The search of a new topological superconducting single crystal has been a challenge nowadays and little progress has been make in the past five years. Recently, a joint research group lead by Prof. ZHANG Yuheng and Prof. ZHANG Changjin made significant progress in the investigation of topological superconducting single crystals. Using flux-melting growth method, researchers successfully intercalated Sr into the prototype topological insulator Bi₂Se₃ and obtained high quality Sr_xBi₂Se₃ single crystals. The Sr_xBi₂Se₃ samples exhibit superconductivity, with a superconducting volume fraction as high as 91.5%. The samples are exposed to air for more than four months but the superconductivity is hardly weakened, suggesting that they are very stable in the air.

Researchers investigated the possible topological properties of the Sr_xBi₂Se₃ samples using the high magnetic field facilities in CHMFL. They found that the Sr_xBi₂Se₃ samples exhibit periodic quantum oscillations between 10 and 35 Tesla. After analyzing these oscillation signals, the researchers found the evidence of the existence of topologically-protected surface state. These results suggest that the Sr_xBi₂Se₃ system is the ideal material for further investigation of topological superconductivity.

The above results have been recently published in Journal of the American Chemistry Society.

This work was supported by the State Key Project of Fundamental Research of China, the Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences, and the National Natural Science Foundation of China.

Relative link to the paper：http://pubs.acs.org/doi/abs/10.1021/jacs.5b06815.

(a) The superconducting transition of the SrxBi2Se3single crystal; (b) The superconducting volume fraction; (c) and (d) The quantum oscillations of the sample; (e) and (f) The transport evidence of the existence of topologically-protected surface state: the intercept of the Landau index and 1/B is close to 0.5.