Low Temperature X-Ray Diffraction Study on CaFe$_{2}$As$_{2}$

For undoped CaFe$_{2}$As$_{2}$ single crystals, we observed that utilizing thermal treatments could stabilize two pure tetragonal phases PI and PII. Both phases are non-superconducting, while the superconductivity with a T$_{c}$ up to 25 K can be induced through proper thermal treatment. Room temperature X-ray studies suggest that the origin of superconductivity arises from the interface of the mesoscopically stacked layers of PI and PII. To further investigate, a systematic low temperature X-ray study was conducted over a series of thermal treated CaFe$_{2}$As$_{2}$ single crystals. From which, we observed the phase aggregation of PI and PII upon cooling, more importantly, an ordered stacking structure exists at low temperature, which closely related to superconducting volume fraction and the ratio of PI and PII. These results further support the proposal of interface-enhanced superconductivity in undoped CaFe$_{2}$As$_{2}$. References L. Z. Deng, B. Lv, K. Zhao, F. Y. Wei, Y. Y. Xue, Z. Wu, and C. W. Chu, Phys. Rev. B 93, 054513(2016) K. Zhao, B. Lv, L. Z. Deng, S. Y. Huyan, Y. Y. Xue, and C. W. Chu, Proc. Natl. Acad. Sci. doi: 10.1073/pnas.1616264113