Investigation of the Possible Mechanism Underlying Superconductivity in the Parent Compounds of IRON-Based 122 Material Systems

Date

2019-12

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Abstract

Among the 122 family of iron-based superconductors, CaFe2As2 (Ca122) has attracted lots of attention due to its sensitivity to the external pressure, complex phase diagram and discovery of non-bulk superconductivity with high Tc (~49 K) with rare-earth doping. When focusing on the phase diagram, Ca122 can be stabilized reversibly at room temperature in two slightly different non-superconducting tetragonal phases, PI and PII, through thermal treatment. Upon proper annealing, superconductivity with Tc of up to 25 K emerges in the samples that are mixed by PI and PII phases. Systematic low-temperature X-ray diffraction (XRD) studies were conducted on undoped Ca122 samples annealed at 350°C over different periods of time. Only in superconducting samples, an extra XRD peak that shifts with annealing time was observed at low temperature. The extra peak was found to be strongly linked to the nano-scale phase mixing of PI and PII, which was verified by computational simulations. High resolution transmission electron microscopy confirmed the existence of these nano-scale phase mixtures in the superconducting samples. We proposed that interfacial inducement is the most reasonable explanation for the emergence of 25 K superconductivity in undoped Ca122. On the other hand, we occasionally found the superconductivity with a Tc of around 17 K in light cobalt doped Ca122 by proper annealing. The chemical composition analysis showed very special relationship between the superconductivity and atomic ratio of As in the compound. Further experiments proved that the superconductivity in A122 (A: Ca, Sr, Ba) could be universally induced by annealing the samples in elemental As. Through systematic study of post-annealing effects in Ca122, we found that superconductivity is located at the surface and tends to be 2D. Hall measurements proved that Ca122 annealing in As includes two different processes, and the emergence of superconductivity is due to the electron doping process. First principles calculations revealed that the electron doping nature could originate from the charge transfer between the Ca122 and other elements.

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Keywords

Iron-based superconductivity, Interfaces, Post-annealing, Micron-structure, Phase distribution simulation, First principles calculations

Citation

Portions of this document appear in: Huyan, S., L. Z. Deng, Z. Wu, K. Zhao, J. Y. Sun, L. J. Wu, Y. Y. Zhao et al. "Low-temperature microstructural studies on superconducting CaFe 2 As 2." Scientific reports 9, no. 1 (2019): 6393.