Microstructure and Superconducting Properties of Na1 – δFe1 – xCoxAs Single Crystals

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We present a comprehensive study of microstructure, magnetotransport properties, and Abrikosov vortex pinning in Na1 – δFe0.955Co0.045As single crystals of overdoped composition with $${{T}_{c}} \approx 21{\kern 1pt} $$ K. Using transmission electron microscopy with atomic resolution, planar defects with structure similar to NaFe2As2 and dimensions in the $$ab$$ -plane about 200–500 nm responsible for sodium deficiency were visualized. It was shown that namely these defects act as effective additional Abrikosov vortex pinning centers, as confirmed by the strong power-law critical current dependence $${{J}_{c}}(H) \propto {{H}^{{ - \alpha }}}$$ . Temperature dependence of the upper critical field $${{\mu }_{0}}H_{{c2}}^{{ab,c}}(T)$$ and its anisotropy at $$T \to {{T}_{c}}$$ between the crystallographic ab- and c-directions is determined.

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