Effect of Grain Boundaries on Superconducting Properties of FeSe Films

The grain boundaries in superconductors affect strongly the electronic transport in the normal and most notably superconducting state. E.g., high-angle grain boundaries in high-Tc cuprates act as weak links and suppress the current-carrying capability even in small magnetic fields [1]. To the contrary, in A15 compounds a high density of grain boundaries acting as dominant pinning sites enhances the critical current density, Jc, even in high magnetic fields [2]. For the iron-based superconductors both effects exist [3]: an exponential decay of Jc with increasing misalignment of adjacent grains (most studies performed on films of Co-doped BaFe2As2 or Te-doped FeSe on bicrystal substrates), as well as the surface pinning by planar defects, such as grain boundaries, dominating at low temperature in magnetic fields.
The effect of the grain boundary density on the electronic transport in an iron-based material is discussed here using the example of sputtered c-axis FeSe films on (110) SrTiO3 substrates [4]. Unexpected anisotropy of the in-plane electronic transport occurs for currents applied along the perpendicular [001] and [1-10] substrate directions. ... mehrSuch anisotropy is not observed for films on (100) SrTiO3. We relate it to the extraordinary grain-boundary architecture of elongated plate-like grains strictly aligned along [001] resulting in a higher grain boundary density in the perpendicular [1-10] direction. Obviously the film micro¬structure can be engineer¬ed by the choice of a proper substrate and its crystallographic orienta¬¬tion.
For instance, the grain boundary enforced anisotropy of normal state resistivity can be explained in terms of the reduced-area and intergrain effects [5]. The anisotropy of self-field Jc(T) likely occurs since an average grain misorientation in the [1-10] direction is closer to the critical angle of the strong-link to weak-link transition than in the [001] direction. The lower mobility for [1-10] current trans¬port, as derived from the normal-state mag¬¬ne¬to¬resistance, also reflects the higher grain-boundary density in that direction. The tem¬pe¬ra¬ture-dependent upper critical field Bc2(T) revealed smaller slope close to Tc and consequently the larger in-plane mean free path for [001] current transport, which is related to higher cleanliness, or better grain alignment as compared to [1-10]. The vortex state can be described in terms of the thermal activated flux flow model over a wide range of temperature and resistivity with an evidence for a two-dimensional pancake-vortex liquid and anisotropic pinning potentials. The in-field critical current density is characterized by a crossover from a single-vortex to a small-bundle pinning regime with increasing field. In particular, the resulted scaling of the pinning-force density vs field implies the pinning on grain boundaries. All these findings demonstrate that the microstructure of our FeSe films dominates the electronic transport via the resulting anisotropic grain-boundary density.

References
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