Unusual two-dimensional behavior of iron-based superconductors with low anisotropy

Abstract

We study angular-dependent magnetoresistance in iron-based superconductors ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{FeTe}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}$. Both superconductors have relatively small anisotropies $\ensuremath{\gamma}\ensuremath{\sim}2$ and exhibit a three-dimensional (3D) behavior at low temperatures. However, we observe that they start to exhibit a profound two-dimensional behavior at elevated temperatures and in applied magnetic field parallel to the surface. We conclude that the unexpected two-dimensional (2D) behavior of the studied low-anisotropic superconductors is not related to layeredness of the materials, but is caused by appearance of surface superconductivity when magnetic field exceeds the upper critical field ${H}_{c2}(T)$ for destruction of bulk superconductivity. We argue that the corresponding 3D-2D bulk-to-surface dimensional transition can be used for accurate determination of the upper critical field.