Surface supercurrent diode effect

We report a new type of supercurrent diode effect on the surface of a superconductor with surface states, such as the (001) surface of iron-based superconductors BaFe$_{2-x}$Co$_x$As$_2$ under in-plane magnetic fields. Surface supercurrent diode effect can lead to a perfect supercurrent diode in a considerably wide range of fields. For comparison, the conventional supercurrent diode effect due to the spin-orbit coupling in a two-dimensional superconductor cannot be perfect in usual cases. Calculations are within the Ginzburg-Landau formalism.