Low energy electron diffraction, scanning tunneling microscopy and spectroscopy, and first-principles spin-dependent density functional theory are utilized to investigate the geometric, electronic, and magnetic structures of the stripe-ordered ($1\times 2$) surface of $\mathrm{Ca}{({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)}_2{\mathrm{As}}_2$ ($x=0$, 0.075). The surface is terminated with a 50% Ca layer. Compared to the bulk, the surface Ca layer has a large inward relaxation ($\sim 0.5\AA$), and the underneath $\mathrm{As}\text{−}{\mathrm{Fe}}_2\text{−}\mathrm{As}$ layer displays a significant buckling. First-principles calculations show that the ($1\times 2$) phase is stabilized by the bulk antiferromagnetic spin ordering through the spin-charge-lattice coupling. Strikingly, a superconducting gap ($\sim 7\text{meV}$ at 7.4 K) is observed to spatially coexist with the ($1\times 2$) phase ($x=0.075$ compound). This implies the coexistence of both superconductivity and AFM ordering at the surface.

• Received 15 July 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.077205