Cracking the Essential Coordination Structure of the Iron- based Superconductor

High-temperature superconductors often contain transition-metal lattice planes coordinated by anions, where novel quantum states of paired electrons emerge. In iron-based high-temperature superconductors, the Fe atoms in the square-lattice are tetrahedrally coordinated by pnictogen or chalcogen anions, forming strong Fe-anion tetrahedral structures that are hard to break. Therefore, usual cleaving techniques only expose the plane of complete anion coverage. We found, utilizing scanning tunneling microspectroscopy, breaking a local tetrahedral Fe-anion coordination structure by a vacancy in this complete anion coverage locally destroys the superconductivity, manifested with the suppressed superconducting coherence peaks and a pair of in-gap states in the local tunneling spectrum. This dramatic spectral behavior clearly indicates the integrity of the anion coordination structure in the iron-based superconductivity. More remarkably, we demonstrate here the hard-to-break coordination structure can be cracked open by using our cryogenic cleaving method, resulting in the exposure of a pristine Fe-plane, allowing us to directly probe this mysterious lattice. We find that the tunneling spectrum on this Fe-lattice exhibits a striking pseudogap feature without superconducting coherence peaks. More surprisingly, by locally decorating this pristine Fe-plane with atomic anions to restore a local tetrahedral structure, the superconducting coherence peaks emerge in the local gap spectrum. All these results emphasize the essential role of the coordination structure in the emergence of the iron- based superconductivity. We also notice that the exposed pristine Fe square-lattice is actually a checkerboard composed of corner-sharing squares. The electronic nature of this checkerboard and the homogeneous pseudogap states formed on this checkerboard both deserve deep theoretical investigations. The anion-decoration enabled local superconducting spectral behavior presents an atomic perspective of the paired quantum states emerged in the essential coordination structures of the iron-based superconductors, which may also provide a significant reference for understanding the cuprate high-Tc superconductivity.