Figure 1: (Left) Crystal structure of the 1111 $FeAs$ material (after [47]). Fluorine (green) replaces oxygen (gray), donating electrons to the $FeAs$ layers. Other atoms shown are iron (yellow), arsenic (purple), and lanthanum (light blue). (Right) Crystal structure of the 122 $FeAs$ material (after [14]). The parent compound of both materials has iron moments (red arrows) that form a striped antiferromagnetic pattern. Calcium atoms are shown in blue.