Antiferromagnetic topological insulators

We consider antiferromagnets breaking both time-reversal (Theta) and a primitive-lattice translational symmetry (T-1/2) of a crystal but preserving the combination S=Theta T-1/2. The S symmetry leads to a Z(2) topological classification of insulators, separating the ordinary insulator phase from the antiferromagnetic topological insulator phase. This state is similar to the strong topological insulator with time-reversal symmetry and shares with it such properties as a quantized magnetoelectric effect. However, for certain surfaces the surface states are intrinsically gapped with a half-quantum Hall effect [sigma(xy)=e(2)/(2h)], which may aid experimental confirmation of theta=pi quantized magnetoelectric coupling. Step edges on such a surface support gapless, chiral quantum wires. In closing we discuss GdBiPt as a possible example of this topological class.