Magnetochiral Dichroism in a Collinear Antiferromagnet with No Magnetization.

We show the directional dichroism in a collinear antiferromagnet MnTiO_{3}. The dichroism between two distinctive antiferromagnetic states with opposite signs of staggered magnetic moments can be regarded as magnetochiral dichroism in the absence of external fields. Electric-field reversal of antiferromagnetic domain causes a change in the absorption intensity of unpolarized light around 2.15 eV. The difference in optical absorption between two antiferromagnetic states is reversed for the light propagating in the opposite direction. The absorption coefficient displays a hysteretic behavior for a cycle of sweeping the external electric or magnetic field.