Strain control of band topology and surface states in antiferromagnetic EuCd${}_2$As${}_2$

Abstract

Topological semimetal antiferromagnets provide a rich source of exotic topological states which can be controlled by manipulating the orientation of the Néel vector, or by modulating the lattice parameters through strain. We investigate via $abinitio$ density functional theory calculations, the effects of shear strain on the bulk and surface states n two antiferromagnetic EuCd${}_2$As${}_2$ phases with out-of-plane and in-plane spin configurations. When magnetic moments are along the $\text{c}$-axis, a $3\%$ longitudinal or diagonal shear strain can tune the Dirac semimetal phase to an axion insulator phase, characterized by the parity-based invariant ${\eta }_{4I}=2$. For an in-plane magnetic order, the axion insulator phase remains robust under all shear strains. We further find that for both magnetic orders, the bulk gap increases and a surface gap opens on the (001) surface up to 16 meV. Because of a nonzero ${\eta }_{4I}$ index and gapped states on the (001) surface, hinge modes are expected to happen on the side surface states between those gapped surface states. This result can provide a valuable insight in the realization of the long-sought axion states.

5 pages, 4 figures