Surface state evolution induced by magnetic order in axion insulator candidate EuIn2As2

Gapping of Dirac surface states through time reversal symmetry breaking may realize the axion insulator state in condensed matter. Despite tremendous efforts, only a few material systems fall into this category of intrinsic magnetic topological insulators. Recent theoretical calculations proposed the antiferromagnetic (AFM) ${\mathrm{EuIn}}₂{\mathrm{As}}₂$ to be a topologically nontrivial magnetic insulator with gapped surface states. Here we use scanning tunneling microscopy and spectroscopy complemented with density-functional theory calculations and modeling to probe the surface electronic states in ${\mathrm{EuIn}}₂{\mathrm{As}}₂$. We find a spin-orbit induced bulk gap of $\ensuremath{\sim}120\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ located only a few meV above the Fermi energy, within which topological surface states reside. Temperature-dependent measurements provide evidence of the partial gapping ($\ensuremath{\sim}40\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$) of the surface states at low temperatures below the AFM order, which decreases with increasing temperature but remains finite above ${T}_{\mathrm{N}}$.