  Topological insulators, in contrast to ordinary semiconductors, accompany
protected metallic surfaces described by Dirac-type fermions. Here, we
theoretically show another emergent two-dimensional metal embedded in the bulk
insulator is realized at a magnetic domain wall. The domain wall has long been
studied as ingredients of both old-fashioned and leading-edge spintronics. The
domain wall here, as an interface of seemingly trivial antiferromagnetic
insulators, emergently realizes a functional interface preserved by zero modes
with robust two-dimensional Fermi surfaces, where pyrochlore iridium oxides
proposed to host condensed-matter realization of Weyl fermions offer such
examples at low temperatures. The existence of ingap states pinned at domain
walls, theoretically resembling spin/charge solitons in polyacetylene, and
protected as the edge of hidden one-dimensional weak Chern insulators
characterized by a zero-dimensional class A topological invariant, solves
experimental puzzles observed in R2Ir2O7 with rare earth elements R. The domain
wall realizes a novel quantum confinement of electrons and embosses a net
uniform magnetization, which enables magnetic control of electronic interface
transports beyond semiconductor paradigm.
