Emergent Andreev Reflection from a Lattice Duality Defect.

Andreev reflection converts an incoming fermion into an outgoing hole and is usually tied to a superconducting interface. We show that an analogous charge-conjugating boundary condition emerges from a purely lattice duality defect. Starting from a Majorana representation of the transverse-field Ising chain, we construct a folded lattice model in which a boundary Majorana impurity implements a one-site translation of a staggered Majorana chain. In the continuum, this translation becomes a chiral fermion-parity defect: it flips the sign of the only left-moving Majorana mode while leaving the right-moving mode unchanged. When the two Majorana modes are recombined into a complex fermion in the folded geometry, this sign flip becomes the Andreev-like boundary condition. Our lattice formulation gives a microscopic interpretation of the Emery-Kivelson boundary of the two-channel Kondo problem and of Maldacena-Ludwig monopole scattering, while identifying the boundary as the interface between a Kitaev-chain symmetry-protected topological phase and a gapless chain. The same Majorana translation defect also provides a lattice realization of an axial U(1)_{A}-symmetric charge-flip boundary.