Topological edge and domain-wall phonons in the mass-dimerized diatomic chain: complex band structure, Zak phase, and spectral unfolding

Abstract

The one-dimensional chain with alternating masses is the simplest mechanical system with a topological band structure. We give a fully analytical account of its localized vibrations built on the complex band structure. The evanescent Bloch waves available at each frequency are matched directly to force balance at a free end or a domain wall, without transfer matrices, which yields closed-form frequencies and decay lengths for every localized mode. The free-end mode sits at a branch point of the complex band structure, where the acoustic and optical sheets join and the decay is fastest. For uniform springs, a sharp wall between the two mass orderings hosts one in-gap mode, symmetric at the light$\mid$light wall and antisymmetric at the heavy$\mid$heavy wall, and an adiabatic construction shows that these are the same topological mode; the light$\mid$light wall also binds a non-topological mode above the optical branch. Spectral unfolding makes this classification visible in momentum space: the sign of the Bloch factor sets the zone-center or zone-boundary character of each mode, and its magnitude sets the width. The Zak phase, quantized by inversion, identifies the modes of topological origin and fixes a half-integer mode count per band at a wall, while the chiral symmetry known to be hidden in strain coordinates explains which frequencies are pinned.