Analytical classification of Majorana zero-mode spatial profiles in extended Kitaev chains: probability maxima can shift inward

TL;DR

This paper analytically studies Majorana zero modes in extended Kitaev chains, revealing that their spatial profiles can have maxima away from edges and depend on Hamiltonian parameters.

Contribution

It derives a recursion relation for MZM spatial structure in extended Kitaev chains, providing closed-form expressions and insights into their decay behaviors.

Findings

MZMs can have maxima at interior sites, not just edges.

Decay behaviors include monotonic, oscillatory, and localized.

Finite chains require specific lengths to reproduce semi-infinite MZM profiles.

Abstract

Topological phases in one-dimensional superconducting systems are commonly characterized by symmetry-protected invariants. These invariants determine the number of Majorana zero-energy boundary modes but do not specify their corresponding spatial structure. In this work, we present an analytical study of Majorana zero modes (MZMs) in an extended Kitaev chain with nearest- and next-nearest-neighbor couplings. By expressing the Hamiltonian in the Majorana basis, we derive a recursion relation whose characteristic roots completely determine the spatial structure of the zero modes and yield closed-form expressions for their amplitudes. We show that, even within a single topological phase, the MZMs can exhibit qualitatively distinct decay behaviors - monotonic decay, oscillatory decay, and perfectly localized states. Remarkably, boundary-origin MZMs need not have their maximum probability at the edge of the chain. They can instead exhibit maxima at interior lattice sites with an exponentially decaying envelope from either side of the maxima. Furthermore, the characteristic roots determine the length scale required for finite chains to reproduce the semi-infinite MZM structure, providing a direct link between Hamiltonian parameters, finite-size effects, and experimentally observable spatial profiles.