Angular momentum and topology in semiconducting single-wall carbon nanotubes

TL;DR

This paper classifies semiconducting single-wall carbon nanotubes based on orbital angular momentum, revealing how valley coupling and boundary shape influence electronic properties and edge states.

Contribution

It introduces a classification scheme using the greatest common divisor of chirality integers and derives an effective lattice model to analyze valley coupling and edge states.

Findings

Different angular momentum classifications lead to distinct degeneracy patterns.

Valley coupling is strong near armchair chiralities.

The number of edge states depends on chirality and boundary shape.

Abstract

Semiconducting single-wall carbon nanotubes are classified into two types by means of orbital angular momentum of valley state, which is useful to study their low energy electronic properties in finite-length. The classification is given by an integer $d$, which is the greatest common divisor of two integers $n$ and $m$ specifying the chirality of nanotubes, by analyzing cutting lines. For the case that $d$ is equal to or greater than four, two lowest subbands from two valleys have different angular momenta with respect to the nanotube axis. Reflecting the decoupling of two valleys, discrete energy levels in finite-length nanotubes exhibit nearly fourfold degeneracy and its small lift by the spin-orbit interaction. For the case that $d$ is less than or equal to two, in which two lowest subbands from two valleys have the same angular momentum, discrete levels exhibit lift of fourfold degeneracy reflecting the coupling of two valleys. Especially, two valleys are strongly coupled when the chirality is close to the armchair chirality. An effective one-dimensional lattice model is derived by extracting states with relevant angular momentum, which reveals the valley coupling in the eigenstates. A bulk-edge correspondence, relationship between number of edge states and the winding number calculated in the corresponding bulk system, is analytically shown by using the argument principle, which enables us to estimate the number of edge states from the bulk property. The number of edge states depends not only on the chirality but also on the shape of boundary.