Endohedral Impurities in Carbon Nanotubes

TL;DR

This paper models endohedral impurities in carbon nanotubes, revealing how vibronic coupling induces symmetry breaking and affects low-energy electronic properties, including potential Kondo behavior suppression.

Contribution

It introduces a generalized Anderson model with pseudo-Jahn-Teller coupling to describe impurity distortions and symmetry breaking in nanotubes.

Findings

Spontaneous axial symmetry breaking occurs when vibronic coupling exceeds a critical value.

The effective potential exhibits O(2) symmetry in the broken state.

Strong vibronic coupling suppresses the Kondo energy scale in metallic nanotubes.

Abstract

A generalization of the Anderson model that includes pseudo-Jahn-Teller impurity coupling is proposed to describe distortions of an endohedral impurity in a carbon nanotube. Treating the distortion within mean-field theory, spontaneous axial symmetry breaking is found when the vibronic coupling strength g exceeds a critical value g$_c$. The effective potential in the symmetry-broken state is found to have O(2) symmetry, in agreement with numerical calculations. For metallic zigzag nanotubes endohedrally-doped with transition metals in the dilute limit, the low-energy properties of the system may display two-channel Kondo behavior; however, strong vibronic coupling is seen to exponentially suppress the Kondo energy scale.