# Carbon nanotubes as excitonic insulators

## TL;DR

This paper demonstrates that zero-gap carbon nanotubes can become excitonic insulators due to electron-hole pairing, leading to a measurable energy gap influenced by tube radius and magnetic field, challenging existing theories.

## Contribution

The study provides first-principles evidence that zero-gap carbon nanotubes exhibit excitonic insulating behavior, supported by many-body calculations and quantum Monte Carlo methods.

## Key findings

- Excitonic order opens a gap in zero-gap nanotubes.
- The gap scales inversely with tube radius.
- Weak dependence of the gap on axial magnetic field.

## Abstract

Fifty years ago Walter Kohn speculated that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons---electron-hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. Here we show that this excitonic insulator is realized in zero-gap carbon nanotubes by performing first-principles calculations through many-body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings call into question the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulator.