Zener tunneling in the electrical transport of quasimetallic carbon nanotubes

We study theoretically the impact of Zener tunneling on the charge-transport properties of quasimetallic (Qm) carbon nanotubes (characterized by forbidden band gaps of a few tens of meV). We also analyze the interplay between Zener tunneling and elastic scattering on defects. To this purpose we use a model based on the master equation for the density matrix, which takes into account the interband Zener transitions induced by the electric field (a quantum mechanical effect), the electron-defect scattering, and the electron-phonon scattering. In the presence of Zener tunneling the Qm tubes support an electrical current even when the Fermi energy lies in the forbidden band gap. In the absence of elastic scattering (in high-quality samples), the small size of the band gap of Qm tubes enables Zener tunneling for realistic values of the the electric field (above 1 V/m). The presence of a strong elastic scattering (in low-quality samples) further decreases the values of the field required to observe Zener tunneling. Indeed, for elastic-scattering lengths of the order of 50 nm, Zener tunneling affects the current-voltage characteristic already in the linear regime. In other words, in quasimetallic tubes, Zener tunneling is made more visible by defects.