Giant magneto-conductance in twisted carbon nanotubes

Using the Landauer-Büttiker formalism, we calculate the effect of structural twist on electron transport in conducting carbon nanotubes. We demonstrate that even a localized region of twist scatters the propagating π electrons and induces the opening of a (pseudo-) gap near the Fermi level. The subsequent conductance reduction may be compensated by an applied axial magnetic field, leading to a twist-induced, giant positive magneto-conductance in clean armchair nanotubes. Carbon nanotubes [1–3] exhibit a range of unusual electronic properties associated with the morphology of these quasi-1D structures. Early one-electron theories successfully associated metallic or semiconducting behaviour with the chiral vector that characterizes a given nanotube [4–6]. Further studies addressed the effect of atomic-level impurities [7–10] and inter-tube interactions [11–14] on electrical conductance, or magneto-transport [7,16,15,17,18]. In parallel with the development of such one-electron theories, intensive studies of electron-electron correlations have been undertaken. Indeed, nonlinear currentvoltage (I − V ) characteristics have recently been observed [19], which are reminiscent of Luttinger liquid behaviour. An intriguing question remains however, namely whether other effects may augment or even dominate such nonlinearities in the transport properties of 1 nanotubes. In this letter we predict that scattering of electrons from twistons may give rise to strongly non-linear I − V characteristics. Twistons [20–22], associated with regions of axial twist in otherwise perfect nanotubes, are intrinsic defects that are frozen into nanotube bundles during their synthesis and hence cannot be ignored when discussing electron transport. Unlike ideal straight and defect-free nanotubes, which have been shown to exhibit conventional magneto-resistive behavior [23], we find that nanotubes containing twistons may behave in a very different way. In twisted tubes, we predict the occurrence of a giant positive magnetoconductance, an unexpected effect that by far exceeds the positive magneto-conductance associated with weak localisation in disordered tubes [24]. To compute the effect of a finite scattering region on transport in an otherwise perfect (n, n) armchair nanotube, we use a parameterized four-state (s, px, py, pz) Hamiltonian, based on a global fit to Density Functional results for graphite, diamond and C2 as a function of the lattice parameter [25]. A finite twiston is treated as a scattering region connecting two semi-infinite (n, n) nanotubes. A recursive Greens function formalism is used to evaluate the transmission matrix t, describing the scattering of electrons of energy E from one end of the semi-infinite nanotube to the other [26]. The differential electrical conductance at bias voltage Vbias is related to scattering properties at energy E = EF − eVbias by the Landauer formula G = G0Tr {