Broken symmetry and pseudogaps in ropes of carbon nanotubes

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Since the discovery of carbon nanotubes1, it has been speculated that these materials should behave like nanoscale wires with unusual electronic properties and exceptional strength. Recently, ‘ropes’ of close-packed single-wall nanotubes have been synthesized in high yield2. The tubes in these ropes are mainly of the (10,10) type3, which is predicted to be metallic4,5,6. Experiments on individual nanotubes and ropes7,8 indicate that these systems indeed have transport properties that qualify them to be viewed as nanoscale quantum wires at low temperature. It has been expected that the close-packing of individual nanotubes into ropes does not change their electronic properties significantly. Here, however, we present first-principles calculations which show that a broken symmetry of the (10,10) tube caused by interactions between tubes in a rope induces a pseudogap of about 0.1 eV at the Fermi level. This pseudogap strongly modifies many of the fundamental electronic properties: we predict a semimetal-like temperature dependence of the electrical conductivity and a finite gap in the infrared absorption spectrum. The existence of both electron and hole charge carriers will lead to qualitatively different thermopower and Hall-effect behaviours from those expected for a normal metal.

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Figure 1: Band crossing and band repulsion.
Figure 2: Perspective view of a rope of (10,10) carbon nanotubes.
Figure 3: Calculated densities of states.


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We thank V. Crespi for discussions and for providing relaxed atomic coordinates. This work was supported by the NSF and DOE. P.D. thanks the NUI for support; H.J.C. and J.I. were supported by the BSRI program of the Ministry of Education of Korea and the SRC program of KOSEF; and S.G.L. acknowledges the hospitality of the Aspen Center for Physics.

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Correspondence to Steven G. Louie.

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