Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Metal-Semiconductor Transition and Fermi Velocity Renormalization in
  Metallic Carbon Nanotubes   Angular perturbations modify the band structure of armchair (and other
metallic) carbon nanotubes by breaking the tube symmetry and may induce a
metal-semiconductor transition when certain selection rules are satisfied. The
symmetry requirements apply for both the nanotube and the perturbation
potential, as studied within a nonorthogonal $\pi$-orbital tight-binding
method. Perturbations of two categories are considered: an on-site
electrostatic potential and a lattice deformation which changes the off-site
hopping integrals. Armchair nanotubes are proved to be robust against the
metal-semiconductor transition in second-order perturbation theory due to their
high symmetry, but can develop a nonzero gap by extending the perturbation
series to higher orders or by combining potentials of different types. An
assumption of orthogonality between $\pi$ orbitals is shown to lead to an
accidental electron-hole symmetry and extra selection rules that are weakly
broken in the nonorthogonal theory. These results are further generalized to
metallic nanotubes of arbitrary chirality.

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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat
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The categories of this paper are: chem-ph
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The categories of this paper are: funct-an
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The categories of this paper are: stat
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The categories of this paper are: quant-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: supr-con
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The categories of this paper are: plasm-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: hep-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-ex
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The categories of this paper are: astro-ph
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The categories of this paper are: math-ph