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Magnetic magic - carbon nanotubes are rolled-up cylinders of graphite with diameters of about 1-10 nm, and they have been the focus of intense research since they were discovered in 1991. (a) A magnetic field (red arrow) introduces a phase factor to the electron wavefunction in the circumferential direction (red circle) via the Aharonov-Bohm effect. As a result, the electronic properties of a nanotube can be modulated by a magnetic field. (b) The energy spectrum (red lines) is a plot of energy (E) versus wavevector (k). As the magnetic flux, φ, increases, the energy-band structure of the nanotube oscillates from that of a metal to that of a semiconductor. The transformation of a semiconducting nanotube into a metallic one is a little more complicated because the band gap first decreases to zero and then widens again.