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The graph on the cover shows the energy spectrum of a 2D electron system subjected to a strong perpendicular magnetic field and a weak 2D periodic electrostatic potential as a function of the applied magnetic flux. The spectrum consists of Landau bands, each of which exhibits the identical, repeating fractal pattern known as a Hofstadter butterfly. [M. C. Geisler, J. H. Smet, V. Umansky, K. von Klitzing, B. Naundorf, R. Ketzmerick, and H. Schweizer, Phys. Rev. Lett. 92, 256801 (2004)] |
