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November 2011 Volume 7 No 11

Time-reversal symmetry makes massless Dirac fermions in topological insulators 'gapless'. When a gap opens, it breaks this symmetry and confers mass to the fermions. But now a quantum phase transition has been observed in a three-dimensional topological insulator that allows these particles to acquire mass without symmetry breaking. Letter p840

IMAGE: T. SATO

COVER DESIGN: KAREN MOORE

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