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February 11th, 2010
Abstract:
Symmetry is at the heart of all physics. Predicting the behavior of a material by studying underlying symmetries is one of the oldest and most powerful theoretical techniques, with quite impressive consequences: the symmetry of time invariance gives rise to energy conservation while rotational symmetry underlies the conservation of angular momentum. What then if symmetry is broken? Broken symmetry often hints at exciting new phenomena such as the emergence of the Higgs boson in particle physics, or ferromagnetism in condensed matter physics.
Very recently, two experimental groups—Yue Zhao, Paul Cadden-Zimansky, Zhigang Jiang, and Philip Kim at Columbia University in the US, reporting in the current issue of Physical Review Letters [1], and Harvard's Benjamin Feldman, Jens Martin, and Amir Yacoby, also in the US [2]—have reported on the eightfold symmetry-breaking of the zero-energy Landau level in bilayer graphene systems (Fig. 1). The Columbia experiment used the typical setup of bilayer graphene on a SiO2 substrate [3] and found that the unusual zero-energy quantum Hall octet, while intact at lower magnetic fields, splits up completely into eight separate Landau levels when exposed to 35 T (generated at the National High Magnetic Field Laboratory in Tallahassee, Florida, and close to the limit of what is currently possible for man-made static magnetic fields). The Harvard group used "suspended graphene," an otherwise identical system, but where additional processing is used to remove the supporting SiO2 substrate [4]. They report that the same symmetry breaking occurs at the more moderate magnetic field of about 3 T.
Source:
The American Physical Society
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