Department of Physics, University of Basel, Basel, Switzerland.
Phys Rev Lett. 2012 Feb 17;108(7):076602. doi: 10.1103/PhysRevLett.108.076602. Epub 2012 Feb 13.
Bilayer graphene bears an eightfold degeneracy due to spin, valley, and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kinds of devices. In bilayers of type B1 the eightfold zero-energy Landau level is partially lifted above a threshold field revealing an insulating ν=0 quantum-Hall state at the charge neutrality point. In bilayers of type B2 the Landau level lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike B1, the minimum conductance in B2 is not exponentially suppressed, but remains finite with a value G is < or approximately equall to e(2)/h even in a large magnetic field. We suggest that this phase of B2 is insulating in the bulk and bound by compressible edge states.
双层石墨烯由于自旋、谷和层对称性而具有八重简并，允许通过磁场或电场、应变甚至通过相互作用自发地诱导出大量的破对称状态。我们研究了经清洁电流退火的悬浮双层石墨烯中的输运性质。我们发现了两种器件。在 B1 型双层中，八重零能朗道能级在一个阈值场以上部分提升，在电荷中性点揭示出一个绝缘 ν=0 量子霍尔态。在 B2 型双层中，朗道能级提升是完全的，即使在零磁场下，微分电导也会出现间隙，表明存在一个自发破对称的绝缘态。与 B1 不同的是，B2 的最小电导不是指数抑制的，而是在很大的磁场下仍保持有限值 G 约等于 e(2)/h。我们认为，B2 的这个相在体相中是绝缘的，由可压缩的边缘态束缚。
