Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08855, USA.
Nature. 2009 Nov 12;462(7270):192-5. doi: 10.1038/nature08522. Epub 2009 Oct 14.
In graphene, which is an atomic layer of crystalline carbon, two of the distinguishing properties of the material are the charge carriers' two-dimensional and relativistic character. The first experimental evidence of the two-dimensional nature of graphene came from the observation of a sequence of plateaus in measurements of its transport properties in the presence of an applied magnetic field. These are signatures of the so-called integer quantum Hall effect. However, as a consequence of the relativistic character of the charge carriers, the integer quantum Hall effect observed in graphene is qualitatively different from its semiconductor analogue. As a third distinguishing feature of graphene, it has been conjectured that interactions and correlations should be important in this material, but surprisingly, evidence of collective behaviour in graphene is lacking. In particular, the quintessential collective quantum behaviour in two dimensions, the fractional quantum Hall effect (FQHE), has so far resisted observation in graphene despite intense efforts and theoretical predictions of its existence. Here we report the observation of the FQHE in graphene. Our observations are made possible by using suspended graphene devices probed by two-terminal charge transport measurements. This allows us to isolate the sample from substrate-induced perturbations that usually obscure the effects of interactions in this system and to avoid effects of finite geometry. At low carrier density, we find a field-induced transition to an insulator that competes with the FQHE, allowing its observation only in the highest quality samples. We believe that these results will open the door to the physics of FQHE and other collective behaviour in graphene.
在石墨烯中，碳原子呈层状排列，其两个显著特性是载流子的二维和相对论特性。石墨烯二维特性的第一个实验证据来自于对其在施加磁场时输运性质的测量中观察到的一系列平台。这些是所谓的整数量子霍尔效应的特征。然而，由于载流子的相对论特性，石墨烯中观察到的整数量子霍尔效应在性质上与半导体类似物不同。作为石墨烯的第三个显著特性，人们推测相互作用和相关性在这种材料中应该很重要，但令人惊讶的是，缺乏石墨烯中集体行为的证据。特别是，在二维中典型的集体量子行为，分数量子霍尔效应（FQHE），尽管有强烈的研究和理论预测其存在，但迄今为止，在石墨烯中仍未观察到。在这里，我们报告了在石墨烯中观察到的 FQHE。我们的观察结果是通过使用悬浮石墨烯器件进行的双端电荷输运测量来实现的。这使我们能够将样品与衬底诱导的扰动隔离，这些扰动通常会掩盖该系统中相互作用的影响，并避免有限几何形状的影响。在低载流子密度下，我们发现了一种与 FQHE 竞争的场诱导转变为绝缘体，这使得只有在高质量样品中才能观察到 FQHE。我们相信这些结果将为石墨烯中的 FQHE 和其他集体行为的物理学开辟大门。
