The 负 refraction of electrons in graphene has been seen for the first time in experiments done by physicists in the US. The work represents an important advance in the fabrication of graphene electronic devices, and could lead to new applications of graphene such as low-power transistors.
当光或其他波穿过两种不同材料之间的界面时，会发生负折射。术语“negative” is used when the direction of the light is bent in the opposite direction to that which occurs for conventional materials such as glass and water. Negative refraction is a property of some artificial metamaterials and can be used to bring diverging rays back to a focus – allowing for the creation of a perfect lens. First proposed by the Russian physicist Victor Veselago in 1968, various types of 负 refraction materials have subsequently been produced and the concept has been applied to the design of invisibility cloaks. However, actually making practical metamaterials has proven to be very difficult.
In principle, it should be much easier to achieve 负 refraction with electron waves in a semiconductor. For electrons in a solid, the equivalent quantity to the optical index of refraction is the Fermi wave vector. This intrinsic property points in the same direction as the electron flow in an n-type semiconductor – in which charge is carried by electron flow. In a p-type semiconductor, however, charge is carried by positive “holes”波向量指向相反的方向。在n型和p型半导体之间的界面“p–n junction”), the Fermi wave vector therefore changes sign and 负 refraction should result.
In practice, however, no one has been able observe 负 refraction at a p–n结. The main reason is that in conventional semiconductors with an energy gap between the valence and conduction bands, an electron has to gain or lose energy to traverse a p–n结. The result is that the vast majority of electrons are reflected at the junction rather than being transmitted across and therefore refracted.
Graphene is a sheet of carbon just one-atom thick and it has no band gap. Therefore p–n结s made from graphene should be much more transparent to electrons than those made from other semiconductors. Nevertheless, previous attempts to see 负 refraction in graphene have failed. In search of an explanation for this failure, 科里·迪恩 哥伦比亚大学的研究人员及其同事对跨石墨烯中p–n边界的电子传输进行了建模。他们得出结论，可能的罪魁祸首是界面处的原子级粗糙度，这是用于制作结的常规光刻工艺的结果。
“假设您将聚焦的激光束照射到一块玻璃上，您会发现它很容易折射并测量方向变化，” explains Dean. “现在想象一下，拿一块砂纸擦拭玻璃表面，光束会散开。”
To get around this problem, the team fashioned a junction using the natural edge of a graphene flake. They attached multiple electrodes to both sides of the junction. By injecting the electrons on one side and placing the junction in a variable transverse magnetic field, they controlled the angle at which electrons approached the boundary. They then used the voltage on the electrodes on the other side to work out where the electrons had ended up after crossing the junction. By comparing their measurements with computer models, they obtained clear evidence of 负 refraction.
该团队认为，其发现可能会导致一些实际应用。迪恩说，原则上，将发散的电子束重新聚焦到两个点之一的能力可以构成电子开关的基础。这样的开关可以使用非常少的能量进行操作，并且可以用于提高电子设备的效率。迪恩还建议，在实际设备中可以利用负折射材料在光学方面的某些相似之处（例如隐身衣）：“I don’t think it’太疯狂了，以至于无法以一种简单的方式将某些相同的概念应用于电气设备’因为技术还没有被考虑过’t been there,” he says.
理论物理学家 卡洛·比纳克 荷兰莱顿大学的工作给人留下了深刻的印象：“最大的技术进步是’我们已经能够建立非常薄，非常突然的PN结，” he says. “就其本身而言，这可能会产生深远的影响，因为我们知道p–n结具有各种电子应用。”他对用电子完美透镜的有用性持怀疑态度：“如果您有电子设备，则您的欧姆接触较大，并且会从各个方向发射电子，并且电子会从另一方向朝各个方向发射，” he says. “We don’在半导体器件中使用角分辨率，可能是因为它’这不是操作设备的可靠方法。”
该研究描述于 科学 .