Strain control over macroscopic quantum states in two-dimensional heterostructures

低温微电子机械系统的最新进展为研究二维(2D)系统的结构、力学和电学性质之间的基本相互作用以及通过应变来操纵它们开辟了道路。在这个项目中，我们计划进一步扩展这一方法，以获得对2D异质结构中的宏观量子态，如量子霍尔态和超导电性的应变控制。具体地说，我们想要解决三个重要的、开放的研究问题。首先，我们的目标是深入了解热涨落、本征皱缩、整体应变和纳米尺度应变变化在石墨烯中的作用。为此，我们将以一种可控和可重现的方式对悬浮的石墨烯器件施加应变，并研究其对量子霍尔效应的影响。在此背景下，我们还将引入应变梯度来探索应变诱导的石墨烯中的伪量子霍尔效应，这将是迈向山谷电子学的一个里程碑。我们要解决的第二个问题是，为什么在衬底上的单层NbSe2异质结中可以观察到超导电性，而在悬浮器件中却观察不到。有人认为这是由于固有的皱缩和热波动，我们将通过施加应变来压平和稳定悬挂的设备来验证这一假说。如果成功，这项实验将通过应变的操纵来控制固定温度下的量子相变，并使超导态和非超导态共存的应变工程器件成为可能。最后，我们研究了基于NbSe_2的单层和少层超导薄膜的本征阻尼。从焦耳加热中提取的有效电阻提供了关于膜中非超导区的存在的信息，从而提供了关于序参数的信息。该项目成功的关键是我们的方法保证了对诱导应变场和低温输运的精确控制，这为探测和控制二维异质结构中的物质相提供了新的途径。

Recent advancements in micro electro-mechanical systems at low temperature opened up the way for studying the fundamental interplay between structural, mechanical and electronic properties in two-dimensional (2D) systems, and to manipulate them by means of strain. In this project we plan to further extend this approach to gain strain control over macroscopic quantum states, such as quantum Hall states and superconductivity, in 2D heterostructures. Specifically, we want to address three important, open research questions. First, we aim at getting a thorough understanding on the role of thermal fluctuations, intrinsic crumpling, overall strain and nanometer-scale strain variations in graphene. To do so, we will apply strain in a controllable and reproducible way to suspended graphene devices, and study its effects on the emergence of the quantum Hall effect. In this context, we will also induce strain gradients to probe the strain-induced pseudo quantum Hall effect in graphene, which will be a milestone towards valley-tronics. The second question we want to address is why superconductivity can be observed in single-layer NbSe2 heterostructures on substrate, but not in suspended devices. It has been suggested that this is due to intrinsic crumpling and thermal fluctuations, and we will test this hypothesis by applying strain to flatten and stabilize suspended devices. If successful, this experiment will give control over a quantum phase transition at a fixed temperature via the manipulation with strain, and enable strain-engineered devices in which both the superconducting and non-superconducting states coexist. Finally, we investigate the intrinsic damping in a superconducting single- and few-layer NbSe2–based membranes. The effective resistance extracted from Joule heating gives information about the presence of non-superconducting regions in the membrane and thereby about the order parameter. The key element for the success of this project is the accurate control over the induced strain field combined with low-temperature transport guaranteed by our approach, which provides new ways of probing and controlling phases of matter in 2D heterostructures.