Strain control over macroscopic quantum states in two-dimensional heterostructures

• Applicant: Professor Dr. Christoph Stampfer
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 406252058

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.

• DFG Programme: Research Grants