Temperature-dependent properties of 2D materials:Direct measurements of electron-phonon coupling and bending rigidity with helium scattering

In this project, we study fundamental properties of 2D materials using beams of atoms and light. These materials, only one or a few atomic layers thick, can span up to several square centimeters, or even meters. They hold great significance for the development of new technologies such as flexible electronics, which can be integrated into the body or clothing. A notable example of a novel 2D material is graphene, which is a single layer of carbon atoms. Recent studies have shown that stacking two graphene layers with a slight twist—forming "magic angle graphene"—can induce superconductivity. This means electricity can be conducted without heat loss. However, the exact mechanisms behind this form of superconductivity are still not well understood. So far, we have measured the flexibility of 2D layers and how it changes with temperature and the addition of nanoholes. For the first time, we have also detected the "boson peak"— a sudden change in heat conductivity known in amorphous materials — in a 2D material. This discovery was published in the prestigious journal Nature Physics. Finally, we have studied how atomic vibrations impact electron movement in electrically conducting and semiconducting 2D materials. Insight into these mechanisms can help us understand the different forms of superconductivity that can be achieved.

2D materials have been extensively researched since the discovery of graphene in 2004. Some 2D materials have attracted particular interest in recent years for two reasons: they can exhibit superconductivity and/or they can be used in flexible electronics. The 2018 «Physics World Breakthrough of the year» showed that when two graphene layers are rotated 1.1° the material becomes superconducting. This year superconductivity was also found in two twisted trilayer graphene systems and sample provider for this project Prof. Stevan Nadj-Perge, Caltech, showed that the superconducting properties of maging angle graphene improves when it is placed on a different substrate. The nature of superconductivity in these 2D materials is not well understood. This hampers the design of new superconducting 2D materials. Flexible electronics is pursued intensively for applications such as foldable displays, wearable biosensors, implants for monitoring life signs, artificial nerves, muscle implants and soft robotics. However, to design flexible electronic components that do not fracture when bent, it is important to know how flexible the different material layers are relative to each other (bending rigidity). For the components to work over a sufficiently large temperature range, it is important to know how the bending rigidity changes with temperature. At present there are no experimental measurements on this for 2D materials and the different theories disagree. In this project, we use helium scattering to measure i) the electron-phonon coupling in the low frequency range believed to be particular important for superconductivity of 2D materials, due to the recently observed substrate influence, and ii) temperature-dependent bending rigidity for a range of 2D materials. These properties cannot be measured directly with any other technique. The project will provide crucial information on how to design future 2D superconductors and flexible electronics components.