Yuan Cao, Harvard University: Moiré Materials and their New Degrees of Freedom

In condensed-matter physics, the introduction of new degrees of freedom (DoFs) often leads to the emergence of complex and fascinating phenomena. Recently, advances in low-dimensional materials have enabled us to tailor electrons in strongly-correlated electron systems through electrostatic gating and the freedom of van der Waals stacking. These developments have raised an important question: are there hidden DoFs in these versatile material platforms that we have yet to discover? In this seminar, I will discuss our exciting discovery of the moiré degree of freedom in 2D materials. This DoF originates from the twist angle in van der Waals stacks and opens up a new arena of correlated electronic phases. One of the most intriguing observations was the unconventional superconducting state that arises when two sheets of graphene are twisted at a magic angle of 1.1 degrees. Furthermore, our research revealed that this is just the beginning of an entire family of moiré superconductors, all of which exhibit rich and unusual phenomenology. These findings have sparked a new field of study known as "twistronics", where we can now use moiré systems to study and create correlated quantum phenomena with unparalleled control. The physics of these moiré systems will also provide valuable insight into the understanding and creation of other quantum materials.