Comprised of a single atomic layer of carbon atoms and approximately 50,000 times thinner than the average strand of human hair, graphene provides an incredibly versatile material platform. Electronic properties of graphene can be quickly and reversibly tuned into metallic, insulating, ferromagnetic, and even superconducting states with experimental knobs such as field-effect doping, electric and magnetic field. This unprecedented tunability enables a multi-dimensional phase space of novel quantum phenomena, which has revolutionized the field of condensed matter physics and quantum material engineering.

The versatility of graphene heterostructures gives rise to new experimental challenges to understand the microscopic mechanism and electronic orders underlying these novel quantum phenomena. For example, the electronic spin degrees of freedom play a crucial role in determining the nature of the associated quantum phases. However, conventional experimental methods, such as the transport technique, are not directly sensitive to the spin order. As such, the development of new experimental tools capable of probing the spin order is key for further advancing our understanding of the physics of graphene and other 2D materials.

Erin Morissette (fourth-year PhD candidate) and collaborators, including her advisor, Professor Jia Li, have recently made progress toward this effort to probe the spin structure in a 2D system by employing a resistively-detected ESR technique in twisted bilayer graphene, with results published this past May in Nature Physics. This approach combines ESR and quantum transport methods by first exciting the carriers in the system with microwave photons, and then measuring how that excitation couples to the resistance of the device.

The report details the first observation of microwave resonance in a high-quality graphene system. The researchers, including scientists from the Center for Integrated Nanotechnologies at Sandia National Laboratories in Albuquerque and theory collaborators from the University of Innsbruck in Austria, were able to identify the microwave resonance as a spin excitation across the twisted bilayer graphene (TBG) device. Furthermore, the dispersion of the resonance modes with magnetic field and microwave frequency uncovered novel information about the ground state of the strongly-correlated system: a parameter of great theoretical interest but never before experimentally extracted, which describes the coupling between electron spins in opposite valleys of the supermoiré lattice in TBG.

In addition to twisted bilayer graphene, microwave resonance can also be utilized to investigate other strongly-correlated 2D systems that exhibit exotic quantum phenomena, such as the superconducting states in Bernal bilayer graphene at high electric fields. Microwave resonance can also be combined with the angle-resolved non-linear transport method, also recently developed by the Li Lab at Brown, which provides even further insight into the valley states of these systems. Taken together, these advancements represent unprecedented understanding and control of the extraordinary electronic states in two-dimensional materials.