In extreme environmental conditions, such as very low temperatures and/or very high magnetic field, interactions between electrons in an otherwise electrically conducting material can cause this material to become in insulator. Understanding the physics that underlies those so called "correlated insulators" is one of the major challenges of condensed matter research. Indeed, making theoretical predictions on a system comprising a huge number of interacting particles is, to begin with, extremely difficult; furthermore, experimentally probing an electrically insulating system evidently rules out the use of electrical measurements.
The goal of the project QUAHQ is to tackle this experimental issue on a material that has driven a very large amount of novel research in the past decade, due to the many promises it holds both for fundamental and applied electronics. Under high magnetic field and at cryogenics temperatures, graphene can host electrically insulating, strongly correlated electronic states. These states are expected to host a number of charge neutral collective modes which can carry heat across the sample, and that directly reflect the intrinsic microscopic nature of the correlated states.
To probe these collective modes, we perform heat transport measurements in ultra-high quality graphene samples, cooled down to extremely low temperatures, and immersed in high magnetic fields. We are focusing our efforts on two classes of correlated states in graphene: the so-called "nu=0" state of the quantum Hall effect, which arises in a very peculiar point of the band structure of graphene where its valence and conduction bands touch one another; and fractional quantum Hall states, where interactions become dominant and give rise to excitations on the edge of the sample which have fractional charges.
The results of our investigations have shown that in contradiction to theoretical predictions, the nu=0 state is both an electrical and thermal insulator (Nat. Phys. 20, 1927 (2024))). Its thermal conduction properties were thought to arise from a spectrum of collective excitations carrying magnetic textures that remained ungapped; our results suggest that this spectrum actually has a significant gap, requiring further theoretical and experimental investigations. We also have demonstrated, again through heat transport, that neutral modes carrying heat in the direction opposite to that of the chiral charge carrying channels circulating along the edges of the sample in the fractional quantum Hall effect can exchange energy with the charged edge channel, regardless of the type of excitation the latter carry: fractional charge with anyonic quantum statistics, or integer charges with fermionic statistics. This result answers questions central to the community, and shows how one can control the electrostatics at the edge of a graphene sample (Phys. Rev. Lett. 129, 116803 (2022)).
These main results, as well as the rest of the work performed during the project, highlight the importance of heat transport measurements as a tool to investigate strongly correlated phases of matter.
