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Reporting period: 2020-08-01 to 2022-01-31

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.
During the first period of the project, we have set up our heat transport measurement platform, and developed and optimized our fabrication process. The measurement platform consists in a cryogen-free dilution refrigerator equipped with a large superconducting magnet, and wired to perform electrical and heat transport measurements down to extremely low temperatures. In particular, a major experimental challenge of the project QUAHQ is the ability to perform ultra-high sensitivity noise measurements so as to resolve local temperature changes of a fraction of milliKelvin (mK), from a base electronic temperature of about 10 mK. These changes are measured on micrometer-sized metallic islands electrically connected to the graphene crystal. We have optimized the fabrication process to enhance the connectivity of these islands while minimizing their size to suppress spurious heat transport due to crystalline phonons.

We now routinely measure the thermal conductance of chiral edge channels of the quantum Hall effect, for fully filled Landau levels, for partially filled Landau levels where spin and valley symmetries are broken, as well as , since very recently for fractional quantum Hall states. The obtained base electronic temperature (11 mK) is very close to the base temperature of the refrigerator, and among the lowest obtained in a cryogen free system under high magnetic field. This places us in a very competitive position with respect to other groups performing heat transport measurement in graphene.
The results obtained during the first half of the project are very promising. We are ready to move to more complex experiments going beyond the state of the art (in particular in the nu=0 quantum Hall state of graphene). I expect to have new breakthrough results for both WP1 and WP2 before the end of the project, as well as additional results related to the physics addressed by the QUAHQ project, going beyond the two WPs.
Thermal conductance of graphene quantum Hall edge channels.