Online Physics Seminar by Prof. Klaas-Jan Tielrooij​ (Ultrafast Dynamics in Nanoscale Systems Group, Catalan Institute of Nanoscience and Nanotechnology (ICN2), Barcelona, Spain) held on 18 April 2022.

Abstract:
Besides the many exciting optical, electrical, magnetic and mechanical properties of quantum materials, their thermal properties are arguably equally fascinating. In this talk I will describe some recent results related to both electronic and phononic heat using a range of quantum materials, including graphene, topological insulators and transition metal dichalcogenides. The first topic [1,2] will be electronic heat spreading in graphene, which we probe using a novel microscopy technique with femtosecond temporal and nanometer spatial accuracy. This technique enables the observation of electronic heat spreading in the diffusive regime, as well as in the hydrodynamic window – before interactions with phonons occur. In this window, we demonstrate tunability between the Fermi liquid and the Dirac fluid regime, where the latter is associated with a giant thermal conductivity up to 40,000 W/m/K. This is an order of magnitude larger than the already extremely large thermal conductivity of graphene, where the heat is carried by phonons. The second topic [3,4,5] deals with terahertz nonlinear photonics, where electronic heat in systems with massless Dirac fermions gives rise to extremely large nonlinearities in the terahertz regime. This includes the generation of third-harmonic signal with a field conversion efficiency around 10%. In the third and final topic [6], I will show our very recent results on the phonon thermal conductivity of the prototypical transition metal dichalcogenide MoSe2, focusing on the effect of crystal thickness, down to the monolayer, and the effect of the environment. In contrast to 3D-bonded materials, such as silicon, layered materials only suffer a small decrease in thermal conductivity all the way down to subnanometer thicknesses – basically because of their layered, 2D-bonded nature. The described topics are relevant for several applications, including thermal management and data communication.

References
[1] A. Block et al. Nat. Nanotechnol. 16, 1195 (2021)
[2] M. Massicotte et al. Nanoscale 13, 8376 (2021)
[3] H.A. Hafez et al. Nature 561, 507 (2018)
[4] J.C. Deinert et al. ACS Nano 15, 1146 (2021)
[5] S. Kovalev et al. npj Quantum Materials 6, 84 (2021)
[6] D. Saleta Reig et al. Adv. Mater. 34, 2108352 (2022)