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PhD Student: Sebin Varghese

Directors: Prof. Klaas-Jan Tielrooij, Ultrafast Dynamics in Nanoscale Systems Senior Group Leader at ICN2.

Short Abstract: It has been a longstanding goal of physicists to understand how heat flows and how it affects the thermal properties of materials. This quest has been driven not only by the pursuit of knowledge but also by the tremendous importance of heat in daily life. The discovery of new materials has introduced challenges in understanding fundamental properties such as heat transport, while also opening avenues for technological advancements. In this thesis, we investigate the heat transport properties of a class of layered materials known as transition metal dichalcogenides (TMDs). We study these materials using two different optical techniques: the conventional technique of Raman thermometry and a novel spatiotemporal pump-probe thermometry technique that we have developed. With this new technique, we directly quantify the in-plane thermal diffusivity in TMDs. The thermal diffusivities obtained for MoSe2, down to a thickness of 3 layers, are consistent with results obtained from Raman thermometry. Interestingly, in monolayer and bilayer samples, we observe a transition in heat transport behaviour—from diffusive in thicker samples to viscous with very low diffusivity in ultrathin samples. We attribute this viscous transport to the hydrodynamic flow of heat, a phenomenon that has never been observed in TMDs at any temperature.