Measuring thermal diffusivity with Scanning Ultrafast Electron Micro-Diffraction.

Ultrafast Electron Diffraction provides direct experimental access to the fundamental time- scales of atomic motion. In this technique, a femtosecond laser pulse excites a sample, and an equally short electron pulse probes the structural response. When the sample is excited far from equilibrium, the complex relaxation process can involve multiple timescales and competing energy-transport mechanisms. We describe a method for probing sample response both picoseconds, microseconds and milliseconds after femtosecond excitation, providing a more complete picture of the structural dynamics. By scanning a micron-sized diffraction probe across the sample surface, this method is able to map out thermal transport in both space and time. We present results of an experiment performed on a Moiré heterobilayer ($MoSe_{2}/WSe_{2}$) that points to the role played by interlayer excitons in heat diffusion.