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The emergence of extreme-ultraviolet free-electron-laser (EUV FEL) sources has made possible transient grating (TG) spectroscopy at nanometer length scales, enabling studies of nanoscale thermal transport and bulk and surface coherent phonons with nanoscale wavelengths [1]. Here, we report very recent EUV TG measurements performed at the FERMI FEL on single-crystal graphite and dense amorphous carbon. We used the reflection-mode TG geometry [2] enabling measurements on thick samples. By changing the pump wavelength between 26.6 – 66.5 nm at a fixed crossing angle, we varied the TG period between 56 – 138 nm, with the probe wavelength set to 13.3 nm.
We observe that the thermal grating relaxation in graphite is two orders of magnitude slower than predicted by the heat diffusion equation. Furthermore, whereas on the macroscale the in-plane thermal conductivity of graphite exceeds that of dense amorphous carbon by a factor of ~400 [3,4], the effective thermal conductivities are found to be almost comparable on the tens of nanometers length scale. We also observe acoustic oscillations at rather high frequencies, 0.6 THz in graphite and 2 THz in amorphous carbon, identified as the back-scattering Brillouin frequency from coherent LA phonons propagating almost normally to the surface [5]. Another interesting observation is the absence of surface phonons on the basal plane of graphite, whereas in amorphous carbon we detect coherent surface phonon oscillations at frequencies up to 150 GHz.
Our results demonstrate the FEL-based reflection-mode EUV transient thermal grating spectroscopy as a powerful approach for probing non-diffusive heat transport, as well as bulk and surface phonon dynamics in the mesoscopic regime between inelastic optical scattering and inelastic x-ray and neutron scattering.
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| Scientific Topics | Solid State Physics |
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