Experimental observation of a fourfold difference in thermal conductivity yet comparable maximum phonon mean free path in α -alumina and α -gallium oxide

Lighter-element materials usually possess higher lattice thermal conductivity (LTC) due to both higher group velocities and longer mean free paths (MFPs) than their heavier-element counterparts. In this work, using broadband frequency-domain thermoreflectance, we measure the LTC and MFP spectra of corundum-structured α-Al2O3 and α-Ga2O3 crystals and report comparable maximum MFPs yet an approximately fourfold difference in LTC. For α-Al2O3, diffusive transport persists down to a thermal penetration depth of 135 nm, yielding an LTC of 34.9 ± 1.1 W m−1 K−1 and a maximum MFP smaller than 135 nm. For α-Ga2O3, ballistic transport appears below 120 nm, exhibiting an LTC of 9.2 ± 0.53 W m−1 K−1 and a maximum MFP of about 120 nm. This counterintuitive finding challenges the conventional expectation, providing experimental insights into the fundamental understanding and design of thermal transport in functional materials.