Thermal conductivity and dynamic heat capacity across the metal-insulator transition in thin film VO₂

The thermal properties of VO₂ thin films, 90-440 nm thick, are measured by time-domain thermoreflectance (TDTR) across the metal-insulator transition (MIT). The thermal conductivity increases by as much as 60% in the metallic phase; this increase in conductivity is in good agreement with the expected electronic contribution to the thermal conductivity. For relatively thick layers, TDTR data are sensitive to the dynamic heat capacity and show a pronounced peak near the MIT temperature created by a contribution to the enthalpy from periodic transformations at the 10 MHz frequency of the thermal waves used in the experiment. The dynamic heat capacity increases as the amplitude ΔT of the thermal waves becomes comparable to the width of the MIT and reaches ≈30% of the bulk latent heat for ΔT≈1.6 K.