Kapitza conductance of Bi/Sapphire interface measured by time-resolved x-ray diffraction

We measure the thermal boundary (Kapitza) conductance at the interface between single crystal thin films of bismuth and sapphire using time-resolved x-ray diffraction. Films of varying thickness (65-284 nm) are grown by molecular beam epitaxy with their c-axis perpendicular to the surface. In the measurements, an ultrafast laser pulse is used to rapidly heat the near-surface region of the film, and x-ray diffraction is used to measure the average lattice constant of the film along the c axis. By comparing the depth dependence of the temporal profile with model calculations of the thermal transport, we extract an average Kaptiza conductance of $\sim$2000$\pm$1000 W/cm$^2$/K. These results do not significantly vary with film thickness or excitation density below the damage threshold.