Pressure-tuning of the thermal conductivity of a layered crystal, muscovite

We study the physics of heat conduction in layered, anisotropic crystals by measuring the cross-plane elastic constant $C_{33}$ and thermal conductivity $\Lambda $ of muscovite mica as a function of hydrostatic pressure. Picosecond interferometry and time-domain thermoreflectance provide high precision measurements of $C_{33}$ and $\Lambda $, respectively, of micron-sized samples within a diamond anvil cell; $\Lambda $ changes from the anomalously low value of 0.46 W m$^{-1}$ K$^{-1}$ at ambient pressure to a value more typical of oxides crystals with large unit cells, 6.6 W m$^{-1}$ K$^{-1}$, at $P$=24 GPa. We find good agreement between the data and a simple theoretical model that takes into account the pressure dependence of the cross-plane sound velocities.