Threefold reduction in thermal conductivity of Vycor glass due to adsorption of liquid ⁴He

We report thermal conductivity measurements of porous Vycor glass when it is empty and when the pores are filled with helium between 0.06 and 0.5 K. The filling of liquid ³He and liquid ⁴He inside the Vycor pores brings about two to three-fold reduction of the thermal conductivity as compared with empty Vycor. This dramatic reduction of thermal conductivity, not seen with solid ³He and ⁴He in the pores, is the consequence of hydrodynamic sound modes in liquid helium that greatly facilitate the quantum tunneling of the two-level systems in Vycor and enhance the scattering of the thermal phonons in the silica network.