Threefold Reduction in Thermal Conductivity of Vycor Glass Due to Adsorption of Liquid 4He

TL;DR

This study demonstrates that filling Vycor glass pores with liquid helium significantly reduces its thermal conductivity due to hydrodynamic sound modes enhancing phonon scattering and quantum tunneling.

Contribution

It reveals the dramatic impact of liquid helium in pores on thermal conductivity, highlighting the role of hydrodynamic sound modes in porous silica.

Findings

Filling with liquid 4He reduces thermal conductivity threefold.

Filling with liquid 3He reduces thermal conductivity twofold.

Solid helium does not cause this reduction.

Abstract

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 respectively two and 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 (TLS) in Vycor and enhance the scattering of the thermal phonons in the silica network.