CPM Seminar

Effect of Helium on Porous Vycor Glass: Anomalous Thermal Conductivity Reduction

Zhigang Cheng

Department of Physics
Pennsylvania State University

The possible supersolid state of solid ⁴He has received enormous focus in the past decade. Various methods have been utilized to detect the normal-solid to supersolid transition, including torsional oscillator experiments, heat capacity measurements, etc. On the other hand, thermal conductivity of normal liquid ⁴He has a diverging anomaly as temperature approaches to the superfluid transition temperature from above. This provides an alternative method to study the possible supersolid state: if there is a genuine normal-solid to supersolid transition, there should be some thermal conductivity anomaly at the transition temperature. In this talk, I will present our recent results of thermal conductivity measurements of solid helium confined in porous Vycor glass between 0.06 and 0.5 K. The measured thermal conductivity does not show any anomaly to indicate the supersolid transition. However, we observed a counter-intuitive phenomenon that the infusion of liquid ⁴He in the Vycor pores brings about a threefold reduction of the thermal conductivity as compared with empty Vycor. By comparing with the results when Vycor is infused with superfluid ⁴He films, liquid ³He and solid helium, we found that the dramatic reduction is caused by the presence of hydrodynamic slow sound modes in liquid ⁴He that greatly facilitate the quantum tunneling of the two-level systems (TLS) in the silica structure of Vycor. This leads to the enhancement of the thermal phonon scattering in Vycor and therefore reduces the thermal conductivity.