Thermodynamic Properties of Helium in Porous Media

Open Access
Author:
Cheng, Zhigang
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 24, 2013
Committee Members:
  • Moses Hung Wai Chan, Dissertation Advisor
  • Jainendra Jain, Committee Member
  • Thomas E Mallouk, Committee Member
  • Julian Decatur Maynard Jr., Committee Member
  • Jorge Osvaldo Sofo, Committee Member
Keywords:
  • helium
  • heat capacity
  • thermal conductivity
  • porous media
Abstract:
The first credible experimental hint of non-classical rotational inertia (NCRI) or supersolidity was reported in 2004 in a torsional oscillator experiment of solid 4He confined in Vycor. Since then numerous studies on the possible novel state have been carried out. While it was shown very recently that the observed drop in the resonant period of the torsional oscillators housing solid helium which was interpreted as a signature of NCRI is more likely a mechanical phenomenon other than a real phase transition, a number of interesting properties of solid 4He have been observed during the last decade in many laboratories. These newly discovered results include a shear modulus anomaly, dc flow through solid helium and a heat capacity peak. Most of these studies focus on bulk crystalline solid 4He. This dissertation focuses on the study of thermodynamic properties of helium in porous media. We have measured the heat capacity of solid 4He grown in aerogel and Vycor. For solid 4He in aerogel, the dependences of heat capacity on pressure and 3He concentration have been systematically studied. We found evidence that 3He atoms tend to reside in the vicinity of silica strands as temperature is decreased forming a 3He rich region. We have also carried out measurements of thermal conductivity of solid 4He embedded in Vycor. The thermal conductivity of Vycor is not significantly changed with the infusion of solid helium. Interestingly, the infusion of liquid 4He in Vycor pores results in a three-fold reduction in thermal conductivity below 0.5 K. The introduction of superfluid 4He films and liquid 3He into the Vycor pores also results in the reduction of thermal conductivity. We propose a model suggesting the origin of the reduction is the presence of hydrodynamic slow sound modes in liquid 4He, as well as in superfluid 4He films and liquid 3He. The slow sound modes facilitate the quantum tunneling of two-level systems (TLS) in silica and dramatically increase the TLS-phonon scattering. The more modest reduction in solid helium-Vycor composite is caused by the presence of phonon excitations in solid helium which also facilitate TLS tunneling in silica.