NMR and Third Sound Studies of Dilute HELIUM-3 - HELIUM-4 Mixture Films.
Abstract
The results of a series of simultaneous third sound and NMR measurements on dilute ^3 He-^4He mixture films adsorbed on Nuclepore at dilution refrigerator temperatures are reported. The third sound speed and dissipation measurements were carried out using a resonator constructed from a single sheet of Nuclepore. The nuclear magnetic susceptibility of the ^3He, and the NMR relaxation times, T_2 and T_1 were measured using pulsed NMR techniques in a 2 Tesla field. Mixture films in the coverage range 0 <=q d_3 <=q 0.44 monolayers and 2.8 <=q d _4 <=q 9.5 layers were investigated where d_3 is the ^3 He coverge in terms of bulk ^3He liquid layers and d_4 is the thickness of the ^4He film in bulk ^4He liquid layers. We present measurements of the fractional change in the square of the third sound speed, delta C_sp{3}{2}, following the addition of small amounts of ^3 He to a ^4He film. For d _4 = 2.8 layers and 0 <=q d_3 <=q 0.088 layers deltaC_sp{3} {2} is a linear function of d _3 consistent with the Lekner formulation of the mixture film problem. For d_3 = 0.088 layers and 2.8 < d_4 < 5.3 layers deltaC _sp{3}{2}/d_3 has an unexpected minimum as a function of d _4. Comparisons to deltaC _sp{3}{2}/d _3 on glass are made. Measurements of the nuclear magnetic susceptibility, chi, of the ^3He in these mixture films as a function of d_3 are presented. At low ^3He coverages, d_3 < 0.1 layers with a 2.8 layer thick ^4He film, the ^3He behaves approximately as a 2D ideal Fermi gas. For larger submonolayer ^3 He coverages, 0.088 <=q d _3 <=q.44 layers, with a 9.5 layer thick ^4He film, the low temperature susceptibility is enhanced, increasing with increasing amounts of ^3He, contrary to the ideal 2D Fermi gas model. No evidence of a 2D gas to liquid -like phase transition in the ^3He as suggested by an interpretation of heat capacity measurements was observed. For d_3 = 0.088 layers, the qualitative temperature dependence of chi varies little with d_4 through the range 2.8 < d_4 < 9.5 layers but its magnitude decreases by about 10-15% as d_4 is increased through this range. For all of the coverages investigated T _2 << T_1. These relaxation times depend only weakly on temperature, d _4 and d_3.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1988
- Bibcode:
- 1988PhDT........15V
- Keywords:
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- Physics: Condensed Matter