The capacity and NMR investigation of two dimensional helium mixtures adsorbed on grafoil
The subject of this thesis is the investigation of two dimensional helium mixtures
adsorbed on Grafoil. Heat capacity and Nuclear Magnetic Resonance (NMR)
measurements were used in the temperature range of 1 mK to 90 mK. This work
represents the first systematic study of monolayer fluid helium mixtures adsorbed
in Grafoil at low mK temperatures, extends previous experimental observations on
solid helium mixtures and explores the ground state of 3He on top of three layers of
The experimental setup consists of a dilution refrigerator with a nuclear de magnetisation
stage with the possibility to achieve ultra low temperatures of 0.3 mK.
An NMR setup was constructed to perform continuous wave NMR measurements
and was satisfactorily used to obtain data for this thesis. A series of automated,
computer controlled data acquisition programmes was also developed to measure
and analyse the signals.
The study involved the investigation of three different types of mixtures on the
second and third layer of Grafoil. The study of fluid mixtures lead for the first time,
to evidence of finite solubility of 2D 3He in fluid 4He. It was also possible to explore,
within the data collected, the phenomenon of phase separation within a monolayer
mixture. Furthermore, the possibility of 3He condensation and stratification effects
in this system were also argued as an alternative scenario.
A study of solid helium mixtures supported previous observations of delocalised
3He tunnelling excitations and a model was constructed to interpret the data. Additional
data in this mixture at various 3He concentrations provided evidence of finite
solubility of 3He in 2D solid 4He.
Finally, additional measurements were undertaken on 3He on top of three layers
of 4He. A number of new and interesting features were observed. The new data
suggest the possibility of a phase transition below 10 mK