A pulsed NMR study of He adsorbed on bare and 4He preplated MCM-41 using DC SUID detection

Abstract

This thesis describes two low field NMR and vapour pressure experiments of helium adsorbed in the pores of the mesoporous powder substrate MCM-41 at temperatures down to 1.5K. A sensitive broadband DC SQUID based spectrometer was developed to facilitate these measurements with a coupled energy sensitivity of 30 h at 1.5K. NMR relaxation times T1 and T∗ 2 were measured as a function of temperature for frequencies from 80 to 240 kHz. MCM-41 is an amorphous silica formed of a hexagonal array of straight regular pores. For our sample the pore diameter is 2.3 nm and length is ∼ 300 nm, equal to the grain size. The substrate was characterised by 4He vapour pressure isotherms performed using an in-situ pressure gauge with a resolution of 1 μbar. Layer completion and full pores were determined from the isothermal compressibility of the adsorbed film. In the first NMR experiment 3He was adsorbed onto the bare pore walls of MCM-41 from coverages approaching monolayer completion up to full pores. The adsorbed helium was found to form a 2D film which exhibited significant motional narrowing of the NMR line at low temperatures due to quantum tunnelling. In the second experiment the substrate was initially preplated with just over a monolayer of 4He before a low 3He coverage of 0.01 monolayer was added. Subsequently the preplating coverage was varied whilst the 3He coverage was held constant at 0.02 monolayer. Under these stringent conditions a transition to a quasi-1D state is expected to occur at temperatures below 100mK where Luttinger liquid behaviour is predicted. Nuclear magnetic resonance is a powerful tool for probing the necessary dynamic and magnetic properties of systems to determine the Luttinger liquid parameters. The 3He in this helium mixtures experiment was also found to behave as a 2D film, where at these low coverages the relaxation times were independent of the 3He density. A correlation was found between the relaxation times and the isothermal compressibility of the film. The results of the NMR experiments have been used to make proposals on the design and fabrication of a new experiment with a coolable cell capable of reaching temperatures low enough to observe a transition to 1D. Cooling this substrate to temperatures far below 1K is not trivial and low resonant frequencies are required to avoid excessively heating the sample during NMR pulsing.