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Low temperature phase of methane Lees, Ronald Milne

Abstract

The phase behaviour of methane at temperatures below 20°K is still not understood. There is recent evidence for a second phase transition at 8°K, in addition to the well known transition at 20.4°K. This second transition requires a long time to reach equilibrium. In our experiment, the nuclear magnetic resonance signal of the methane protons was monitored for a period of many hours at 1.2°K, in order to; determine whether a slow phase change occurred in the approach to equilibrium. This change would manifest itself in the width of the resonance line, which is sensitive to the local environment of the proton, and thus able to provide information about the crystal structure. No definite time variation of line width was observed. The line width increased by 15% from 4.2°K to 1.2°K. The second moment of the line also rose sharply as the temperature increased, indicating an increasing contribution to the spin-spin interaction from intra-molecular broadening. An approximate model yielded a correlation time for the intra-molecular term of the order of a microsecond. Significant differences between different methane samples indicate that sample preparation is important, and that the above results were for samples in some sort of metastable phase.

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