Elastic Low Temperature Anomalies of Solid Hydrogen Crystallites

The property of solid hydrogen (H₂, HD and D₂) not to wet an attractive substrate, can be used to prepare micron sized hydrogen crystals. We used surface acoustic waves (SAW) in the range from 100 MHz to 1 GHz to study the elastic properties of the crystallite-substrate system. The growth of crystallites at temperatures around 3 K can be monitored by the resonant coupling of elastic eigenmodes to the SAW. We observed drastic changes of the resonant coupling for all isotopes when the temperature was varied between 0.15 K and 2.4 K. Possible origins of this unexpecied effect are discussed including a temperature variation of the wetting behavior. For H₂ the impact of the ordering transition of ortho-H₂ was also investigated.