Elastic Softening in Synthetic Diamonds
Abstract
This study reveals a previously unreported phenomenon: elastic softening of synthetic diamonds at temperatures below 1 K. We present ultrasonic measurements on single-crystalline, non-irradiated synthetic diamonds--namely, type-IIa (colorless) and type-Ib (yellow) diamonds grown by high-pressure high-temperature (HPHT) synthesis, as well as type-IIa diamond grown by chemical vapor deposition (CVD). A pronounced, divergent decrease in the elastic stiffness constant C44 was observed in all samples down to 20 mK. We attribute this softening to electric quadrupolar degrees of freedom with irreducible representation T2 in diamond. The microscopic origin of this effect, however, remains unresolved. By analogy with similar behavior observed in silicon, we suggest the presence of an as-yet-unidentified, defect-derived quantum ground state with T1 or T2 symmetry at ppb-level concentrations in all three diamonds studied.