Elastic Softening in Synthetic Diamonds

This study reveals a novel phenomenon demonstrating the elastic softening of synthetic diamonds when cooled to very low temperatures below 1 K. We present the results of ultrasonic measurements of single-crystalline synthetic diamonds without irradiation, namely type-IIa (colorless) and Ib (yellow) diamonds grown by high-pressure-high-temperature synthesis as well as type-IIa diamond grown by chemical vapor deposition. We observed a divergent decrease of the elastic stiffness constant $C_{44}$ in all samples at low temperatures down to 20 mK. We argue for the existence of electric quadrupolar degrees of freedom with irreducible representation $T_2$ in diamond as the origin of this elastic softening. The possible ground state preserves local $T_d$ symmetry presumably down to absolute zero. Of the ground states of atomic vacancies in diamond proposed thus far, the only relevant one is the positively charged single atomic vacancy V$^+$, which has not been observed in previous optical measurements in diamonds.
Comment: 8 Pages, 3 figures with supplemental materials