Mysterious Softening Discovered in Synthetic Diamonds
© The Physical Society of Japan
This article is on
J. Phys. Soc. Jpn.
94,
073602
(2025)
.
Diamonds are the hardest known material, widely used in jewelry, cutting tools, and high-technology applications. They also have remarkable physical properties, such as a wide bandgap and excellent thermal conductivity, making them ideal for scientific and industrial applications. Recently, synthetic diamonds, which are artificially grown diamonds with very few impurities, have attracted attention in quantum science. A notable example is the nitrogen vacancy (NV) center, which is a tiny defect in a crystal where a nitrogen atom sits next to a missing carbon atom. NV centers can behave like quantum bits (qubits) or ultrasensitive sensors, even at room temperature.
In most previous studies, such defects have been deliberately introduced into diamonds using high-energy particles. However, very little is known about how ultrapure, undamaged diamonds behave at extremely low temperatures, especially in terms of their mechanical (elastic) properties.
Recently, a surprising phenomenon was discovered: synthetic diamonds become slightly softer when cooled below 1 K, just above absolute zero. This softening of approximately 0.01% in elastic stiffness is small but significant. It was observed in all three types of synthetic diamond that were studied using ultrasensitive ultrasound techniques, specifically the ultrasonic phase comparison method.
A similar softening was observed in ultrapure silicon crystals, which share the same diamond structure. In silicon, this effect is thought to arise from the electric quadrupole moments of atomic vacancies, which are extremely small imperfections where atoms are missing in the lattice. It is believed that the same quantum mechanical phenomenon could be responsible for the softening observed in diamond.
This discovery is even more intriguing because it cannot be explained by known impurities, such as nitrogen or boron, which have well-established quantum ground states. This suggests that new, previously unidentified types of lattice defect or quantum degrees of freedom may be involved.
Understanding this hidden behavior could open new possibilities for improving quantum devices, such as sensors or qubits based on diamond. As research progresses, synthetic diamonds are expected to play an even more central role in the development of next-generation quantum technologies.
(Written by Tatsuya Yanagisawa on behalf of all authors.)
J. Phys. Soc. Jpn.
94,
073602
(2025)
.
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