最も硬い物質「ダイヤモンド」が極低温で軟らかくなる～鍵を握るのは電子！〜

2025-06-18 北海道大学,新潟大学

ダイヤモンドの結晶構造と様々な原子空孔

＜関連情報＞

• https://www.hokudai.ac.jp/news/2025/06/post-1927.html
• https://www.hokudai.ac.jp/news/pdf/250618_pr.pdf
• https://journals.jps.jp/doi/10.7566/JPSJ.94.073602

人工ダイヤモンドの弾性ソフト化 Elastic Softening in Synthetic Diamonds

Tatsuya Yanagisawa, Ruo Hibino, Hiroyuki Hidaka, Hiroshi Amitsuka, Toshiyuki Tashima, Mitsuhiro Akatsu,  …
Journal of Physical Society of Japan  Published: June 11, 2025
DOI:https://doi.org/10.7566/JPSJ.94.073602

Abstract

This study reveals a previously unreported phenomenon: elastic softening in synthetic diamonds at temperatures below 1 K. We present ultrasonic measurements on single-crystalline, non-irradiated synthetic diamonds — specifically, type-IIa (colorless) and type-Ib (yellow) samples grown by high-pressure high-temperature (HPHT) synthesis, as well as type-IIa diamonds 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 an irreducible representation T₂ in the diamonds. However, the microscopic origin of this effect remains unclear. Drawing an analogy to behavior observed in silicon, we suggest the presence of an as-yet-unidentified, defect-derived quantum ground state with T₁ or T₂ symmetry, present at ppb-level concentrations in all three diamonds studied.