2022-07-18 アルゴンヌ国立研究所(ANL)
何十年もの間、科学者は古典力学の法則を用いて、アモルファスカーボンの原子の動きをモデル化してきた。周期律表の重い原子については、この古典力学の方程式は、材料の特性の多くを正確にとらえるのに適した近似式である。しかし、多くの種類の炭素、特にアモルファス炭素については、原子の動きを記述するためにこれらの古典的方程式を用いても不十分であることが、研究チームによって明らかにされた。
炭素原子の電子と原子核の動きを記述するために、今度は量子力学的な原理を用いてアモルファスカーボンの電子特性のシミュレーションを新たに行った。その結果、アモルファスカーボンの特性を正確に予測するためには、原子核に古典力学を用いるのではなく、電子と原子核の両方に量子力学を用いることが重要であることが判明した。
＜関連情報＞
- https://pme.uchicago.edu/news/new-look-disordered-carbon
- https://www.pnas.org/doi/full/10.1073/pnas.2203083119
アモルファスカーボンの電子物性に及ぼす核量子効果の影響 Influence of nuclear quantum effects on the electronic properties of amorphous carbon
Arpan Kundu, Yunxiang Song, and Giulia Galli
Proceedings of the National Academy of Sciences
Published:July 15, 2022
DOI:https://doi.org/10.1073/pnas.2203083119
Significance
In crystalline solids, atoms are arranged in periodic patterns on regular lattices. Amorphous solids, instead, lack long-range order; namely, a regular array of atoms beyond first or second nearest neighbors is absent. The lack of periodicity influences many properties of amorphous materials, including the coupling of electronic and nuclear motion. Here we study amorphous carbon, a system composed of a relatively light atom. We show that to understand its electronic properties, a quantum mechanical treatment of electron–nuclear coupling is essential, and we illustrate a simulation framework based on first principles to do so. We also discuss the role of specific defect states in the disordered network in determining the physical properties of amorphous carbon.
Abstract
We carry out quantum simulations to study the physical properties of diamond-like amorphous carbon by coupling first-principles molecular dynamics with a quantum thermostat, and we analyze multiple samples representative of different defective sites present in the disordered network. We show that quantum vibronic coupling is critical in determining the electronic properties of the system, in particular its electronic and mobility gaps, while it has a moderate influence on the structural properties. We find that despite localized electronic states near the Fermi level, the quantum nature of the nuclear motion leads to a renormalization of the electronic gap surprisingly similar to that found in crystalline diamond. We also discuss the notable influence of nuclear quantum effects on band-like and variable-hopping mechanisms contributing to electrical conduction. Our calculations indicate that methods often used to evaluate electron–phonon coupling in ordered solids are inaccurate to study the electronic and transport properties of amorphous semiconductors composed of light atoms.
