npj Computational Materials

Quantum effects on dislocation motion from ring-polymer molecular dynamics (量子效应对环-聚合物分子动力学中位错运动的影响)
Rodrigo Freitas, Mark Asta & Vasily V. Bulatov
npj Computational Materials 4:55 (2018)
doi:s41524-018-0112-9
Published online:22 October 2018
Abstract| Full Text | PDF OPEN

摘要:原子的量子运动也称为零点振动，最近被提出来用以解释铁和其它高原子质量金属中长期存在的理论计算和实验测量的低温塑性强度之间的误差。这一理论挑战了传统观念，即在重原子组成的固体中，原子的量子运动通常来说并不重要。本研究通过环-聚合物分子动力学（Ring-Polymer Molecular Dynamics,RPMD）对位错运动的量子效应作了量子动力学模拟。为了将量子原子模拟扩展到普适缺陷的相干长度和时间尺度上，我们在开源代码LAMMPS中实现了RPMD，从而使RPMD方法可广泛应用于该领域。我们使用RPMD/LAMMPS方法直接计算了位错迁移率及其对α-Fe屈服强度的影响。模拟结果表明，在温度低于50 K时，存在较为明显的量子效应，但也仅仅降低了大约13%的Peierls势垒。而相比于基于谐波过渡态理论的模拟结果，两套方法在Peierls相变势垒的降低程度上给出了截然不同的结果。本研究证实，零点振动为原子运动提供了充足的额外扰动，但其效果随着温度的升高而降低，即零点振动对位错迁移率的增强作用在很大程度上被升温后增加的有效原子尺寸所抵消，出现量子弥散效应，而这在以前的工作中一直都被忽略了。　　

Abstract:Quantum motion of atoms known as zero-point vibration was recently proposed to explain a long-standing discrepancy between theoretically computed and experimentally measured low-temperature plastic strength of iron and possibly other metals with high atomic masses. This finding challenges the traditional notion that quantum motion of atoms is relatively unimportant in solids comprised of heavy atoms.Here we report quantum dynamic simulations of quantum effects on dislocation motion within the exact formalism of Ring-Polymer Molecular Dynamics (RPMD).To extend the reach of quantum atomistic simulations to length and time scales relevant for extended defects in materials, we implemented RPMD in the open-source code LAMMPS thus making the RPMD method widely available to the community.We use our RPMD/LAMMPS approach for direct calculations of dislocation mobility and its effects on the yield strength of α-iron.Our simulation results establish that quantum effects are noticeable at temperatures below 50？K but account for only a modest (≈13% at T？=？0？K) overall reduction in the Peierls barrier, at variance with the factor of two reduction predicted earlier based on the more approximate framework of harmonic transition state theory.Our results confirm that zero-point vibrations provide ample additional agitation for atomic motion that increases with decreasing temperature, however its enhancing effect on dislocation mobility is largely offset by an increase in the effective atom size, an effect known as quantum dispersion that has not been accounted for in the previous calculations.

Editorial Summary

Dislocations: ring-polymer molecular dynamics incorporates quantum effects（位错：环-聚合物分子动力学包含量子效应）

环-聚合物大分子动力学可以准确地模拟量子效应对位错运动的影响。来自美国加州大学伯克利分校和劳伦斯利弗莫尔国家实验室的Rodrigo Freitas领导的研究小组，研究了原子的量子运动对Peirels应力的影响，即材料的耐低温位错运动。与实验相比，经典分子动力学对Peirels应力有所高估，但通过并行计算对150000个原子进行环-聚合物分子动力学模拟与实验值差异较小。这表明，早期的量子修正高估了零点振动对体系的扰动，同时低估了近邻原子的约束效应。采用有效的环-聚合物分子动力学模拟可以帮助我们研究材料中普适的缺陷，同时可以准确地计算原子动力学的量子修正。

Large ring-polymer molecular dynamics can accurately simulate quantum effects on dislocation motion. A team led by Rodrigo Freitas at the University of California, Berkeley and Lawrence Livermore National Laboratory, U.S.A., investigated the effect of atomic quantum motion on the Peirels stress, i.e., the low-temperature resistance to dislocation motion. While classical molecular dynamics yielded the expected overestimation of the Peirels stress compared to experiments, ring-polymer molecular dynamics on 150,000 atoms using parallel computing showed a smaller discrepancy between simulations and experiments.This indicated that earlier quantum corrections overestimated the agitation effect of zero-point-vibrations and downplayed the effect of atomic neighbor confinement.Implementing efficient ring-polymer molecular dynamics can help us study extended defects in materials, while accurately accounting for quantum corrections to atom dynamics.