Quantum effects on dislocation motion from Ring-Polymer Molecular Dynamics

TL;DR

This study uses Ring-Polymer Molecular Dynamics to simulate quantum effects on dislocation motion in iron, revealing only a modest reduction in Peierls stress and showing quantum effects do not fully explain experimental discrepancies.

Contribution

First direct quantum simulations of screw dislocation motion in iron using RPMD, clarifying quantum effects on Peierls stress and resolving previous theoretical-experimental gaps.

Findings

Quantum RPMD predicts ~13% reduction in Peierls stress.

Zero-point energy accounts for ~50% of stress reduction.

Quantum dispersion offsets some quantum effects at low temperatures.

Abstract

Quantum motion of atoms known as zero-point vibrations is recognized to be important at low temperatures in condensed matter systems comprised of light atoms or ions, affecting such properties and behaviors as proton-transfer reactions, vibrational spectra of water and ice, and mechanical properties of low temperature helium. Recently, quantum motion of atoms was proposed to explain a long-standing discrepancy between theoretically computed and experimentally measured low-temperature resistance (Peierls stress) to dislocation motion in iron and possibly other metals with high atomic masses. Here we report the first direct simulations of quantum motion of screw dislocations in iron within the exact formalism of Ring-Polymer Molecular Dynamics (RPMD) that rigorously accounts for quantum effects on the statistics of condensed-phase systems. Our quantum RPMD simulations predict only a modest ($\approx\!13\%$) reduction in the Peierls stress in iron compared to its fully classical prediction. Our simulations confirm that reduction in the Peierls stress solely due to the zero-point energy is close to $50\%$ predicted earlier, but its effect is substantially offset by an increase in the effective atom size with decreasing temperature, an effect known as quantum dispersion. Thus, quantum motion of atoms does not resolve the notorious discrepancy between theoretical and experimental values of the Peierls stress in iron.