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.
Highlights
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,[1,2] vibrational spectra of water[3,4], and ice,[5,6] and mechanical properties of low temperature helium.[7–9]
Here we re-examine the effect of zero-point vibrations on dislocation motion in α-iron using Ring-Polymer Molecular Dynamics[22,23] simulations (RPMD)
To take full advantage of RPMD accuracy, in the following we make no assumptions on the character of dislocation motion and measure the effect of zero-point vibrations on the Peierls stress in the most direct manner possible, akin to the very method by which the Peierls stress is estimated in experiment
Summary
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,[1,2] vibrational spectra of water[3,4], and ice,[5,6] and mechanical properties of low temperature helium.[7–9] Recently, quantum motion of atoms was proposed to explain a long-standing discrepancy between theoretically computed and experimentally measured lowtemperature resistance (Peierls stress) to dislocation motion in iron and possibly other metals with high atomic masses.[10–12]. In ref.[10] the effect of zeropoint vibrations was accounted for by computing an approximate quantum correction to the rate of dislocation motion predicted within the classical transition state theory (TST). Here we re-examine the effect of zero-point vibrations on dislocation motion in α-iron using Ring-Polymer Molecular Dynamics[22,23] simulations (RPMD). To take full advantage of RPMD accuracy, in the following we make no assumptions on the character of dislocation motion and measure the effect of zero-point vibrations on the Peierls stress in the most direct manner possible, akin to the very method by which the Peierls stress is estimated in experiment. Using the same interatomic model of α-iron as in ref.[10], here we compute the resistance to dislocation motion twice—first using fully classical Molecular Dynamics (MD) and using RPMD simulations—and compare the results in the limit of zero temperature to assess the differences. To persuade the dislocation to move along the horizontal ð112Þ plane, the blue atoms in the top and in the bottom layers were moved along the bx axis at constant and opposite velocities
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