Crystal Dislocations as Atomic Scale Ratchets

Abstract

The symmetry of a system's response to external stimuli is a fundamental concept in physics and materials science. At the microscopic scale, breaking this symmetry to achieve a rectified response is exceptionally difficult to engineer and remains rare in nature. Conventional micromechanics models of crystalline solids assume a symmetric response to applied stress, where reversing the load simply inverts the direction of defect velocity without altering its magnitude. In this work, we report an atomic-scale, geometry-rooted mechanism that breaks this symmetry. Molecular dynamics simulations of face-centered cubic nickel reveal that dislocations containing atomic-scale jogs exhibit asymmetric mobility under opposite applied stresses, reversing the loading direction triggers significantly higher drag. This asymmetry arises from an unconventional coupling between an atomic displacement vector and the second-order tensorial eigenstrain of the jog motion mechanism. Because jogs are ubiquitous structures in plastic deformation, this discovery challenges classical descriptions of plastic deformation mechanisms, with direct implications to cyclic creep, and opens new pathways for defect engineering to enhance fatigue resistance.