Subharmonic Shapiro steps of sliding colloidal monolayers in optical lattices

TL;DR

This paper theoretically predicts the occurrence of subharmonic Shapiro steps in sliding colloidal monolayers under periodic forcing, highlighting the role of soliton dynamics and offering experimentally accessible phenomena.

Contribution

It introduces the prediction of subharmonic Shapiro steps caused by localized solitonic slips in colloidal monolayers, extending understanding of dynamical mode locking in 2D systems.

Findings

Prediction of subharmonic Shapiro steps due to soliton dynamics

Observation of colloids jumping across patches during AC cycles

Potential for experimental verification in colloid monolayers

Abstract

We investigate theoretically the possibility to observe dynamical mode locking, in the form of Shapiro steps, when a time-periodic potential or force modulation is applied to a two-dimensional (2D) lattice of colloidal particles that are dragged by an external force over an optically generated periodic potential. Here we present realistic molecular dynamics simulations of a 2D experimental setup, where the colloid sliding is realized through the motion of soliton lines between locally commensurate patches or domains, and where the Shapiro steps are predicted and analyzed. Interestingly, the jump between one step and the next is seen to correspond to a fixed number of colloids jumping from one patch to the next, across the soliton line boundary, during each AC cycle. In addition to ordinary "integer" steps, coinciding here with the synchronous rigid advancement of the whole colloid monolayer, our main prediction is the existence of additional smaller "subharmonic" steps due to localized solitonic regions of incommensurate layers executing synchronized slips, while the majority of the colloids remains pinned to a potential minimum. The current availability and wide parameter tunability of colloid monolayers makes these predictions potentially easy to access in an experimentally rich 2D geometrical configuration.