Shell formation and two-dimensional nanofriction in three-dimensional ion Coulomb crystals

TL;DR

This study investigates 3D ion Coulomb crystals' shell formation and their 2D nanofriction properties, revealing complex behaviors like stick-slip motion, hysteresis, and coexisting shear regimes, advancing understanding of nanoscale friction mechanisms.

Contribution

It extends nanofriction studies from 1D and 2D ion crystals to 3D shell structures, providing new insights into rotational barriers, friction regimes, and system-dependent behaviors.

Findings

Shell formation depends on ion number and trap aspect ratio.

Changing ion number significantly alters the rotational barrier.

Multiple friction regimes, including stick-slip and smooth sliding, are observed.

Abstract

Self-organized three-dimensional (3D) ion Coulomb crystals in linear Paul traps naturally form concentric shells that provide a curved, atomically resolved interface for studying two-dimensional (2D) nanofriction. Building on earlier studies of one-dimensional nanofriction and orientational melting in 2D ion crystals, we extend friction studies from linear chains and planar rings to 3D shell structures. Using molecular-dynamics simulations, we map shell formation as a function of ion number N and trap aspect ratio and obtain a simple scaling relation that can aid ion-number estimation in experiments. We compute a Peierls-Nabarro-type potential for rotating the outer shell against a static inner core, using the rotation angle as a collective coordinate. Changing N by one can alter the effective rotational barrier by up to a factor of ~7, while changes by only a few ions can lead to variations up to a factor of ~60. Combining geometric commensurability analysis with energy decomposition, we show that the barrier is governed by a system-dependent interplay between inter-shell interaction, outer-shell response and trap confinement. Dynamical simulations with applied torques reveal pinned, stick-slip and smooth-sliding regimes with depinning thresholds that depend on ion number, inner-shell geometry and trap aspect ratio. Some configurations show hysteresis due to torque-induced metastable states. We further find that spatially varying coupling to the inner-core corrugation can create coexisting fast and slow domains within the rotating outer shell, realizing multidimensional friction in which intra-shell shear and inter-shell nanofriction act simultaneously. Our results establish self-organized ion Coulomb crystals as model systems for 2D nanofriction and suggest routes toward ion-based nanorotors, torque sensors and ultra-low-friction nanomechanical systems.