Atomic Friction and Symmetry-Breaking Transitions in Ion Coulomb Systems

Abstract

Trapped ion Coulomb crystals are regularly used as analogue simulators for physical systems, for which the access to the dynamics of the individual particles is lacking or which are hard to simulate using classical computers. One area of interest are solid-state friction models with two atomically flat surfaces sliding against each other. Typically, the access to the dynamics of the individual particles is lacking in realistic interfaces, therefore, ion crystals have been proposed in order to test friction models. While a model system of an ion chain sliding over a rigid optical potential has been demonstrated, a model system that implements back action between the two sliding surfaces does not exist.

In this cumulative thesis, an atomic system with intrinsic back action and access to individual particles that allows the study of nanofriction is presented. The system consists of an ion Coulomb crystal in the two-dimensional zigzag phase, into which a topological defect is introduced. The defect leads to a mismatch between the ion chains, which allows for the observation of the pinning-to-sliding phase transition for a finite system. The transition shows symmetry breaking and the existence of a soft mode at zero temperature, which is a localized topological defect mode. The influence of the defect's position and type on the existence of the soft mode is studied. It is found that breaking the intrinsic symmetry of the topological defect in the sliding phase by external forces prevents the observation of the soft mode. In the presented experiments, mode frequencies are determined with resonant excitation of the collective motions of the ions via amplitude modulation of a Doppler cooling laser. A non-zero soft mode frequency at the transition is measured, which is attributed to the finite crystal temperature.

Furthermore, the linear-to-zigzag transition and the zigzag mode, i.e., the soft mode of this transition, under thermal noise are investigated. An increase in the mode frequency with temperature, as well as fast switching between the two possible ground states of the two-dimensional zigzag phase is found. An analytical model is derived that explains the observed temperature dependence of the low-frequency spectrum at the linear-to-zigzag transition. This analysis has important consequences for the cooling of a soft mode near a symmetry-breaking transition. In the future, this model could be adaptable to the pinning-to-sliding transition in order to further the understanding of the thermal effects of friction and heat transport.