Thesis Type:

PhD thesis

Abstract:

In recent years, cold-atom experiments have moved towards atomic systems with increasingly stronger interactions. One goal is to emulate condensed-matter phenomena in an ultimately controlled system by studying the motion of atoms in optical lattices. Trapped ions are the epitome of a strongly-interacting cold-atom system, but until now have been limited to simulating spin systems. In this thesis work, a toolbox is developed for combining trapped ions with optical lattices and for studying problems of atomic crystals in periodic potentials. One such problem of tremendous technological and economic importance is friction - a ubiquitous phenomenon that is poorly understood even at the atomic level (nanofriction), where stick-slip processes are known to be the dominant source of dissipation and wear. Friction is studied in this thesis work with unprecedented spatial resolution and control at the individual-atom level in the synthetic frictional interface between crystals of trapped ions (moving object) and an optical lattice (rigid corrugated substrate). These experiments address, at the atomic scale, four quintessential questions about friction: the dependence of friction on the load (corrugation depth), on material properties (object-substrate lattice mismatch), on the contact area (number of atoms at an atomically smooth contact) and on velocity and temperature. In particular, we observe the elusive regime of superlubricity - the vanishing of stick-slip friction - for ion crystals mismatched to the lattice. With increasing load, we observe superlubricity to break and stick-slip friction to reappear as a result of a long-theorized sliding-topinned structural transition known as the Aubry transition. Although these effects were initially predicted to occur in the infinite-atom limit, we find them to arise already at the level of two or three atoms in our system. The presented results could potentially lead to ways of engineering friction in nanomaterials or even at the macroscopic scale, and the system can further be used to study quantum many-body physics of solids in periodic potentials, potentially relevant to friction and surface physics at the nanoscale and at cold surfaces.

PhD thesis