Workshops Superconductivity in atomically thin materials and heterostructures November 20, 2017 to November 23, 2017 Location : CECAM-Lugano, Lugano, Switzerland

Since the discovery of graphene, two-dimensional (2D) materials have become the core of the modern material physics. Typically, the 2D limit is reached in layered materials with strong in-plane bonds and weak, van der Waals-like coupling between layers, which enables atomically-precise cleaving or their layer-by-layer growth. Tremendous effort has been put into advances to not only fabricate, but also functionalize 2D materials, in order to meet the technological grand challenges in sustainable energy solutions and device miniaturization. In that respect, it is now well established that 2D materials are very susceptible to chemical functionalization by dopant atoms, electric gating, strain, and/or interface interplay with the substrate, making them far more versatile than their 3D counterparts. Driven by a similar idea, a separate lane of research over the past two decades has been devoted to tailoring the properties of superconductors by confinement - when reduced in size to mesoscopic scale, comparable to characteristic length-scales of superconductivity, but also beyond - to true nanoscale. In the former case, the critical properties (critical magnetic field and current) of the given superconductor could be significantly enhanced, which is of both fundamental and technological interest. In the latter case, new quantum-mechanical effects were foreseen in the direction of strong confinement. It soon became obvious that ultrathin films harbor the best of both worlds, so that longitudinally the almost macroscopic superconductivity can be preserved, while being strongly confined and quantum-engineered in transverse direction. The problem was that at such small thicknesses, thermodynamic fluctuations, proximity effects, scattering, could all break Cooper-pairs and destroy superconductivity. It was thus a great surprise that superconductivity survived in few-monolayer crystalline films of Pb [1], followed by observation of conventional superconductivity and vortices in one monolayer Pb and In on Si(111) [2]. However, it was immediately shown that the latter is not pure surface-state superconductivity, since the Si substrate played an essential role. This was an indication that very little can be done theoretically unless first-principles studies based on density-functional theory (DFT) are undertaken. Further experimental advances and observation of 2D superconductivity in van der Waals materials, e.g. an isolated single layer of doped graphene [3], (gated) transition metal chalcogenides and carbides [4], and iron-selenide on SrTiO3 (with surprisingly high critical temperature Tc>100K [5]), created an enormous buzz in the community and raised a serious challenge for the theory to (a) explain the observed features, and (b) make further predictions towards functional alterations of those materials and possible ultra-small devices. Unfortunately, while modern experimental techniques are enabling increasingly multifold studies of superconductivity (in-situ synthesis, functionalization, transport and/or scanning-probe measurements), the community has witnessed an increasing gap between the ab initio calculations and those on mean-field levels, and even more to the desired device modelling at the ultrathin limit. This workshop aims to change this unsatisfying picture: to bring together some of the experts in the field, on both theoretical and experimental end, and discuss the needed multi-scale characterization of atomically thin superconductors, in order to identify the main challenges and further exploration avenues in this booming field of research, with envisaged applications in ultra-low power and ultra-light electronics, as well as novel functional materials.