Phase transitions in two-dimensional ultrathin magnetic films

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Condensed matter physics makes a direct impact on society through the discovery and understanding of new materials with novel properties that might be suitable for applications. In the last couple of decades, fundamental research into two dimensional (2D) materials that are only one or two atoms thick, has been a very productive source of materials with the potential to change society. Some examples are graphene (one atomic layer of graphite), high temperature superconductors, the use of atomically thin magnetic films for data storage, and the 2D surface properties of topological insulators that are a candidate for use in quantum computers. These are real 2D systems "living" in the 3D world. Understanding the properties of these 2D systems involves understanding how these properties change at phase transitions (the most famous phase transition is the ice to liquid transition of water). Very powerful theorems in theoretical physics show that there are a small number of universal types of phase transition, and that the same quantitative theory describes the phase transitions and properties in the huge variety of different materials that fall within each of these types (or classes). An equally powerful theorem shows that no classes exists in uniform 2D systems: it is because of non-uniformities that ordered systems with reliable materials properties exist in 2D at all. This is one way of understanding why 2D materials are such a productive source of novel materials - small non-uniformities can have a profound effect when they "tip the balance" in unforeseen and unusual ways that allow a phase transition to a state with stable properties that are themselves unusual. Although theoretical and computational studies of 2D phase transitions are well-advanced, quantitative experimental studies of the same transitions in real 2D materials is not. Many detailed predictions of the theories are well-known but have not yet been shown to apply to real materials. I propose experimental studies of ultrathin 2D magnetic films to make these quantitative comparisons. Films of magnetic materials, such as iron, a few atoms thick can be grown on the surface of a non-magnetic substrate crystal, so that the system is magnetically 2D. Even though I will study only magnetic films, almost all of the different universal classes of phase transition in 2D can be studied by carefully choosing the combination of film and substrate - therefore the results are much more widely applicable than it might at first seem. Specifically, I propose to study three classes of 2D transitions: one that occurs when the isolated parts of a film that partially covers a substrate connect together to form a continuous film; one where long-range interactions disrupt the ordered phase and lead to pattern formation; and the Kosterlitz-Thouless transition, the subject for which the 2016 Nobel Prize in physics was awarded.