Pioneering the reproducible growth of superconducting ruthenate films to develop usable materials for the future of topological quantum computing

Abstract

Dr Masaki Uchida, at the University of Tokyo, is interested in uncovering how the topological quantum phenomenon of superconductivity works. By studying superconducting ruthenate films and quasiparticle Majorana fermions, Uchida hopes to unveil the characteristics of both superconductivity and topological quantum phenomena. This, in turn, can help unify the basic concepts of topological quantum phenomena across the field of physics and provide a new, versatile platform that can treat all topological phenomena within the same framework. Furthermore, it can aid in the development of topological quantum computers and change the way in which computers process information. Superconductivity is characterised as a topological quantum phenomenon. To understand such a complicated subject matter, it is best to break it down. Topology is a type of mathematics that studies shapes. Unlike geometry, topology is not concerned with the exact definitions of shapes such as triangles and rectangles but focuses on the properties of space that are preserved even when a shape undergoes continuous deformations such as stretching, crumpling and bending. This means that certain shapes that do not look the same, such as a doughnut and a coffee mug, actually have the same relationship to space due to their singular hole, and thus are considered ‘topologically’ equivalent. Quantum mechanics is a branch of physics that investigates quanta, the extremely small parts of the physical world such as atoms, electrons and light waves. With these ultra-small objects, the basic rules of physics no longer apply. Instead, atoms, electrons and light waves follow another set of physical laws called the laws of quantum mechanics. In certain quantum occurrences, or quantum phenomena, topology plays a fundamental role. Hence the term ‘topological quantum phenomena.’ By focusing on the topological nature of certain materials, Uchida hopes to elucidate the underlying physics of topological quantum phenomena. In his research, Uchida is studying superconducting strontium ruthenate (Sr2RuO4) films and quasiparticle Majorana fermions. Ruthenate is a perovskite, or a layered mineral that is made up mostly of calcium titanium oxide (CaTiO3) and has the same crystal structure as CaTiO3. For physicists, ruthenate is particularly fascinating due to its two-dimensional chiral p-wave superconductor. Chiral states are those that have mirror image versions of themselves that are not identical. Chirality is unique to very limited superconductive materials and therefore a special feature of ruthenate films. However, ruthenate films also have a very low superconducting critical temperature of 1.5 degrees Kelvin (-271.65 degrees Celsius). Because this temperature is difficult to reach, scientists struggle to properly grow these superconducting films. The study of the topological nature of ruthenate films has led to new insights in particles called Majorana fermions. Majorana fermions are quasiparticles, meaning they are not technically a ‘particle’ but are an emergent phenomenon that behaves like a particle. For example, in a group of electrons, there are spaces absent of electrons that have a positive charge, called ‘holes’. Although the hole is not a physical particle, it can still carry an electric charge and be passed from one atom to another like a normal electron. Similarly, Majorana fermions act with both matter and antimatter characteristics. They have zero charge and the chiral superconductors have been shown to host Majorana fermions. Therefore, the growth of ruthenate films is key in harnessing the potential of Majorana fermions.