Extraordinary behaviour of extraordinary materials
Within the framework of the IMAX (Improvement of materials with X-rays) project, scientists used scanning micro X-ray diffraction to investigate the subtle changes in the structure of a superconducting cuprate material at high temperature. Specifically, they were able to probe the electron distribution in great detail. Instead of uniform striping that was expected, the scientists observed a mishmash of particle clumps of various shapes and sizes. Further research revealed that the density of clumps was related to the amount of doping needed to create this particular superconductor. Complicating things, the distribution of the clumps' sizes followed a power law, suggesting that they form in a way similar to fractals. This fractal-like self-organisation had striking similarities with the spatial inhomogeneity of another superconducting cuprate material at a lower temperature. These findings suggest that the path to understanding superconductivity may be more complex than initially thought. They also offered possibilities that had not yet been explored. Superconductors are systems where quantum mechanics can be reconciled with the laws of classical physics. Specifically, magnetic fields penetrate superconducting materials in the form of tiny vortices that display both classical and quantum properties. This led IMAX scientists to study them in order to shed light on one of the most enigmatic phenomena in modern condensed matter physics: the Mott transition. This complex phenomenon is controlled by the interactions of many quantum particles, and it is not clear if it is a classical or quantum phenomenon. Moreover, a Mott transition, in which a phase transition from an insulating to a metallic state is induced by an electrical current flowing through the material, has never been directly observed. Against this backdrop, the scientists built a system containing 90 000 superconducting niobium nano-sized islands on top of a gold film. In this configuration, the vortices settled in energy dimples and made the material act as a Mott insulator. When they applied a large enough electric current, a dynamic Mott transition was directly observed as the system turned into a conducting metal. In other words, a phase transition was induced from a state of locked vortices to a state of itinerant vortices as the current pushed the material out of equilibrium. This is a classical system that is easy to experiment with and can be used to enhance our understanding of out-of-equilibrium physics. Remarkably, such sharp phase transitions are also observed in complex oxide films upon increasing their thickness. The IMAX team grew thin films of a perovskite oxide on a non-magnetic material, using a technique called pulsed laser deposition. By adding a sixth layer of the specific perovskite oxide, the material changed from antiferromagnetic to ferromagnetic. Such an abrupt transition has never been seen before. Importantly, scientists expect that this exceptional feature of the perovskite oxide is limited to changes in the film thickness, but possibly also to other manipulations, such as applying electric fields. Further research is already being carried out, aiming to find use of the effect in information and sensing technology.
Keywords
Superconductor, IMAX, X-ray, cuprate material, Mott transition, perovskite oxide
