Magnetic resonance imaging (MRI) is best known as a medical imaging tool, but it can also be applied to visualize microscopic features of quantum materials like superfluids and superconductors. A high-resolution MRI technique has now allowed Yutaka Sasaki of Kyoto University, Japan, and colleagues to uncover a previously hidden structure of chiral domains in superfluid helium-3 (3He). The result suggests that MRI might be used to visualize vortices and other topological structures in a variety of quantum materials. When 3He is cooled below a few millikelvin, it becomes a superfluid—a fluid that can flow with zero viscosity. Previous experiments have led physicists to suspect that as 3He enters this phase, it breaks up into macroscopic domains. Each domain contains superfluid atoms with a common angular momentum, so that there is a handedness, or chirality associated with the domain. No one had yet seen these domains, but doing so would help physicists test their theoretical understanding of not only superfluidity but also related forms of superconductivity. Sasaki’s team investigated a thin film of superfluid 3He at 2 mK with an MRI technique that they previously developed to acquire images of ultracold quantum condensates with 10 μm spatial resolution. Analysis of the MRI data showed that the sample was divided into two or more millimeter-sized chiral domains separated by parallel walls, seen as dips in the MRI signal. The number and location of the domains changed each time the helium was cooled below the temperature at which it becomes a superfluid. This implies that the domains arise spontaneously as the superfluid forms, unlike domains in other materials, which are usually determined by internal impurities or external boundary conditions. This research is published in Physical Review Letters.
