A new probe of multipole physics in Pr-based compounds

Electrons are like humans---the behavior of a single electron on its own can be very different from its behavior when surrounded by other electrons. This concept is known as emergent phenomena. Emergent behavior leads to new properties in quantum materials that have applications in different sectors, ranging from medical and information sciences to electronic and automotive industries. Before the emergent properties of quantum materials can be used in everyday life, a deep understanding of their behavior and the proper tools for detecting it are required. Multipolar ordering is an example of such phenomena that requires special materials and new tools for detecting and understanding it. In order to visualize multipoles, we begin with the monopole. A monopole can be imagined as a single small sphere that does not change with angle or by rotating it in space. Earths gravitational field and the electric field around a positive or negative charge are simple examples of monopolar fields. Two opposite charges separated by a small distance or the north and south pole of a bar magnet are examples of dipoles---the second type of multipole in a long list of possibilities. If a dipole is rotated by 180 degrees, its sign changes once. Higher order multipoles, such as quadrupoles and octupoles, vary more quickly with angle, requiring a technique to study them that is sensitive to changes in rotation angle. We have well known techniques to detect monopoles and dipoles. For example, sand may be brought near a bar magnet in order to align dipoles within the sand and confirm the presence of iron particles. On the contrary, the rapid angular variation of higher order multipoles smears out such simple alignment possibilities, making similar tests impossible. Furthermore, higher order multipoles never appear on their own, always being overshadowed by a strong dipolar contribution that makes it difficult to study the effects of the multipolar contribution without complication. We propose to study praseodymium-based compounds because they lack the usual strong dipolar contribution, providing a perfect opportunity to study higher order multipoles in isolation. Using a technique that is sensitive to abrupt angular changes in magnetic properties, we will address fundamental questions related to the role that higher order multipoles play in forming new material properties, such as superconductivity and quantum spin liquid states.