Light reads hidden octupolar magnetism from the handedness of crystal vibrations

A dipole magnet has two poles, north and south ^[1]. In octupolar order, a pattern of particles in a crystal behaves as if it had eight ^[2]. The university's account calls detecting and controlling such states a significant challenge ^[3]. "We identified new signatures of a hidden type of magnetic state which cannot be detected using ordinary probes," said Arun Paramekanti, a University of Toronto physics professor and the study's senior author ^[6][5].

The method is indirect. It uses light to probe the atomic vibrations produced as electrons spin ^[4]. Those vibrations move through the crystal lattice as packets of energy called phonons, and the team looked for chiral phonons, ones that do not match their own mirror image ^[7]. "Just as a left hand cannot fit cleanly over a right hand even though they are mirror images, chiral phonons have a 'handedness' and exist in distinct, non-matching forms," said lead author Rory Sutcliffe, a Toronto physics doctoral candidate ^[8].

Handedness links the vibrations to the hidden order. "We found that the onset of octupolar order can impart a distinct handedness to certain phonon modes," said Swati Chaudhary, a project research associate at the University of Tokyo and a co-author ^[10]. "These vibrations behave differently from those found in conventional magnets, so we call them 'pseudo-chiral' phonons, and they provide a new way of identifying and studying hidden magnetic states," she said ^[11]. To read that handedness, the team aimed what the release describes as a special type of rotating light at magnetic materials ^[9].

The design has one property I like a great deal. The handedness arrives with the onset of the order ^[10], so a sample can be its own control. I'd expect the persuasive version of this result to be one crystal measured on both sides of the point where the order sets in, with the pseudo-chiral signal present only on the ordered side. According to the university, the team observed "a clear optical fingerprint of the otherwise hidden magnetic order" ^[9]. The release does not name the material studied or report how large that signal was, and another lab would need both before trying the method on its own compounds.

The applications are several steps further on. The university calls the work a critical first step toward harnessing multipolar magnetism for data storage and computing ^[14]. "Our research opens up the possibility for using higher-order magnets in several applications including controllable read-write memory elements found in everyday computers," Paramekanti said ^[13]. Kathleen Hart, a Toronto physics doctoral candidate and co-author, said the work "lays the foundation for how such octupolar magnetism might eventually be controlled through atomic vibrations within a material" ^[12].