When heat is added to a crystal, its atoms vibrate, generating tiny quantum excitations known as “phonons”. Understanding the behavior and properties of these phonons is crucial for advancing materials science and technology. When a phonon causes atoms to move in circles (see attached image), the phonon has an associated handedness, or chirality. Essentially, this means that phonons wherein the atoms rotate counter-clockwise can have distinct properties from those wherein the atoms move clockwise. These “chiral phonons” have only been intensely studied recently, and much remains unknown about how they affect the bulk properties of crystals.
One of the most fascinating aspects of chiral phonons is that they can generate magnetic fields. If the atoms are electrically charged, their circular motions act like a tiny electric induction coil, producing magnetism. Since the charges of atoms are generally small in comparison to their mass, it was thought that this effect would be almost undetectably small. However, recent experiments have indicated much larger phonon-generated magnetic fields than initially thought, although the reason for this is still not perfectly understood.
Since chiral phonons are still a relatively unstudied phenomenon, our research into them is principally fundamental in nature. If the properties of chiral phonons, and how they affect, for example, the magnetism or heat transfer of a material, are better understood, this could lead to new applications which cannot currently be foreseen. Indeed, one of these applications was unexpectedly discovered during the project, as described below.
The first major goal of this research project was to use computational and theoretical techniques to study the magnetism of chiral phonons in a variety of materials, in order to determine if careful selection of the crystal structure could enhance the magnetic fields generated by phonons. A second goal was to study phonon magnetism in chiral materials (materials which have an inherent handedness). In these materials, phonons with non-zero momentum can naturally have a handedness and magnetism, whereas most previous studies had focused on phonons with near-zero momentum since these can be created using light. A third goal was to use the resulting theoretical predictions to detect chiral phonons experimentally.
