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Spinning Sound

Acoustic atomic vibrations in solid materials may result in spin, one of the most fundamental properties of quantum physics

In solids, the vertical or horizontal oscillations of an atom (the yellow ball) spread through each other as if they were connected by springs. It has now been shown that these vibrations can rotate around the atom's axis, producing spin

Léo Ramos Chaves

Reverberations between atoms within a solid material produce excitations that are dispersed by the material. In certain quantum mechanics situations, these vibrations create waves that act like a flow of particles, called phonons. Responsible for propagating heat and sound through the material, these oscillations can occur in two directions: vertically and horizontally. The atoms act as if they are connected by springs, with a disturbance in one of them spreading through the others. Now, a team of physicists from the Federal University of Pernambuco (UFPE) has experimentally demonstrated that under specific conditions, these vibrations can propagate in another way: as well as moving up or down and side to side, they can revolve around the atom’s own axis.

In quantum mechanics, the spinning motion of the waves inside the solid material corresponds to the spin or angular momentum exhibited by elementary particles, such as electrons, and by atomic nuclei in the presence of magnetic fields. In short, the researchers found that a phonon—a packet of energy produced by vibrational waves—can have spin. The result of the study was published in the scientific journal Nature Physics in April 2018.

The experiments that led to the discovery were divided into two stages. First, the physicists used a field of microwaves to generate a type of atomic excitation called a magnon, which is known to have spin, and in which the electrons of an atom vibrate in response to the magnetic properties of a material. They then used magnetic fields to convert the magnons into phonons. This process of transforming magnons into phonons, called coupling, is a known scientific technique. “The main objective of our experiment was to convert one form of excitation into another, transforming a magnon into a phonon,” explains Sergio Machado Rezende, coordinator of the group of physicists from UFPE who conducted the study.

The surprise came at the end of the experiment, when the researchers noticed an unanticipated result. “We saw that as well as turning into phonons, the magnons had unexpectedly transferred their spin,” says Jose Holanda, a PhD student supervised by Rezende and lead author of the paper based on the study. In this type of conversion, the magnons’ spin is not usually maintained in the phonons. Due to the unprecedented nature of the findings, Rezende and his colleagues constructed an optical scattering system to confirm whether the spin had really been preserved. Light scattered in a material provides precise information on the presence or absence of angular momentum in phonons—and the system confirmed the spin in the phonons.

The type of material used seems to have been a decisive factor in the unexpected outcome of the UFPE experiments. Instead of using large solid materials such as rods or cylinders (as is the norm in this type of study), the researchers used a thin film of yttrium iron garnet (Y3Fe5O12), which is just a few atoms thick and reduces magnetic loss. “Within this film, excitations travel distances to the order of centimeters,” says physicist Antônio Azevedo, also from UFPE, who manufactured the material used in the experiment. The use of this material was evidently a determining factor in the spin of the magnons transferring to the phonons. “This effect does not occur in other materials,” explains physicist Matthias Benjamin Jungfleisch, from Argonne National Laboratory, USA, who did not participate in the article but wrote a commentary on it in the same issue of Nature Physics.

The finding that acoustic atomic vibrations in solid materials can present one of the most important properties of quantum physics could lead to further research into phonon spin as a way of encoding information. This could eventually form a new branch of spintronics, using the spin of electrons to transmit signals and store data. In conventional electronics, as the name implies, signals are conducted through the charge of the electron. One of the advantages of using phonons in spintronics is that magnetic materials, which are more complicated to produce, would only be needed in the initial phase of the process, to convert magnons into phonons. “Once produced, the phonons could carry the spin to other nonmagnetic materials,” suggests Rezende.

Scientific article
HOLANDA, J. et al. Detecting the phonon spin in magnon-phonon conversion experiments. Nature Physics. Apr. 2, 2018.