Spin waves, which are caused by a shift in electron spin propagating through a material, could revolutionize the way gadgets store and transmit data. Magnons are waves that do not scatter or pair with other particles.
They can even behave like a superfluid, flowing through a substance with no energy loss under the correct conditions. However, the features that make them so potent also make measuring them extremely hard.
Superfluidity, the frictionless flow and other exotic behavior found in liquid helium at temperatures approaching absolute zero (-273.15 °C, or -459.67 °F), and comparable frictionless behavior of electrons in a superconducting solid (less commonly mentioned). Quantum mechanical processes cause the strange behavior in each situation.
Researchers at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) established the capacity to excite and detect spin waves in a two-dimensional graphene magnet in a prior study, but they were unable to determine any of the wave’s precise properties.
SEAS researchers have now established a new method for measuring the fundamental features of graphene spin waves.
“In previous experiments, we only knew that we could generate spin waves, but we didn’t know anything about their properties in a quantitative way,” said Amir Yacoby, Professor of Physics and of Applied Physics at SEAS and senior author of the paper.
“With this new work, we can determine all these quantitative numbers, including the energy and number of spin waves, their chemical potential, and temperature. This is an extremely important tool that we can use to explore new ways of generating magnons and get closer to achieving spin superfluidity.”
Spin waves don’t like to interact with anything but by using electrons and this energy cost as a proxy to probe the properties of a spin waves, we can determine the chemical potential, which combined with knowing the temperature and a few other properties, gives us a full description of the magnon.
Yonglong Xie
The research is published in Nature Physics.
In 1938, Soviet physicist Pyotr Leonidovich Kapitsa and Canadian physicists John F. Allen and A.D. Misener found superfluidity (in the form of frictionless flow via thin capillaries) in 4He below 2.17 K (290.98 °C, or 455.76 °F).
Assessing the qualities of a spin wave is comparable to measuring the properties of a tidal wave if water were undetectable. How would you know the speed, height, or number of tidal waves if you couldn’t see them?
One method is to bring something measurable into the system, such as a surfer. By monitoring the speed of the surfer, the tidal wave’s speed may be determined. Yacoby and his team used an electron surfer in this situation.
To begin, the researchers used a quantum Hall ferromagnet. Quantum Hall ferromagnets are magnets built from two-dimensional materials, such as graphene, in which all electron spins point in the same direction. If an electron with a different spin is put into this system, it will try to flip the spins of its neighbors with its own energy.
However, the researchers discovered that injecting an electron with a different spin into the system and then generating spin waves reduced the energy required for the electron to flip its neighbors.
“It’s striking that somehow the electrons that we’re putting into the system are sensitive to the presence of spin waves,” said Andrew T. Pierce, a graduate student at SEAS and co-first author of the study.
“It’s almost as it these electrons are grabbing onto the wave and using it to help flip the spins of their neighbors.”
“Spin waves don’t like to interact with anything but by using electrons and this energy cost as a proxy to probe the properties of a spin waves, we can determine the chemical potential, which combined with knowing the temperature and a few other properties, gives us a full description of the magnon,” said Yonglong Xie, a postdoctoral fellow at SEAS and co-first author of the study.
“This is critical to knowing whether the wave is approaching the limit where it achieves superfluidity.”
The findings could potentially be applied to other difficult-to-measure exotic systems, such as the recently found moiré materials, which are believed to accommodate a wide range of waves, including the spin wave investigated here.
This research was co-authored by Seung Hwan Lee, Patrick R. Forrester, Di S. Wei, Kenji Watanabe, Takashi Taniguchi, and Bertrand I. Halperin.
The U.S. Department of Energy, Basic Energy Sciences Office, Division of Materials Sciences and Engineering under award DE-SC0001819, the Gordon and Betty Moore Foundation, and the National Science Foundation, under grant DMR-1231319 supported it in part.
