By Emily Hubbell

 

Models developed by Anh Tuan Ngo and Sergio Ulloa helped a University of Cincinnati researcher control spin with an electrical field.

 For decades, the transistors inside radios, televisions and other everyday items have transmitted data by controlling the movement of the electron’s charge. Scientists have now discovered that transistors could use less energy, generate less heat and operate at higher speeds if they exploited another property of the electron: its spin.

In 1921, scientists discovered that each electron has spin. Since then, researchers around the world and at Ohio University have been developing electronic devices that embed data inside an electron’s spin. The emerging field of spin electronics—or spintronics—could revolutionize memory storage devices and quantum computers.

Until now, scientists in spintronics have controlled spin by attaching an external magnet directly to the devices. But with the demand for smaller transistors on the rise, a bulky magnet is not an efficient or practical way to manipulate spin’s orientation, said Sergio Ulloa, professor of physics.

“The holy grail in spintronics is to address spin with something other than magnets,” he said. “An electrical field is portable and easy to switch on and off.”

Ulloa and graduate student Anh Tuan Ngo helped solve this issue by providing theoretical modeling for a recent experiment that was the first to successfully control  an electron’s spin using purely electrical fields. These findings were published in the article, “All-Electric Quantum Point Contact Spin-Polarizer." (Nat. Nanotechnol., 2009). 

The team collaborated with an University of Cincinnati research group led by Philippe Debray and Marc Cahay. Debray conceived and designed the experiments, while the OU researchers provided calculations explaining the behavior of the electrons in Debray’s experiment and predicted how strong the electric field’s control over the spin would be.

Their models revealed one key to the experiment—that the tiny connection along which the electrons travel in the device must be asymmetrical. 

Asymmetry allows the electrons to recognize which direction they are traveling along the wire. This, due to relativistic effects, helps their spin determine which way is up, thus allowing the electrons to engage in spin-orbit coupling and polarization. The coupling triggers the spin and the electron-electron interaction enhances it. This enabled the scientists to control the spin current electrically.

Controlling spin electronically has major implications for the future of novel devices such as transistors, but this experiment is only the first step of many, Ulloa said. The next step would be to rework the experiment so that it could be performed at a higher, more practical temperature not requiring the use of liquid helium.

“The fundamental physics in this experiment were already known. We used our imaginations to use the fundamentals in a different way,” Debray said. “But to be able to have practical applications, the next step will be to go to a higher temperature with new materials.” 

This research is supported by the Materials World Network and a National Science Foundation PIRE grant.

 

Micrographs of the experimental device.  The lower left image is a 3.7 x 4 square micron blowup of the quantum point contact (QPC) region in the center. The applied voltages on the lower (LG) and upper (UG) gates result in a strongly asymmetric QPC potential depicted in the upper left inset.  The combination of lateral spin-orbit coupling in the system as well as strong electron interactions result in the 100% spin polarization in the QPC shown in the right inset.