In thin, two-dimensional semiconductors, electrons move, spin and synchronize in unusual ways. For researchers, understanding the way these electrons carry out their intricate dances— and learning to manipulate their choreography—not only lets them answer fundamental physical questions, but can yield new types of circuits and devices.
One correlated phase that such electrons can take on is magnetic order, in which they align their spin in the same direction. Traditionally, the ability to manipulate magnetic order within a 2-D semiconductor has been limited; scientists have used unwieldy, external magnetic fields, which limit technological integration and potentially conceal interesting phenomena.
Now, researchers from the University of Chicago’s Pritzker School of Molecular Engineering (PME) have discovered how to use nanoscale, low-power laser beams to precisely control magnetism within a 2-D semiconductor. Their approach, described online in the journal Science Advances, has implications for both studying the emergence of the correlated phase as well as designing new optoelectronic and spintronic devices.
“The fact that we can now use light to manipulate electrons in this way means we have unprecedented control over this magnetic order,” said Asst. Prof. Alex High, the senior author of the new work.
High’s lab focused on transition metal dichalcogenides (TMDs), a family of semiconductors that can be exfoliated into single, two-dimensional flakes, measuring just three atoms thick. Scientists had previously hypothesized that electrons within TMDs could assume a correlated phase, with their spin aligned in the same direction to lower the system energy—this ferromagnetic phase is what we colloquially call magnetism. Generating or modeling this transition to the correlated state, however, has been difficult.
High has long been interested in how light can be controlled and, in turn, can alter states of matter. His team wondered whether, instead of external magnetic fields, miniscule beams of light could be used to create a correlated magnetic phase. They aimed a tightly-focused laser beam, less than a micron (one-thousandth of a millimeter) in diameter at a monolayer TMD. They flashed the laser for nanoseconds at a time, while also monitoring the TMD with an optical probe that let them track the activity of its electrons.
The probe revealed that the pulsing laser was impacting the spin-polarization of electrons within a 5 micron by 8 micron area of the TMD, spreading a correlated phase outward from the laser. In other words, the electrons were aligning their spin; the researchers could control the magnetic order of electrons within the tiny area.