Most modern electronic devices rely on tiny, finely tuned electrical currents to process and store information. These currents dictate how fast computers run, how regularly pacemakers tick, and how securely money is stored in the bank. Researchers have demonstrated a new way to precisely control such electrical currents by leveraging the interaction between an electron's spin (the quantum magnetic field it inherently carries) and its orbital rotation around the nucleus.
Broadly, all materials can be categorized as metals or insulators, depending on the ability of electrons to move through the material and conduct electricity; however, not all insulators are created equally. In simple materials, the difference between metallic and insulating behavior stems from the number of electrons present — an odd number for metals and an even number for insulators. In more complex materials, like Mott insulators, the electrons interact with each other in different ways, with a delicate balance determining their electrical conduction.
In a Mott insulator, electrostatic repulsion prevents the electrons from getting too close to one another, which creates a “traffic jam” and limits the free flow of electrons. Until now, there were two known ways to free up the traffic jam: reducing the strength of the repulsive interaction between electrons or by changing the number of electrons. The researchers found a third method: altering the very quantum nature of the material to enable a metal-insulator transition to occur.
Using a technique called angle-resolved photoemission spectroscopy, the team examined the Mott insulator Sr2lrO4, monitoring the number of electrons, their electrostatic repulsion, and finally the interaction between the electron spin and its orbital rotation. They found that coupling the spin to the orbital angular momentum slows the electrons down to such an extent that they become sensitive to one another's presence, solidifying the traffic jam. Reducing spin-orbit coupling in turn eases the traffic jam. A transition from an insulator to a metal was observed using this strategy.