Quantum switch turns metals into insulators

Technology News | February 5, 2020

By Rich Pell

The researchers achieved this by demonstrating a new way to precisely control electrical currents in a material. They did so by leveraging the interaction between an electron’s spin – the quantum magnetic field it inherently carries – and its orbital rotation around the nucleus.

“We have found a new way to switch the electrical conduction in materials from on to off,” says Berend Zwartsenberg, a Ph.D. student at UBC’s Stewart Blusson Quantum Matter Institute (SBQMI) and lead author of a paper on the work. “Not only does this exciting result extend our understanding of how electrical conduction works, it will help us further explore known properties such as conductivity, magnetism, and superconductivity, and discover new ones that could be important for quantum computing, data storage, and energy applications.”

While all materials can be generally categorized as either metals or insulators depending on how freely electrons move through them, say the researchers, 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 – such as so-called Mott insulators – the electrons interact with each other in different ways, with a delicate balance determining their electrical conduction.

In such an insulator, say the researchers, 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 this traffic jam: by reducing the strength of the repulsive interaction between electrons, or by changing the number of electrons.

The researchers, however, explored a third possibility: altering the quantum nature of the material to enable a metal-to-insulator transition to occur. Using a technique called angle-resolved photoemission spectroscopy, the team examined the Mott insulator Sr₂IrO₄, monitoring the number of electrons, their electrostatic repulsion, and finally the interaction between the electron spin and its orbital rotation.

“We 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.” says Zwartsenberg. “Reducing spin-orbit coupling in turn eases the traffic jam and we were able to demonstrate a transition from an insulator to a metal for the first time using this strategy.”

This work, say the researchers, expands the potential of modern electronics.

“If we can develop a microscopic understanding of these phases of quantum matter and their emergent electronic phenomena,” says co-author Andrea Damascelli, principal investigator and scientific director of SBQMI, “we can exploit them by engineering quantum materials atom-by-atom for new electronic, magnetic, and sensing applications.”

For more, see “Spin-orbit-controlled metal–insulator transition in Sr₂IrO₄.”