Physicists Pioneer New Way to Turn Metal into Insulator

A team of physicists led by the University of British Columbia has demonstrated a novel way to precisely control electrical currents by leveraging the interaction between an electron’s spin and its orbital rotation around the nucleus.

An artist’s impression of the dissolving of the electronic ‘traffic jam:’ the red atoms are different in their quantum nature and allow transport of electrons in their surroundings. Image credit: Berend Zwartsenberg.

An artist’s impression of the dissolving of the electronic ‘traffic jam:’ the red atoms are different in their quantum nature and allow transport of electrons in their surroundings. Image credit: Berend Zwartsenberg.

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 so-called 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: by reducing the strength of the repulsive interaction between electrons, or by changing the number of electrons.

Berend Zwartsenberg, a Ph.D. student in the Stewart Blusson Quantum Matter Institute at the University of British Columbia, and colleagues explored a third possibility: was there a way to alter the very quantum nature of the material to enable a metal-insulator transition to occur?

Using a technique called angle-resolved photoemission spectroscopy, the researchers 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,” Zwartsenberg said.

“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 is a really exciting result at the fundamental physics level, and expands the potential of modern electronics,” said Professor Andrea Damascelli, principal investigator and scientific director of the Blusson Quantum Matter Institute at the University of British Columbia.

“If we can develop a microscopic understanding of these phases of quantum matter and their emergent electronic phenomena, we can exploit them by engineering quantum materials atom-by-atom for new electronic, magnetic and sensing applications.”

The team’s work is published in the journal Nature Physics.

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B. Zwartsenberg et al. Spin-orbit-controlled metal–insulator transition in Sr2IrO4. Nat. Phys, published online January 27, 2020; doi: 10.1038/s41567-019-0750-y

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