A Quantum Metal Model System

Under the right conditions, a strontium-lattice atomic clock becomes a quantum metal

Exciting new theory from the Rey group reveals the profound effects of electron interactions on the flow of electric currents in metals. Controlling currents of strongly interacting electrons is critical to the development of tomorrow’s advanced microelectronics systems, including spintronics devices that will process data faster, use less power than today’s technology, and operate in conditions where quantum effects predominate.

The group’s secret to better understanding electron interactions comes from controlling arrays of cold strontium (Sr) atoms inside the crystal made of light, or optical lattice, at the heart of the Ye group’s Sr-lattice atomic clock. The Ye group’s atomic clock should be an ideal simulator of the complex quantum interactions associated with the flow of electrons in metals. Metals themselves are much too complicated and “messy” for understanding novel quantum interactions and their effects.

The new theory explaining the flow of electrons exploits the ability of lasers to couple the spin and velocity of atoms, thereby engineering “spin-orbit” coupling. Spin-orbit coupling occurs in materials like metals where electrons (which have spin) move naturally through crystals of positively charged atoms. In the Sr-lattice clock, two atomic clock states play the role of electrons in either a spin-up or spin-down state. The motion of Sr atoms through the crystal of light simulates the flow of electrons in metals.

In this work, research associates Leonid Isaev and Johannes Schachenmayer as well as Fellow Ana Maria Rey investigate how the behavior of Sr atoms changes when the atoms are no longer independent, but rather interact with each other. The new theory predicts that the state of the cold Sr atoms can change from an insulator, where the atoms don’t move at all, to a quantum metal with mobile, interacting Sr atoms. Changing from an insulating state to a quantum metal is simply a matter of manipulating the intensity of the laser responsible for spin-orbit coupling.

Spin-orbit coupling is just one of three key ingredients for creating the exotic metallic state with Sr atoms. The other two ingredients are atomic spin and strong interatomic interactions.  Here’s how all three work together: The spin-orbit coupling generated by the laser creates a landscape of energy barriers that stop all atom movement. However, when the intensity of the laser is “just right,” the atoms start moving again, creating a “mass” current.

At the same time, a spin current is generated when pairs of atoms in different spin configurations are in a superposition.1 In a superposition, the atoms can exchange spins with their partners, inducing a spin current. This phenomenon is sensitive to even the slightest variations in laser intensity, which gives researchers a lot of control over the process. This ability to create and control spin transport may be a key ingredient in the development of spintronics.

All that’s left to do now is to verify that this theory works in real experiments. Efforts are already underway to do just that in the Ye labs. Results are expected soon.––Julie Phillips


1. Superposition is a state in which a particle holds two different properties—such as spin-up or spin-down—at the same time. 


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