Electron Spin Technology Seen Feasible

Researchers at UC Santa Barbara and at Pennsylvania State University have reported in a recent issue of Nature on experiments that show high-efficiency electron spin transfer between two different semiconductor materials. The paper, "Persistent Sourcing of Coherent Spins for Multifunctional Spintronics," also announced the discovery of a new "persistent" mode of spin currents that makes semiconductor reservoirs act, in effect, as "spin batteries."

In his review of the paper, Caltech physicist Michael L. Roukes said of the demonstration of spin transfer across semiconductor interfaces: "This achievement is an important milestone in the race to build futuristic devices that exploit the true quantum nature of electrons."

All semiconductor technology is based on charge. But electrons also spin, or rotate. The results reported in Nature answer affirmatively the key question of whether a new spin-based technology is feasible.

Physicist David Awschalom headed the research team that conducted the experiments. He is director of the UCSB Center for Spintronics and Quantum Computation, a key component of the new California NanoSystems Institute located jointly at UCSB and UCLA. The experiments were the result of a long-standing collaboration between Awschalom and Nitin Samarth, a materials physicist at Penn State.

"Spin appears to be remarkably robust and moves relatively easily between semiconductors," according to Awschalom. "Previously, theories of electron transport from one material to another suggested that the spin would lose its orientation or scatter from impurities or structural effects. These experiments point out that this is not the case."

The question was whether a cloud or bundle of electrons, spinning the same way, would retain that same direction when the cloud is moved to an adjacent semiconducting material. The spins stayed aligned. What astonished Awschalom, his graduate student Irina Malajovich (first author on the Nature paper), and her Penn State collaborators is that the spins not only stayed aligned but did so as the temperature of the materials was raised to room temperature.

Certain semiconductors were found to work as spin reservoirs because spins survive there for long times. In analogy with conventional, charge-based electronics, this work shows that electrons can be withdrawn from such reservoirs with their spin intact, using electric fields. Spin reservoirs are thereby "sourcing" a spin current.

"The implication of these results is that there is no fundamental reason one can't move forward and fabricate spin transistors," said Awschalom. "At present, transistors are the building blocks of electronics. It's exciting to think about future technologies that exploit the electron spin and function in a completely different manner. Imagine devices that could combine photonics, electronics, and magnetics in a single structure."

--Jacquelyn Savani