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Symmetry boosts spin's lifetime

Physicists in the US have invented a new way to increase the lifetime of electron spins flowing in a semiconductor device. Their technique involves fine–tuning the properties of a tiny gallium-arsenide structure and could make it possible to create logic circuits that use electron spin.

Electron spins can assume one of two values — up or down — and this property is already used to store data in computer hard disks and magnetic memories. In the future, more advanced "spintronic" devices could exploit both the spin and charge of the electron to create a wider range of digital devices that are faster are more energy efficient than conventional silicon chips.

However, such devices would rely on electrons maintaining their spin as they travel around a circuit. This has proven difficult because each time such an electron scatters from a defect or lattice vibration in a metal or semiconductor there is a small chance that its spin could flip direction. This occurs because a change in the motion of the electron affects the direction of its spin thanks to an effect called spin-orbit (SO) coupling.

In most materials electrons experience many collisions and this flipping occurs randomly and very rapidly. This can be minimized by using defect–free materials kept at very low temperatures — but this is not really possible in practical devices.

Now Jake Koralek of the Lawrence Berkeley National Lab; David Awschalom of the University of California Santa Barbara; and colleagues have tackled the problem by working out a way to tune the SO interaction in a tiny gallium arsenide structure called a quantum well (Nature 458 610).

The team focussed on two aspects of how spatial symmetries within a semiconductor affect SO coupling — the Dresselhaus and Rashba effects. The former is related to the "inversion asymmetry" that occurs in gallium arsenide crystals and was adjusted by changing the width of the quantum well. The Rashba effect is caused by the application of an electric field to the quantum well and the team controlled this by adding impurities (dopants) to certain regions of the quantum well.

"We tuned the Rashba and Dresselhaus terms to be equal", explained Koralek. This meant that the resultant SO interaction has a much higher spatial symmetry than in a typical semiconductor or metal. Although individual spins are still affected by spin-orbit coupling, they are able to rotate in unison in a long-lasting collective state called a "persistent spin helix" (PSH).

By using a laser technique called transient spin-grating spectroscopy, the team measured how long a PSH persisted in the quantum well. Two "pump" laser pulses are fired at the quantum well where the resulting interference pattern of light creates alternating stripes of spin up and spin down electrons — called a spin-grating. The wavelength of the spin-grating can be adjusted by simply changing the angle between the two pump pulses.

The amplitude and wavelength of the spin–grating are then measured by firing a third "probe" laser pulse at the sample and observing the resulting diffraction pattern. By varying the time between the pump and probe pulses, the team were able to measure how long the spin-grating endured.

They found that when the wavelength of the spin-grating matched the expected wavelength of the persistent spin helix, the spin-grating endured for hundreds of picoseconds — compared to just a few picoseconds when the wavelengths did not match.

This 100-times improvement was seen at the relatively low temperature of 5 K and the team found that it dropped off rapidly as the temperature increased to room temperature. This strong temperature dependence was not expected and could mean that the technique is not appropriate for use in practical spintronic devices.

Although a few hundred picoseconds doesn't sound like a long time, Awschalom told physicsworld.com that such “spin engineered” materials could someday be used in devices that perform large numbers of spin operations on electrons before their spins decayed.

Koralek added that the Rashba interaction can also be controlled by applying a voltage to the quantum well — and this could lead to a spin transistor that could turn a spin current on and off.

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