Lasers spin electrons into motion

By Kimberly Patch, Technology Research News

A group of scientists from Germany and Russia is using polarized light and the spin of electrons to coax current to flow.

Because the method can be used to detect the spin of electrons at room temperature, it is a crucial step toward using electron spin in sensors and electronic devices. It could also prove useful as an analytical tool to investigate the dynamics of electron spin in quantum wells, which would help efforts to build quantum computers.

Any given electron has one of two types of spin -- spin up or spin down. While today's electronics use the flow or absence of flow of electrons to indicate the ones and zeros of binary computing, researchers investigating spintronics are looking to use the spin of electrons to indicate a one or zero.

Quantum wells are thin layers of semiconductor that trap electrons, limiting their motion in specific directions depending on the electrons' spin. Photons of circularly polarized light have angular momentum, and when an electron absorbs a circularly polarized photon, the angular momentum reverses the electron's spin.

This momentum comes from the organization of the lightwaves. The electric and magnetic fields in unpolarized light are oriented in many different directions within the plane perpendicular to the direction the light is traveling. In polarized light, the electric and magnetic fields have the same orientation. In circularly polarized light, these fields are still oriented the same way, but the orientation rotates around the axis of the direction the light is traveling.

The key to the researchers' device is that right-handed circularly polarized light leads to more spin up electrons, while left-handed circularly polarized light leads to more spin down electrons. Thus, by shining a certain type of light at the electrons trapped in a quantum well, the researchers are able to change the mix of electrons spins.

When electrons in a quantum well are spin polarized, meaning as a group they're knocked out of spin equilibrium, more electrons flow in one direction than another, resulting in a current.

When the light is switched off, the electron spin imbalance disappears, and so does the current.

This makes it "possible to to determine in a simple way the state of polarization of the light," said Wilhelm Prettl, a physics professor at the University of Regensburg in Germany. Because the effect is faster than one billionth of a second, it can be used to analyze very short laser pulses in real-time, he added.

The method works in temperatures ranging from 2 degrees Kelvin, to room temperature, said Prettl. Two degrees Kelvin is -275 degrees Celsius.

The method should prove useful in condensed-matter physics research, where understanding various manifestations of spin-dependent phenomena is important in areas like giant magnetoresistance and superconductivity, Prettl said. Giant magnetoresistance occurs when the spin polarized material is in a magnetic field. The magnetic field causes the material's resistance to electric current to drop dramatically. The effect is used to make read-heads for disk drives.

Being able to determine spin is especially important for developing spintronics-based and quantum computers, said Prettl. "The spin of electrons and holes in solid-state systems is the decisive ingredient for active spintronic devices and several schemes of quantum computation," he said.

Practical spintronics devices will have to be able to transport electrons with their spins -- and thus the information they are carrying -- intact. The researchers' method should make it easier to investigate spin lifetime in quantum wells, which are the transistors of spintronics devices and quantum computers.

Prettl's research colleagues were Sergey D. Ganichev and Sergey N. Danilov from the A.F. Ioffe Physico-Technical Institute in Russia, Eugenious L. Ivchenko, Jonathan Eroms, Werner Wegscheider and Dieter Weiss from the University of Regensburg in Germany.

They published the research in the May 7, 2001 issue of Physical Review Letters. The research was funded by the German Science Foundation (DFG) and the Russian Fund of Basic Research (RFFI).

Timeline:   Unknown
Funding:   Government
TRN Categories:  Quantum Computing; Integrated Circuits
Story Type:   News
Related Elements:  Technical paper, "Conversion of Spin into Directed Electric Current in Quantum Wells," Physical Review Letters, May 7, 2001.


July 18, 2001

Page One

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Cartoons loosen up computer interfaces

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Bioengineers aim to harness bacterial motion

Lasers spin electrons into motion


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