spin electrons into motion
Technology Research News
A group of scientists from Germany and
Russia is using polarized light and the spin of electrons to coax current
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
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,
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
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).
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.
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