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Topic Name: Spintronics
Category: Electronics
Research persons: S. G. Carter, Z. Chen, and S. T. Cundiff,Zhigang Chen, Sam G. Carter, Rudolf Bratschitsch, Philip Dawson, and Steven T. Cundiff
Location: JILA,University of Colorado,440 UCB,Boulder, CO 80309-0440, United States
Details
Researchers are investigating a new kind of
microelectronics called spintronics. These devices will rely on the
spindependent behavior of electrons in addition to (or even instead of)
conventional charge-based circuitry. Researchers in physics and engineering
anticipate that these devices will process data faster, use less power than
today's conventional semiconductor devices, and work well in nanotechnologies,
where quantum effects predominate. Spin-FETs (field effect transistors), spin-LEDs
(light-emitting diodes), spin-RTDs (resonant tunneling devices), terahertz
optical switches, and quantum computers are some of the multifunctional
spintronic devices being envisioned. The realization of this vision, however, is
predicated on a much deeper understanding of fundamental spin interactions in
semiconductors.
That's where researchers in Fellow Steve
Cundiff's lab come in. Postdoc Sam Carter and graduate student Zhigang Chen are
collaborating on laser studies of two electron spin properties that are
essential for the development of spin-based electronics: (1) spin transport and
(2) spin coherence. Spin transport depends on spin mobility and spin diffusion.
Spin mobility is a measure of how fast a spin packet moves in response to an
electric field, and spin diffusion describes how rapidly the spin packet spreads
out. The coherence time is how long the phase of spin-synchronized electrons
remains predictable or, in other words, how long such electrons can carry
information. The group studies spin properties in quantum wells in
semiconductors because confinement in the wells increases the strength of the
light-electron interaction and allows the electrons to be isolated.
In the process of probing spin transport and
coherence time, the researchers are also gaining new insights into optical spin
control. Although the Cundiff group currently has no plans to actually build a
spintronic device, it seeks a better understanding of the fundamental physics
that will make such devices possible.
Transport
Sam Carter's studies of spin transport in
semiconductor quantum wells have shown that the optical pulses he uses to
manipulate and probe electron spin also affect the transport of spin packets.
The figure above displays the optical excitation of a spin packet and how the
spins move and spread out in an electric field. Carter believes that, with
further study, it may even be possible to use lasers to control spin transport.
Optical control could well play a role in the design of spin-based quantum
computers. However, other spintronic devices, such as the spin transistor, will
likely not rely on lasers.
Carter recently performed systematic studies of
spin diffusion using transient spin gratings. In this technique, two laser
pulses interfere on the semiconductor to generate a spin grating - alternating
regions of spin up and spin down electrons, separated by a few microns. Spin
diffusion causes this grating to wash out, so measuring how long the grating
lasts determines how fast diffusion occurs.
Carter has shown that increasing the power of
the lasers used to excite the electrons increases spin diffusion. Stronger laser
excitation can free electrons from their local environment, allowing them to
move more freely about the quantum well. Raising the temperature of the
semiconductor had a similar effect. The difference between diffusion of electron
spins was also compared to diffusion of bound electron-hole pairs called
excitons, which diffuse much more rapidly.
The Cundiff group's research is helping
physicists better understand spin transport experiments and aiding in the
development of a better theory to explain them.
Law &Order
Zhigang
Chen investigates how disorder in quantum wells affects spin coherence. His goal
is figuring out what parameters influence spin coherence and why. He's
discovered that weak electron localization (due to disorder) plays a key role in
lengthening spin coherence times. One form of disorder is defects in the
interface between two different materials that constitute a semiconductor
quantum well. These defects lead to fluctuations in the quantum confinement
energy, which trap electrons and prevent them from moving around freely in the
quantum well. The figure on the right shows localized and delocalized electrons
in a quantum well.
When an electron is delocalized and moving
freely in the quantum well, it rapidly loses its spin coherence because of
decreased momentum scattering. A strongly localized electron also loses
coherence, but for different reasons. For example, the influence of stronger
nuclear interactions will relax spin coherence in strongly localized electrons.
Also, when electrons are localized at different defect sites, these different
environments will cause a relaxation of the ensemble spin.
The longest spin coherence times occur when
electrons are weakly localized, i.e., near the crossover point between the
localized and delocalized states. Spintronic device designers will likely take
these findings into account as they look for a compromise between increasing the
spin coherence time and improving transport properties.
In this research, Chen and his colleagues
optically adjusted the environment inside a quantum well to favor either
localization or delocalization. By using a specially designed sample, they were
able to continuously vary the electron density in particular quantum wells. Chen
also worked out a new technique for characterizing the localization of
electrons. These discoveries will guide future device design.
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