Santa Barbara, Calif. -- Researchers at the University
of California at Santa Barbara (UCSB) report in the
Dec. 6 issue of Nature the first demonstration of
continuous electrical tunability of spin coherence in
semiconductor nanostructures.
The six person research team is headed by physicist
David Awschalom, director of the Center for
Spintronics and Quantum Computation. Awschalom's
research is conducted under the auspices of the
California NanoSystems Institute (CNSI), a
multi-million-dollar State initiative conceived by Gov.
Gray Davis to develop the science and technology
that will propel the state's economic future. Funding
for Awschalom's research is provided by the Defense
Advanced Research Projects Agency (DARPA), an
agency which promotes speculative but potentially
groundbreaking research projects.
An approximate understanding of the nature of spin
can be gleaned by analogy with the orbit of planets in
the solar system. In this analogy, electrons orbit a
nucleus in a fashion similar to the Earth¹s orbit
around the Sun. Just as the Earth rotates about it¹s
axis during the orbit, electrons have a quality of
rotation called Œspin.¹ The spin of electrons is
characterized by the direction of rotation, so that spin
Œup¹ or Œdown¹ electrons rotate in opposite directions
(i.e., clockwise or counter-clockwise).
While magnetic fields are conventionally used to
manipulate spins in familiar magnetic devices like
hard-disk drives, this demonstration of electrical
control of aligned spins represents a significant step
towards making new spin-based technologies. One
future technology is quantum computing, where many
schemes make use of electron spin states as bits of
information analogous to the 0¹s and 1¹s of binary
computing. Unlike ordinary bits, Œquantum bits¹ can be
any combination of both 0 and 1 simultaneously,
corresponding to a continuous range of possible
directions.
Magnetic fields can change the direction of spins by
inducing "precession" which is an additional rotation
of the spin orientation about the magnetic field,
similar to the periodic movement of the axis of a top
after it is spun. While the speed of electron spin
precession in a magnetic field is generally fixed by
the particular materials used, the research reported in
Nature has shown that both the speed and direction
of precession can be continuously adjusted by
applying electric fields in specially engineered
quantum structures.
Said Awschalom, "We would like to electrically
manipulate the electron spin because that's the
bridge to a scalable technology. Today's
charge-based electronics all use electrical gates--a
sandwich of electrical plates--to guide electrons. We
want to use the electrical control methods of today's
technology to fabricate a spin gate. This paper
reports spin gates that can make the electron spin go
one way or the other or just stay put. And the gate
works at room temperature."
Awschalom refers to the invention as a gate, rather
than a switch because it performs continuous tuning
of electron spin. Instead of the "off" and "on" options
for a switch, a gate operates across a continuum the
way lights can be dimmed by a rheostat, for instance.
The spin gate device is made of sandwiches of the
semiconductor materials Gallium Arsenide (GaAs)
and Aluminum Gallium Arsenide (AlGaAs) only a
hundred nanometers thick.
Semiconductor heterostructures operate by trapping
electrons in a Œquantum well¹ that is shaped like a
square box. The trick that Awschalom's research
team devised to construct their device was to use a
parabolically shaped quantum well instead of the
usual square box.
A decade ago Awschalom's UCSB colleague, Art
Gossard, professor of electrical and computer
engineering, led a research group that conceived and
used Molecular Beam Epitaxy (MBE) techniques to
build semiconductor heterostructures with parabolic
quantum wells. Klaus Ensslin, then a postdoc in the
Materials Department at UCSB, described how the
trapped electrons behaved in the parabolic structure.
Ensslin is now a physics professor at Switzerland's
Eidgenössische Technische Hochschule (ETH) in
Zurich, which entered last June into a research
agreement with UCSB.
At UCSB on sabbatical from ETH Zurich, Ensslin
returned to collaborate with his old mentor Gossard.
Both assisted the Awschalom research effort and are
authors of the Nature paper. Two of the other three
authors are affiliated with Awschalom: his postdoc
Gian Salis, the first author and now a permanent staff
member at IBM Zurich, and his physics graduate
student Yuichiro Kato both worked on measuring the
devices. Dan Driscoll is Gossard's materials graduate
student who helped in the device fabrication.
"It was our colleagues ability to fabricate the
specially-engineered structures that made these
experiments possible, said Awschalom."
Why does a parabolic quantum well enable electrical
control of spin coherence when a box-shaped well
does not?
Salis explained, "Mathematically speaking, when you
add a line to a parabola the parabola is displaced, but
doesn't change shape. But when you add a line to a
box, you only distort the box into a trapezoidal
structure. This is essentially what happens when we
apply a voltage to our device: the voltage tilts the
whole structure like pushing down on a see-saw. So
we used the two different semiconductors (GaAs,
AlGaAs) to form the parabola and to trap the
electrons. We then applied electrical voltages to
displace the parabola and thereby moved the pooled
electrons in the well from one material to another.
The effects were large! We were able to control spin
electrically exactly as we had thought we could when
we conceived the experiments."
"With the application of just a few volts," added
Awschalom, "the electrons begin to sample different
regions of space, and that's when their spin
precesses faster or slower or stops. We are moving
electrons out of Gallium Arsenide into Alumnium
Gallium Arsenide continually without changing their
wave function or profile in space, and that's what is
unique."
The spin-gates discussed in the Nature report are an
example of the rapidly developing field of Œspintronics,¹
which studies electronic devices that are based on
electron spin.
This raises the question: What might spintronics do
that electronics can't?
In addition to the longer-term goal of quantum
computing, spintronics offer the near-term possibility
of revolutionizing the way we think about piecing
together different technologies.
"Think of one combined unit that integrates logic,
storage, and communication for computing," said
Awschalom. "We envision using a mixture of optical,
electronic, and photonic techniques to prepare and
manipulate spin-based information. The spin could be
stored in semiconductors, run at frequencies many
times faster than today's technology and work at
room temperature. And all in a single nanostructure.
Then imagine millions of these nanostructures
working together in a device small by human
standards. What such devices will do is up to
scientists and engineers to determine. But the most
exciting prospects are the revolutionary ones rather
than simple extrapolations of today¹s technology."
In addition to applications in the emerging field of
spintronics, the last sentence of the Nature paper
points to possible advances in fundamental physics
using the findings: "Furthermore, the large tunability
and quenching of the electronic spin splitting offers
the potential for new insights into other phenomena,
such as ferromagnetic quantum Hall states or the
dynamics of electrically inverted spin populations
through non-adiabatic gating." Studies of the
quantum Hall effect are very important, and have
already garnered two Nobel physics prizes for its
explorers. What's being offered here is a new way of
looking at some tantalizing aspects of condensed
matter physics.
Note: This story has been adapted from a news release issued
by University Of California, Santa Barbara - Engineering for
journalists and other members of the public. If you wish to quote
from any part of this story, please credit University Of California,
Santa Barbara - Engineering as the original source. You may also
wish to include the following link in any citation:
http://www.sciencedaily.com/releases/2001/12/011206073914.htm
|
|