This diagram illustrates how lasers can be used to control an electric current on these new materials. Electrons (blue spheres) travel, as if on a highway, in different directions, with their axis of spin (arrows) aligned differently according to the direction of travel. A circularly polarized laser beam (left) affects only electrons going in one direction, removing them from the flow, leaving a net flow—an electric current—going the other way. Image: Gedik Group |
Exotic
materials called topological insulators, discovered just a few years ago, have
yielded some of their secrets to a team of Massachusetts Institute of
Technology (MIT) researchers. For the first time, the team showed that light
can be used to obtain information about the spin of electrons flowing over the
material’s surface, and has even found a way to control these electron
movements by varying the polarization of a light source.
The
materials could open up possibilities for a new kind of devices based on
spintronics, which makes use of a characteristic of electrons called spin,
instead of using their electrical charge the way electronic devices do. It
could also allow for much faster control of existing technologies such as
magnetic data storage.
Topological
insulators are materials that possess paradoxical properties. The 3D bulk of
the material behaves just like a conventional insulator (such as quartz or
glass), which blocks the movement of electric currents. Yet the material’s
outer surface behaves as an extremely good conductor, allowing electricity to
flow freely.
The
key to understanding the properties of any solid material is to analyze the
behavior of electrons within the material—in particular determining what
combinations of energy, momentum and spin are possible for these electrons,
explains MIT assistant professor of physics Nuh Gedik, senior author of two
recent papers describing the new findings. This set of combinations is what
determines a material’s key properties—such as whether it is a metal or not, or
whether it is transparent or opaque. “It’s very important, but it’s very
challenging to measure,” Gedik says.
The
traditional way of measuring this is to shine a light on a chunk of the solid
material: The light knocks electrons out of the solid, and their energy,
momentum, and spin can be measured once they are ejected. The challenge, Gedik
says, is that such measurements just give you data for one particular point. In
order to fill in additional points on this landscape, the traditional approach
is to rotate the material slightly, take another reading, then rotate it again,
and so on—a very slow process.
Gedik
and his team, including graduate students Yihua Wang and James McIver, and MIT
postdoc David Hsieh, instead devised a method that can provide a detailed 3D
mapping of the electron energy, momentum, and spin states all at once. They did
this by using short, intense pulses of circularly polarized laser light whose
time of travel can be precisely measured.
By
using this new technique, the MIT researchers were able to image how the spin
and motion are related, for electrons travelling in all different directions
and with different momenta, all in a fraction of the time it would take using
alternative methods, Wang says. This method was described in a paper by Gedik
and his team that appeared in Physical
Review Letters.
In
addition to demonstrating this novel method and showing its effectiveness,
Gedik says, “we learned something that was not expected.” They found that
instead of the spin being precisely aligned perpendicular to the direction of
the electrons’ motion, when the electrons moved with higher energies there was
an unexpected tilt, a sort of warping of the expected alignment. Understanding
that distortion “will be important when these materials are used in new
technologies,” Gedik says.
The
team’s high-speed method of measuring electron motion and spin is not limited
to studying topological insulators, but could also have applications for
studying materials such as magnets and superconductors, the researchers say.
One
unusual characteristic of the way electrons flow across the surface of these
materials is that unlike in ordinary metal conductors, impurities in the
material have very little effect on the overall electrical conductivity. In
most metals, impurities quickly degrade the conductivity and thus hinder the
flow of electricity. This relative imperviousness to impurities could make
topological insulators an important new material for some electronic
applications, though the materials are so new that the most important
applications may not yet be foreseen. One possibility is that they could be
used for transmission of electrical current in situations where ordinary metals
would heat up too much (because of the blocking effect of impurities), damaging
the materials.
In
a second paper,
appearing in Nature Nanotechnology, Gedik and his team show that a
method similar to the one they used to map the electron states can also be used
to control the flow of electrons across the surface of these materials. That
works because the electrons always spin in a direction nearly perpendicular to
their direction of travel, but only electrons spinning in a particular
direction are affected by a given circularly polarized laser beam. Thus, that
beam can be used to push aside all of the electrons flowing in one direction,
leaving a usable electric current flowing the other way.
“This
has very immediate device possibilities,” Gedik says, because it allows the
flow of current to be controlled completely by a laser beam, with no direct
electronic interaction. One possible application would be in a new kind of
electromagnetic storage, such as that used in computer hard drives, which now
use an electric current to “flip” each storage bit from a 0 to a 1 or vice
versa. Being able to control the bits with light could offer a much quicker
response time, the team says.
This
harnessing of electron behavior could also be a key enabling technology that
could lead to the creation of spintronic circuits, using the spin of the
electrons to carry information instead of their electric charge. Among other
things, such devices could be an important part of creating new quantum
computing systems, which many researchers think could have significant
advantages over ordinary computers for solving certain kinds of highly complex
problems.