The edge currents of a topological insulator serve as a source of spin-polarized electrons. Graphics: Luis Maier |
Electrons
have an intrinsic angular momentum, called spin. As a consequence, not
only do they carry charge, but they also behave like tiny magnets, which
can be aligned. In our everyday use of computers, however, so many
electron magnets point randomly in all directions as to cancel out as a
whole. But if the spin were to be controlled, conventional computers
might suddenly become a lot faster: In the field of so-called
spintronics, the magnetic orientation of the electrons is used for
information transfer, which generates much less heat than is produced by
continually switching the current on and off as is required in
conventional electronics.
Metal and insulator at the same time: Topological insulators
Topological
insulators represent a very promising class of materials for the
implementation of spintronic devices. They conduct electricity only on
their surface, but not in their interior. In the thin layers of some of
these materials, the edge current consists of exactly two channels in
which the individual electrons flow. The flow direction in the two
channels is opposite to each other as is the spin orientation. This
behavior is called the quantum spin Hall effect due to its analogy to
the quantum Hall effect. The QSH effect was discovered in 2007 by the
research group of Professor Laurens Molenkamp at the University of
Würzburg.
Physicists
at the department of Laurens Molenkamp and the research group of
Professor Ewelina Hankiewicz now demonstrate—together with researchers
of Stanford University in California—how the spin polarization of the
channels can be experimentally verified. They also present an electronic
device that can generate and measure spin-polarized currents and thus
possesses some basic qualities required for spintronics. The results are
published in the prestigious journal Nature Physics (“Spin polarization
of the quantum spin Hall edge states”).
From theory to experiment: Successful with an H-shaped nanostructure
Until
recently, the spin-polarization of the channels was just mathematically
described; experimentally, it could only be indirectly inferred.
“However, the quantum spin Hall effect requires an actual spin-polarized
transport as a condition for its existence,” says research group leader
Hartmut Buhmann of Molenkamp’s department.
Würzburg
physicist Christoph Brüne managed to furnish the desired experimental
proof with an ingenious experimental set-up. Critical to the success was
an H-shaped nanostructure, consisting of mercury telluride and fitted
with an additional gold electrode at each leg.
With
this configuration, it is possible to induce a quantum spin Hall state
in one leg of the H-structure by means of an applied gate voltage. The
other leg causes an imbalance between the two spin currents at the
connection point, the cross bar of the H. As a consequence, only
electrons with magnetic alignment can be extracted and measured. This
also works in the reverse direction so that you can inject a
spin-polarized current and measure the induced voltage in the QSH
material.
Electron microscopic image of the circuit: The semiconductor H is shown in red, the gate contacts in yellow. The picture shows a section of about three by three micrometers. Photo: Luis Maier |
The
theory required for the clear identification of the measured values as
spin-currents, including some sophisticated simulations, comes from the
group of Ewelina Hankiewicz and her colleagues in the research group of
Professor Shou-Cheng Zhang in Stanford: “It wasn’t easy to calculate how
the spin edge currents get into the metal of the second leg,” Professor
Hankiewicz says.
However,
all the hard work paid off in the end. The editors of Nature Physics
even dedicated a “News & Views” review article to the Würzburg
research (“Quantum spin Hall effect: Left up right down”). “This is
equivalent to a high distinction, classifying our results as
particularly important,” explains Laurens Molenkamp.
Next research steps: Development of the concept
So
far, the configuration presented by the Würzburg physicists only works
at extremely low temperatures of typically -271 C. To
make it work at room temperature, the scientists still need to find
suitable materials. In the future, the Würzburg researchers intend as a
first step to develop the concept into a spin transistor, thus providing
all the basic elements required for application in spintronics.
In
addition, topological insulators have even more potential: They are a
safe bet for further exotic discoveries, such as Majorana fermions, i.e.
particles that are their own anti-particles. So it doesn’t come as a
surprise that the German Research Foundation (DFG) intends to establish a
new priority program for “topological insulators” this year.
