click to enlarge Electrons on the surface of a topological insulator can flow with little resistance. Their spin and direction are intimately related; the direction of the electron determines its spin and in turn is determined by it.

Strange new materials experimentally identified
just a few years ago are now driving research in condensed-matter physics
around the world. First theorized and then discovered by researchers at the
U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory
(Berkeley Lab) and their colleagues in other institutions, these “strong 3D
topological insulators”—TIs for short—are seemingly mundane semiconductors with
startling properties. For starters, picture a good insulator on the inside
that’s a good conductor on its surface—something like a copper-coated bowling
ball.

A topological insulator’s surface is not an
ordinary metal, however. The direction and spin of the surface electrons are
locked together and change in concert. And perhaps the most surprising
prediction is that the surface electrons cannot be scattered by defects or
other perturbations and thus meet little or no resistance as they travel. In
the jargon, the surface states remain “topologically protected”—they can’t
scatter without breaking the rules of quantum mechanics.

“One way that electrons lose mobility is by
scattering on phonons,” says Alexis Fedorov, staff scientist for beamline
12.0.1 of Berkeley Lab’s Advanced Light Source (ALS). Phonons are the quantized
vibrational energy of crystalline materials, treated mathematically as
particles. “Our recent work on a particularly promising topological insulator
shows that its surface electrons hardly couple with phonons at all. So there’s
no impediment to developing this TI for spintronics and other applications.”

The TI in question is bismuth selenide, Bi₂Se₃,
on whose surface electrons can flow at room temperature, making it an
attractive candidate for practical applications like spintronics devices, plus
farther-out ones like quantum computers. Much of the research on
electron-phonon coupling in Bi₂Se₃ was conducted at
beamline 12.0.1 by a team including Fedorov, led by Tonica Valla of Brookhaven
National Laboratory. Their results are reported in Physical Review Letters.

The right
tool for the job

To study a TI’s surface conductivity, electron transport on its surface has
to be separated from total conductivity, including the poorly conducting bulk.
One experimental technique, called angle-resolved photoemission spectrometry
(ARPES), is adept at doing just this.

ARPES shines bright light, like that produced by
the Advanced Light Source, on a sample and captures the electrons that the
energetic photons knock free. By recording the angle and energy of these
photoemitted electrons on a CCD detector, ARPES gradually builds up a direct
graphic visualization of the sample’s electronic structure. 

click to enlarge ARPES maps the electronic properties, including the band structure and Fermi surface, of the topological insulator bismuth selenide (left). Like graphene, the lower energy valence band of a topological insulator meets the higher energy conduction band at a point, the Dirac point, with no gap between the bands (center). Unlike graphene, however, the Fermi surface does not usually pass through the Dirac point. Distinct spin states (red arrows) are associated with each different orientation in momentum space (right).

“Of the several ARPES beamlines at the ALS,
beamline 12.0.1 seems to have an ideal balance of energy, resolution, and flux
for research on topological insulators,” says Fedorov. “This beamline was used
for some of the first experiments establishing that 3D TIs actually occur in
nature, and several teams have worked here validating the characteristics of
TIs.”

The photoemitted electrons in an ARPES experiment
directly map out such features as the material’s band structure—the energy
difference, or gap, between electrons bound in atoms’ outer shells, the valence
band, and charge carriers that are free to rove, the conduction band.
Insulators have wide band gaps, semiconductors have narrower ones. 

The band structure of the surface states of a
topological insulator like Bi₂Se₃ appear as two cones
that meet at a point, called the Dirac point. There’s no gap at all between the
valence and conduction bands, only a smooth transition with increasing energy.
This is similar to the band structure of the fascinating material graphene, a
single sheet of carbon atoms, the thinnest possible surface. ARPES diagrams of
band structures like these look like slices through the cones, an X centered on
the Dirac point. 

Although graphene and topological insulators have
similar band structures, other electronic characteristics are very different.
The combinations of different speeds and orientations equivalent to a
material’s highest particle energies (at zero degrees) make up its momentum
space, mapped by the Fermi surface. While the Fermi surface of graphene lies
between the conical bands at the Dirac point, this is not true of TIs. The
Fermi surface of Bi₂Se₃ cuts high across the conical
conduction band, mapping a perfect circle. It’s as if the circular Fermi
surface were drawn right on the surface of the topological insulator, showing
how spin-locked surface electrons must change their spin orientation as they
follow this continually curving path. 

Values including electron-phonon coupling can be
calculated from the diagrams that ARPES builds up. ARPES measures of Bi₂Se₃
show that coupling remains among the weakest ever reported for any material,
even as the temperature approaches room temperature.

Says Fedorov, “Although there’s still a long way to
go, the experimental confirmation that electron-phonon coupling is very small
underlines Bi₂Se₃’s practical potential.” With continued
progress, the spin-locked electronic states of room-temperature topological
insulators could open a gateway for spintronic devices—and for more exotic
possibilities as well. 

For example, by layering a superconducting material
onto the surface of a topological insulator—a feat recently achieved by a group
of Chinese scientists working at beamline 12.0.1—it may be possible to create a
theoretical but yet unseen particle that is its own antiparticle, one that
could persist in the material undisturbed for long periods. Discovery of these
so-called Majorana fermions would be an achievement in itself, and could also
provide a way of overcoming the main obstacle to realizing a working quantum
computer, a method of indefinitely storing data as “qubits.”

The experimental examination of strong, 3D
topological insulators is a field hardly more than five years old, and the
potential rewards, both for fundamental and applied science, have only begun to
be explored.

Source: Lawrence Berkeley National Laboratory