In the search for new materials with improved electrical conductivity, a team of researchers led by Tonica Valla of Brookhaven National Laboratory has found a potential candidate in the topological insulator Bi2Se3. Electrons on the metal surface of a topological insulator can flow with little resistance. Using angle-resolved photoemission spectroscopy (ARPES) at Brookhaven’s National Synchrotron Light Source and at the Advanced Light Source at Lawrence Berkeley National Laboratory (LBNL), the researchers discovered that the surface electrons of Bi2Se3 can flow at room temperature, making it an attractive candidate for practical applications like spintronics devices, plus farther-out ones like quantum computers.
As reported in the May 4 issue of Physical Review Letters (10.1103/PhysRevLett.108.187001), Valla, Alexei Fedorov of LBNL, Young Lee of the Massachusetts Institute of Technology, and their colleagues generated a direct graphic visualization of the sample’s electronic structure. 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 is no gap between the valence and conduction bands, only a smooth transition with increasing energy. This is similar to the band structure of graphene in which ARPES diagrams look like slices through the cones, an X centered on the Dirac point.
Although graphene and topological insulators have similar band structures, their 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 topological insulators. The Fermi surface of Bi2Se3 cuts high across the conical conduction band, mapping a perfect circle. It is 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.
“One way that electrons lose mobility is by scattering on phonons,” said Fedorov. Phonons are the quantized vibrational energy of crystalline materials, treated mathematically as particles. “Our recent work on a particularly promising topological insulator [TI] 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.”
Values including electron–phonon coupling can be calculated from the diagrams that ARPES builds up. ARPES measures of Bi2Se3 show that electron–phonon coupling remains among the weakest known to have been reported for any material, even as the temperature approaches room temperature.
Fedorov said, “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 even quantum computing.
For example, by layering a superconducting material onto the surface of a topological insulator, 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.”
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 of a topological insulator does not usually pass through the Dirac point. For surface electrons, distinct spin states (red arrows) are associated with each different orientation in momentum space (right).
