Researchers in the US say that they have made the best 3D topological insulator to date. The material is called bismuth antimony tellurium selenide (BiSbTeSe2) and could be of fundamental importance for testing a number of condensed-matter and particle-physics theories. The material could also find use in spintronics devices and be used to build robust topological quantum bits (qubits) for quantum computers.
Topological insulators are materials that are electrical insulators in the bulk but can conduct electricity on their surface via special surface electronic states. “Most topological insulators made to date have not been completely insulating in the bulk, because of impurities (unintentionally introduced during material synthesis or processing) that doped the bulk and made it conducting,” explains Yong Chen of Purdue University, who led the research. “Our topological insulator appears not to conduct at all in the bulk but does so only at its surface.”
The researchers worked this out by measuring how thin flakes of BiSbTeSe2 of various thicknesses conducted electricity. They found that the conductance of different samples was almost independent of their thicknesses. Such behaviour is completely different to that seen in normal 3D materials, in which conductance is proportional to sample thickness.
“Our result is consistent with the picture that bulk BiSbTeSe2 is only conducting at its surface,” Chen explains. “It is like you were to keep cutting and reducing the material to ever smaller thicknesses and strangely never finding the conductance to change much. This is because every time you create a new surface, you get the same conduction.” Indeed, the researchers observed this topological surface conduction even at room temperature for samples thinner than around 100 nm – properties that could lead to practical applications.
And that is not all: Chen and colleagues also found evidence for a well-defined “half-integer” quantum Hall effect (QHE), where the top and bottom surfaces of their thin-slab samples each contribute a half-integer unit of quantum conductance (e2/h), where e is the charge on the electron and h is the Planck constant. These two half-integer units make up the measured Hall conductance plateau – quantized at integer units of e2/h. Such half-integer QHE is another unique signature of topological surface-state charge carriers, which are, in fact, spin-polarized massless Dirac fermions.
These massless Dirac fermions are analogous to the massless Dirac fermions that exist in graphene – one-atom-thick sheets of carbon – and give that material its exceptionally large electrical conductivity. In graphene, the charge carriers are not spin-polarized and exist in four degenerate states, whereas on a topological insulator surface there is only one state – the simplest, or “1/4 graphene” state.
High magnetic fields
The researchers observed the QHE in BiSbTeSe2 flakes that were between tens to hundreds of nanometres thick, at cryogenic temperatures of below 30 K and at high magnetic fields applied perpendicular to the top and bottom surfaces of the samples.
“For this part of our experiments, we used a powerful magnet (where we can get up to 33 T) at the National High Magnetic Field Lab in Tallahassee, Florida,” says Chen. Together, these results suggest that BiSbTeSe2 is a “perfect” topological insulator that behaves just how theory says it should. It is thus an excellent material platform on which to look for the exotic physics phenomena that are predicted to exist in topological insulators.
One phenomenon involves collective excitations – or quasiparticles – that resemble Marjorana fermions. First predicted by the Italian physicist Ettore Majorana in 1937, the Majorana fermion has zero charge and is its own antiparticle. While Majorana fermions have never been seen as free particles, there is some evidence that Majorana-like quasiparticles can exist at the interface between an ordinary superconductor and a topological insulator. If Majorana quasiparticles could be created reliably, they could be used to make “topological qubits”. Unlike conventional qubits, such topological qubits would be immune to being destroyed by environmental noise and could therefore form the basis of fault-tolerant quantum computers.
Spintronics and “topological magnetoelectronics”
Another promising application for BiSbTeSe2 is to use the spin-polarization of the charge carriers to create spintronic devices. This is a relatively new technology that seeks to use electron spin to create devices that are smaller, faster and more energy-efficient than conventional electronics.
Another possible application is “topological magnetoelectronics”, which would involve creating effective magnetic monopoles in the material. This could be done by exploiting an unusual form of electromagnetism (different from that described by conventional Maxwell’s equations) that is predicted for such 3D topological insulators.
The team, which includes physicists from Purdue University, Princeton University and the University of Texas at Austin, reports its work in Nature Physics.