Topological insulators (TIs) are artificially constructed materials that are insulating in the bulk but can conduct electricity on their surface via special Dirac-like surface electronic states. These surface states are said to be topologically protected because they are robust to background environmental noise thanks to the electronic properties of the bulk material.

A team led by Dmitri Basov at Columbia University in New York has now performed Faraday rotation and cyclotron resonance magneto-transmission spectroscopy on thin films of the topological insulator bismuth antimony telluride (Bi1-xSbx)2Te3 (or BST) placed on an indium phosphate (InP) substrate. These two techniques, when combined, provide information about the behaviour of the topological surface states both at the top and bottom surfaces of the sample. From the data obtained, the researchers discovered that the two surface states are completely different and host electron- and hole-type Dirac fermions.

Exotic quantum phenomena in 3D TIs

“Observing such separated n- and p-type Dirac fermions paves the way for detecting exotic quantum phenomena in 3D TIs, like topological exciton condensation,” says team member and lead author of the study Yinming Shao. “Our result may even help researchers in their efforts to make vertical p-n junctions, which are expected to exhibit exotic behaviour too, and provide an interesting test ground for the recently verified quantized Faraday and Kerr rotations in TI surface states.”

The team obtained its result by coupling a Fourier-transform infrared spectrometer to an 8 Tesla split-coil magnet to first obtain the magneto-transmission spectra of both the bare InP substrate and the BST thin film on the substrate. “Free charge carriers in the BST film are forced to undergo cyclotron motion in a magnetic field, resulting in cyclotron resonance features in the spectra we obtain,” explains Shao. “We then added two polarizers, one in front and one behind the sample, to measure the rotation of light in the magnetic field. This is the Faraday rotation.”

Experimental results combined with theoretical models

“The complimentary data set of cyclotron resonance and Faraday rotation encodes the charge dynamics of BST films,” he adds. “We are thus able to identify the surface state carriers and distinguish between the different n- and p-type Dirac fermions present at the opposite sides of the sample.”

To confirm their result, the researchers combined the data from their measurements with a magneto-optical Drude model to extract the cyclotron frequency, “Drude weight” and charge carrier scattering rates of different conducting channels in the BST films. “In particular, we used RefFIT software, developed by Alexey Kuzmenko and colleagues at the University of Geneva, in the modelling,” says Shao.

The Columbia team, reporting its work in Nano Letters DOI: 10.1021/acs.nanolett.6b04313, says that it would now like to further reduce the bulk conductivity of its BST films or similar TI systems to make measurements easier. “We will also be using magneto-optical probes to study whether there are observable interaction effects between the n- and p-type surface-state carriers,” Shao tells nanotechweb.org. “The BST films we have made thus far have weak but still finite bulk charge carriers, effectively screening any interaction between the top and bottom surface state carriers. With further reduced bulk conductivity and/or reduced film thickness, the top and bottom surface could ‘talk’ to each other. Such interactions between coupled n- and p-type surface state carriers could give rise to novel quantum phenomena that have so far remained elusive.”

Indeed, Peter Armitage of Johns Hopkins University and Liang Wu of the University of California, Berkeley, who were not involved in this study, say that the fact that there is a breaking of the symmetry between top and bottom surfaces in the sample studied is an important observation and will be a crucial ingredient in incorporating these materials in magnetoelectric devices – if the bulk conduction could be eliminated.