#Unexpectedly fast conduction electrons in Na3Bi

“We acquire ‘mappings’ of the quantum tunneling current between the tip and the sample at different biases,” explains Iolanda.
The derivatives of these mappings show very typical patterns, originating from the scattering of the electrons with the disorder in the sample.
This scattering process mixes electrons that are on the same constant-energy contours in reciprocal space, which is made visible by taking a Fourier transform of the mappings.

“In our case, this yielded circles corresponding to cuts along a Dirac-cone like dispersion.” (see figure).
This analysis technique allowed the team to reconstruct the (linear) band dispersion in the material and extract the charge carriers’ velocities, both in the valence and the conduction bands.
But when these measured band dispersions were compared with theoretical predictions, there was a problem: the measured velocities for the lowest-lying conduction and valence bands were significantly higher than theoretical predictions.
However, the team found one way to significantly improve agreement between measurement and theory:
“We used increasingly complicated models to describe our system, and discovered that as we improved the treatment of the exchange and correlation potential in the model (going from PBE to GW methods), we could get closer to the experimental values—even though we still observed some discrepancies,” explains Iolanda.
While the origin of these unexpectedly strong interactions is still unclear, the new study demonstrates that exchange-correlation effects are likely at the base of the high velocity of electrons in Na₃Bi.
Understanding the ultra-high mobilities of carriers in topological Dirac semimetals is a step toward the successful implementation of these materials in devices for low-energy electronics.
The study, titled “Importance of interactions for the band structure of the topological Dirac semimetal Na₃Bi,” was published in July 2020 in Physical Review B.