Transport Experiments on Topological Insulators
Abstract
The ARO-supported research focused on uncovering novel materials and phenomena associated with topological phases. A new area is the Dirac semimetals, Cd3As2 and Na3Bi, in which the electrons occupy 3D bulk Dirac states much like graphene, with protected nodes. In a magnetic field, the nodes separate into Weyl nodes. The most important achievement is the observation in Na3Bi of the chiral anomaly which appears as a field-induced enhanced conductivity when the magnetic field is strictly aligned with the electric field (observed as a negative, longitudinal magneto resistance LMR). The observation, which confirms a prediction from 1983, provides a powerful probe of the chiral current and transport properties of Weyl states in general. A second material (a half-Heusler) has also been found to display the chiral anomaly. Application of a magnetic field leads to the appearance of 2 or 4 Weyl nodes. Again, a large negative LMR is observed. In addition, a large suppression of the thermoelectric coefficient is observed when the chiral anomaly is present. These findings broaden significantly the range of compounds that may exhibit Weyl physics and the chiral anomaly. In a different direction, Murakami has proposed that Weyl nodes always appear when the bulk energy gap in a semiconductor lacking inversion symmetry is forced to close. High pressure has been used to close the gap in PbSnTe. Careful tuning of the pressure allows the quantum oscillations in the MR to be closely monitored as the Fermi sphere expands out of the charge vacuum. An anomalous Hall effect was observed when the lowest Landau level is occupied. This system is an especially attractive platform for exploring Weyl physics in the gap-closing scenario. Finally, the Berry curvature which underpins transport features in Weyl systems, has been shown to produce a Hall effect in neutral spin excitations.
Document Details
- Document Type
- Technical Report
- Publication Date
- Aug 16, 2016
- Accession Number
- AD1027910
Entities
People
- Nai-phuan Ong
Organizations
- Princeton University