Transport Experiments of Topological Insulators and Dirac Semimetals

Abstract

The progress in understanding the Berry phase of Bloch electrons in crystals has triggered tremendous interest in discovering novel topological phases of solids. The integration of the Berry curvature in the Brillouin zone can categorize solids into phases such as topological insulators (TI), Dirac semimetals (DSM) and Weyl semimetals (WSM). These new phases have unconventional electronic states at the boundaries, such as the spin polarized electrons on the surface of a three-dimensional TI. Under proper engineering, such edge states can carry a dissipationless current, leading to a great application potential in low-power devices and topological quantum

computers.

Besides TI, the newly discovered Dirac and Weyl semimetals represent another example in which electrons have a linear energy-momentum dispersion. The paired Weyl nodes have opposite chiralities, and can be regarded as positive and negative monopoles of the Berry flux. Under the time-reversal, inversion and certain crystal symmetries, as in the cases of Cd3As2 and Na3Bi, the Weyl nodes with different chiralities can coexist at the same point in the Brillouin zone and the crystal becomes a Dirac semimetal. Such semimetals provide platforms for some phenomena in high

energy physics, such as the chiral anomaly effect.

The above predictions lie at the heart of our experimental study of topological materials. We synthesized a topological insulator, Bi2Te2Se, with a suppressed bulk carrier density. Analysis of the prominent Shubnikov-de Haas oscillations in Bi2Te2Se demonstrates clear evidence for the Dirac surface electrons and their \pi Berry phase. We also leveraged the ionic liquid gating technique to bring the chemical potential 50% closer to the Dirac point. Additionally, we studied two types of Na3Bi, a DSM. The first type with a high chemical potential exhibits a large and linear magnetoresistance (MR), implying a transport lifetime steeply tuned by the magnetic field. In the second

type of Na3Bi with a low chemical potential, we observed a novel, negative and highly anisotropic magnetoresistance. By rotating both the electric and magnetic fields, we demonstrate that the negative MR pattern is consistent with the theoretical prediction for the chiral anomaly effect in a DSM.