Anomalous electronic structure and magnetoresistance in TaAs$_2$

TL;DR

This paper reports the discovery of giant negative magnetoresistance in TaAs$_2$, a topological semimetal, and explores its electronic structure, challenging previous assumptions about the mechanisms behind negative magnetoresistance.

Contribution

The study identifies TaAs$_2$ as a new topological semimetal with unusual magnetoresistance, and clarifies that negative magnetoresistance can occur without Dirac/Weyl fermions, expanding understanding of topological materials.

Findings

TaAs$_2$ exhibits -98% magnetoresistance at 3 T

Density functional calculations show it is a topological semimetal without Dirac dispersion

Negative magnetoresistance is not solely due to Dirac/Weyl fermions

Abstract

The resistance of a metal in a magnetic field can be very illuminating about its ground state. Some famous examples include the integer and fractional quantum Hall effects\cite{Klitzing-QHE,Tsui-FQHE}, Shubnikov-de Haas oscillations\cite{SdH}, and weak localization\cite{Lee-WL} \emph{et al}. In non-interacting metals the resistance typically increases upon the application of a magnetic field\cite{Pippard-MR}. In contrast, in some special circumstances metals, with anisotropic Fermi surfaces\cite{Kikugawa-PdCoO2LMR} or a so-called Weyl semimetal for instance\cite{Nielsen-ABJ,Son-ChirAnom}, may have negative magnetoresistance. Here we show that semimetallic TaAs$_2$ possesses a gigantic negative magnetoresistance ($-$98\% in a field of 3 T at low temperatures), with an unknown mechanism. Density functional calculations illustrate that TaAs$_2$ is a new topological semimetal [$\mathbb{Z}_2$ invariant (0;111)] without a Dirac dispersion. This demonstrates that the presence of negative magnetoresistance in non-magnetic semimetals cannot be uniquely attributed to the Adler-Bell-Jackiw anomaly of bulk Dirac/Weyl fermions. Our results also imply that the OsGe$_2$-type monoclinic dipnictides are likely a material basis where unconventional topological semimetals may be found.