Anomalous electronic structure and magnetoresistance in TaAs$_2$
  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. 
Yongkang Luo, R. D. McDonald, P. F. S. Rosa, B. Scott, N. Wakeham, N.   J. Ghimire, E. D. Bauer, J. D. Thompson, and F. Ronning
cond-mat.str-el cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.supr-con